= AIVDM/AIVDO protocol decoding
Eric S. Raymond <esr@thyrsus.com>
v1.58, 24 June 2023
:author: Eric S. Raymond
:description: The AIVDM Marine AIS protocol demystified, for programmers.
:email: <esr@thyrsus.com>
:keywords: AIS, AIVDM, AIVDO, standard, protocol, Automatic Identification System
:robots: index,follow
:sectlinks:
:toc: macro

include::inc-menu.adoc[]

This document is mastered in asciidoc format.  If you are reading it in HTML,
you can find the original at the GPSD project website

// If you are reading the asciidoc master of this document, the
// occasional comments led with '//!' are hints for GPSD developer tools
// that use the table layouts to generate code.

If you find this document useful - and especially if it helps you make
money - please contribute to maintaining it by supporting the author's
full-time open-source work through <<PATREON>>. Even a few dollars a
week can make a difference.

== Introduction

This is a description of how to decode AIVDM/AIVDO sentences.  It
collects and integrates information from publicly available sources
and is intended to assist developers of open-source software for
interpreting these messages.

AIVDM/AIVDO sentences are emitted by receivers for AIS, the marine
Automatic Identification System. AIS transmitters are fitted to
vessels, navigation markers, and certain types of shore station. They
periodically squawk their position (and course, when applicable), using
TDMA (Time Division Multiple Access) technology similar to the way
cellphones do to avoid mutual interference. AIS receivers make this data
available for navigation, anti-collision systems, and other uses.

The International Maritime Organization's (IMO) International
Convention for the Safety of Life at Sea (SOLAS) requires operating AIS
transmitters on all international cargo vessels of more than 300 tons
displacement, all cargo vessels of more than 500 tons displacement,
and all passenger vessels; see <<SOLAS>> for details.  Individual
maritime nations may have stricter and more detailed rules: for those
obtaining in U.S. waters, see <<US-REQUIREMENTS>>.

AIS receivers report ASCII data packets as a byte stream over serial
or USB lines, using the NMEA 0183 or NMEA 2000 data formats. The RS422
variant of serial specified as a physical layer by NMEA 0183 is common
in marine navigation systems; there may be a "pilot plug"
<<PILOTPLUG>> which converts to USB. Alternatively, newer AIS
receivers may report directly over RS232 or USB.

AIS packets have the introducer "!AIVDM" or "!AIVDO"; AIVDM packets
are reports from other ships and AIVDO packets are reports from your
own ship.

A lengthy description of AIS, focusing on the goals and history of the
system, but not describing the data protocols in any detail, can be
found at <<AIS>>.

== Standards

Multiple standards bear on the AIVDM/AIVDO format.  This document
exists because assembling them into a complete picture is technically
difficult and was long impeded by legal barriers as well.

ITU Recommendation M.1371, "Technical Characteristics for a Universal
Shipborne Automatic Identification System Using Time Division Multiple
Access" <<ITU1371>>, issued in 2001, first described the bit-level
format of AIS radio messages. This standard was proprietary and
expensive when issued. I did not have access to it or any of its
followup revisions while assembling versions of this document up to
1.29.

ITU-R M.1371 was expanded and clarified by "IALA Technical
Clarifications on Recommendation ITU-R M.1371-1" <<IALA>>, which
is freely available.

There have been three minor revisions of ITU-R M.1371 since it was
originally issued.  These add interpretations to packet bit fields that
were previously marked "spare" and "regional reserved". ITU-R M.1371
revision 4 became available for free download, apparently at some
point in early 2011, well after most of this document was assembled.

The ASCII format for AIVDM/AIVDO representations of AIS radio messages
seems to have been set by IEC-PAS 61162-100, "Maritime navigation and
radiocommunication equipment and systems" <<IEC-PAS>>. It is
proprietary and I have not looked at it.  Various public sources
indicate that it has been "harmonized" with some version of NMEA 0183,
which I also have not looked at because it too is proprietary and
expensive, and surrounded by rapacious attack lawyers.

== Information Sources

Together, the IALA Technical Clarifications at <<IALA>> and the
Coast Guard's AIS pages at <<NAVCEN>> describe AIS message payloads
type 1-24 almost completely. Certain specialized binary messages of
types 6 and 8 defined by the International Maritime Organization are
described in <<IMO236>> and <<IMO289>>. The detail information on
payload formats in this document is mostly derived from these public
sources.

Kurt Schwehr is a research scientist at the Center for Coastal and
Ocean Mapping at the University of New Hampshire. Much of his research
involves AIS.  His work blog at <<Schwehr>> contains sample messages
and descriptions of AIS operation in the wild that shed light on
various obscure corners of the specification.  He has explained the
otherwise undocumented Repeat Indicator field and USCG extended AIVDM
to me by email. He also communicated some critical information from
<<IEC-PAS>>, and supplied information about new messages and fields in
ITU-1371-3.

Descriptions of messages 25 and 26 are based on AIS transceiver vendor
documentation was originally forwarded to me by a source wishing to
remain anonymous; I later checked them against ITU1371-4.  Message
type 27 was described in ITU1371-4 and added here after that became a
freely available download.

Should you set out to write a decoder using this document, you are
likely to find that one of your challenges is finding enough AIS
packet data to make an effective regression test -- especially if you
live out of line of sight of any ship traffic and would get nothing
from running an AIS receiver.  Fortunately, various AIS sites offer
live feeds over the Internet that aggregate AIS streams from all over
the world. Some charge subscriptions; others offer time-delayed access
for free and charge for a real-time feed. Still others are pool sites;
you join by contributing your feed and receive all feeds.

AIS Hub (<<AISHUB>>) is a free, public AIS feed pool. It provides
exchange of AIS data in raw NMEA format; all AISHub members share
their own received AIS data and receive the merged feed from all other
participating parties.  It is open-source friendly, offering a Linux
port in source of its software for collecting and forwarding AIS
data. Peter Stoyanov and the other AIS Hub principals have generously
donated a live feed to the GPSD project despite the fact that I live
60 miles inland and cannot send them anything interesting.

Some of what this document explains about the quirks of real-world
encoders comes from examples provided by Kurt Schwehr. Other such
information, especially for the less common sentences, comes from
inspection of sentences forwarded to me from AIS Hub by various
interested parties, and more recently from AIS Hub itself.

== Improving This Document

To avoid copyright difficulties, I rely only on freely redistributable
public documents and paraphrased reports from people who have seen the
relevant proprietary standards.  If you are such a person, please help
by reporting the following to be included in future versions of this
document:

* Sample sentences of types 16-17, 22-23, and 25-27.

* Sample sentences of type 6 and 8 conforming to <<IMO236>>,
  <<IMO289>>, and <<INLAND>>.

For verification purposes, I need the raw sentences together with
decoded dumps of their field values.  Please note that sample
sentences not accompanied by field dumps are *not* useful; I can get
plenty of those.

== Open-Source Implementations

The GPSD project ships an AIVDM/AIVDO sentence decoder as part of the
daemon. This document was developed to be the specification for it,
and it will decode all sentence type described herein.

The source-code repository of the GPSD project holds a conforming
standalone Python decoder, ais.py, that is not included in shipped
releases. It may be useful for developers working in that language.

<<Schwehr>> includes links to a collection of Python scripts for
decoding and analyzing AIVDM sentences.  Kurt Schwehr warns that this
is research code rather than a production tool.

There is a http://gnuais.sourceforge.net/[GNU AIS] project at
SourceForge.  It seems intended primarily to work directly
with AIS radios.

http://www.maritec.co.za/aisvdmvdodecoding[The Maritec decoder]
looks to be pretty high-quality and can be exercised through a
public web form. This is a useful resource for anyone qualifying
an AIS decoder.

== AIVDM/AIVDO Sentence Layer

AIVDM/AIVDO is a two-layer protocol.  The outer layer is a variant of
NMEA 0183, the ancient standard for data interchange in marine
navigation systems; NMEA 0183 is described at <<NMEA>>.

Here is a typical AIVDM data packet:

--------------------------------------------------------------------
!AIVDM,1,1,,B,177KQJ5000G?tO`K>RA1wUbN0TKH,0*5C
--------------------------------------------------------------------

And here is what the fields mean:

Field 1, !AIVDM, identifies this as an AIVDM packet.

Field 2 (1 in this example) is the count of fragments in the currently
accumulating message.  The payload size of each sentence is limited by
NMEA 0183's 82-character maximum, so it is sometimes required to split
a payload over several fragment sentences.

Field 3 (1 in this example) is the fragment number of this
sentence. It will be one-based.  A sentence with a fragment count of
1 and a fragment number of 1 is complete in itself.

Field 4 (empty in this example) is a sequential message ID for
multi-sentence messages.

Field 5 (B in this example) is a radio channel code. AIS uses the high
side of the duplex from two VHF radio channels: AIS Channel A is
161.975Mhz (87B); AIS Channel B is 162.025Mhz (88B).  In the wild,
channel codes '1' and '2' may also be encountered; the standards
do not prescribe an interpretation of these but it's obvious enough.

Field 6 (177KQJ5000G?tO`K>RA1wUbN0TKH in this example) is the data
payload. We'll describe how to decode this in later sections.

Field 7 (0) is the number of fill bits requires to pad the data
payload to a 6 bit boundary, ranging from 0 to 5.  Equivalently,
subtracting 5 from this tells how many least significant bits of the
last 6-bit nibble in the data payload should be ignored. Note that
this pad byte has a tricky interaction with the <<ITU1371>>
requirement for byte alignment in over-the-air AIS messages; see the
detailed discussion of message lengths and alignment in a later
section.

The \*-separated suffix (\*5C) is the NMEA 0183 data-integrity checksum
for the sentence, preceded by "*".  It is computed on the entire
sentence including the AIVDM tag but excluding the leading "!".
This checksum is the XOR of all bytes in the payload.

For comparison, here is an example of a multi fragment sentence with a
nonempty message ID field:

--------------------------------------------------------------------
!AIVDM,2,1,3,B,55P5TL01VIaAL@7WKO@mBplU@<PDhh000000001S;AJ::4A80?4i@E53,0*3E
!AIVDM,2,2,3,B,1@0000000000000,2*55
--------------------------------------------------------------------

Technically, NMEA0183 does not actually require that a !-led sentence
be AIS.  This format can be used for any encapsulated data. The
syntax and semantics of fields 1-4 are fixed, and the fill-bit field
and NMEA checksum are required, but the payload fields may contain any
encapsulated data.

It is, however, a safe bet that any such sentence containing an A or B
channel code in field 5 is AIVDM/AIVDO.

== Talker IDS

The AI prefix commonly found on these sentences is an NMEA talker ID
for a mobile AIS station.  Other possible values are as listed:

.AIS talker IDs
[width="25%",frame="topbot"]
|======
| !AB | NMEA 4.0 Base AIS station
| !AD | NMEA 4.0 Dependent AIS Base Station
| !AI | Mobile AIS station
| !AN | NMEA 4.0 Aid to Navigation AIS station
| !AR | NMEA 4.0 AIS Receiving Station
| !AS | NMEA 4.0 Limited Base Station
| !AT | NMEA 4.0 AIS Transmitting Station
| !AX | NMEA 4.0 Repeater AIS station
| !BS | Base AIS station (deprecated in NMEA 4.0)
| !SA | NMEA 4.0 Physical Shore AIS Station
|======

== AIVDM/AIVDO Payload Armoring

The data payload is an ASCII-encoded bit vector.  Each character
represents six bits of data.  To recover the six bits, subtract 48
from the ASCII character value; if the result is greater than 40
subtract 8. According to <<IEC-PAS>>, the valid ASCII characters for
this encoding begin with "0" (64) and end with "w" (87); however, the
intermediate range "X" (88) to "\_" (95) is not used.

.ASCII payload armoring
[width="25%",frame="topbot",options="header"]
|======
| Char    |ASCII   |Decimal   |Bits
|"0"      | 48     | 0        |000000
|"1"      | 49     | 1        |000001
|"2"      | 50     | 2        |000010
|"3"      | 51     | 3        |000011
|"4"      | 52     | 4        |000100
|"5"      | 53     | 5        |000101
|"6"      | 54     | 6        |000110
|"7"      | 55     | 7        |000111
|"8"      | 56     | 8        |001000
|"9"      | 57     | 9        |001001
|":"      | 58     |10        |001010
|";"      | 59     |11        |001011
|"<"      | 60     |12        |001100
|"="      | 61     |13        |001101
|">"      | 62     |14        |001110
|"?"      | 63     |15        |001111
|"@"      | 64     |16        |010000
|"A"      | 65     |17        |010001
|"B"      | 66     |18        |010010
|"C"      | 67     |19        |010011
|"D"      | 68     |20        |010100
|"E"      | 69     |21        |010101
|"F"      | 70     |22        |010110
|"G"      | 71     |23        |010111
|"H"      | 72     |24        |011000
|"I"      | 73     |25        |011001
|"J"      | 74     |26        |011010
|"K"      | 75     |27        |011011
|"L"      | 76     |28        |011100
|"M"      | 77     |29        |011101
|"N"      | 78     |30        |011110
|"O"      | 79     |31        |011111
|"P"      | 80     |32        |100000
|"Q"      | 81     |33        |100001
|"R"      | 82     |34        |100010
|"S"      | 83     |35        |100011
|"T"      | 84     |36        |100100
|"U"      | 85     |37        |100101
|"V"      | 86     |38        |100110
|"W"      | 87     |39        |100111
|"`"      | 96     |40        |101000
|"a"      | 97     |41        |101001
|"b"      | 98     |42        |101010
|"c"      | 99     |43        |101011
|"d"      |100     |44        |101100
|"e"      |101     |45        |101101
|"f"      |102     |46        |101110
|"g"      |103     |47        |101111
|"h"      |104     |48        |110000
|"i"      |105     |49        |110001
|"j"      |106     |50        |110010
|"k"      |107     |51        |110011
|"l"      |108     |52        |110100
|"m"      |109     |53        |110101
|"n"      |110     |54        |110110
|"o"      |111     |55        |110111
|"p"      |112     |56        |111000
|"q"      |113     |57        |111001
|"r"      |114     |58        |111010
|"s"      |115     |59        |111011
|"t"      |116     |60        |111100
|"u"      |117     |61        |111101
|"v"      |118     |62        |111110
|"w"      |119     |63        |111111
|======

Concatenate all six-bit quantities found in the payload, MSB first.
This is the binary payload of the sentence.

== AIS Payload Data Types

Data in AIS message payloads (what you get after undoing the AIVDM/AIVDO
armoring) is encoded as bit fields in the sentence.  Bit fields may be
interpreted in one of the following ways:

- Signed or unsigned integer
- Float (scaled from signed integer)
- Flag or Boolean
- Index into a controlled vocabulary
- Reserved bits
- Spare bits
- Strings

Numeric bit fields are interpreted as big-endian twos-complement
integers; when signed, the sign bit is the highest.

Float fields have an associated divisor which should be applied
to convert to the correct units. In one case, the ROT field in message
types 1-3, the scaling operation involves a more elaborate formula.

Flags are encoded as 1 for true/yes/on, 0 for false/no/off.

Indices into controlled vocabularies are numeric bit fields which
must be interpreted using per-field string lists specified in the
standards.

Spare fields generally seem to have been inserted in order to put
certain field starts on 8-bit boundaries, and should be ignored.  Decoders
should not, however, assume that spare fields will be all zeroes.

Reserved fields should not be ignored, as they may be assigned for
extension data in minor revisions of the AIS standard; it is noted in
the message descriptions where this has already occurred.  It is good
practice for a decoder to make reserved fields available to client
applications as uninterpreted bit fields.

Character-string fields within AIS messages are encoded in a special
way, referred to as "six-bit" in the tables below. First, chop the
string field into consecutive six-bit nibbles without padding (each
span of three 8-bit bytes includes 4 of these). Each six-bit nibble
maps to an ASCII character.  Nibbles 0-31 map to the characters "@" (
ASCII 64) through "\_" (ASCII 95) respectively; nibbles 32-63 map to
characters " " (ASCII 32) though "?"  (ASCII 63). Lowercase ASCII
letters, the backtick, right and left curly brackets, pipe bar, tilde
and DEL cannot be encoded.

.Sixbit ASCII
[width="25%",frame="topbot"]
|======
|000000 | 0  |"@"    |010000 |16  |"P"            |100000 |32  |" "     |110000 |48  |"0"
|000001 | 1  |"A"    |010001 |17  |"Q"            |100001 |33  |"!"     |110001 |49  |"1"
|000010 | 2  |"B"    |010010 |18  |"R"            |100010 |34  |"""     |110010 |50  |"2"
|000011 | 3  |"C"    |010011 |19  |"S"            |100011 |35  |"\#"    |110011 |51  |"3"
|000100 | 4  |"D"    |010100 |20  |"T"            |100100 |36  |"$"     |110100 |52  |"4"
|000101 | 5  |"E"    |010101 |21  |"U"            |100101 |37  |"%"     |110101 |53  |"5"
|000110 | 6  |"F"    |010110 |22  |"V"            |100110 |38  |"&"     |110110 |54  |"6"
|000111 | 7  |"G"    |010111 |23  |"W"            |100111 |39  |"\'"    |110111 |55  |"7"
|001000 | 8  |"H"    |011000 |24  |"X"            |101000 |40  |"("     |111000 |56  |"8"
|001001 | 9  |"I"    |011001 |25  |"Y"            |101001 |41  |")"     |111001 |56  |"9"
|001010 |10  |"J"    |011010 |26  |"Z"            |101010 |42  |"\*"    |111010 |58  |":"
|001011 |11  |"K"    |011011 |27  |"["            |101011 |43  |"\+"    |111011 |59  |";"
|001100 |12  |"L"    |011100 |28  |"{backslash}"  |101100 |44  |","     |111100 |60  |"<"
|001101 |13  |"M"    |011101 |29  |"]"            |101101 |45  |"-"     |111101 |61  |"="
|001110 |14  |"N"    |011110 |30  |"\^"           |101110 |46  |"."     |111110 |62  |">"
|001111 |15  |"O"    |011111 |31  |"\_"           |101111 |47  |"/"     |111111 |63  |"?"
|======

According to the standard, trailing unused characters in six-bit
fields will be represented by "@" (six-bit zero); however, real-world
encoders are not careful about this and often have nonzero garbage
after the "@". The terminating "@" should not be considered part of
the text, and any non-"@" characters after it should be discarded.  It
is also common to space-fill short fields such as ship and station
name, so a decoder should strip trailing spaces after stripping
at-signs and the garbage after them.

Trailing string fields are often specified as "up to" a certain number of bits.
Decoders should be prepared to handle any field length up to that
number, including zero.

== AIS Payload Interpretation

The following table describes message types that are international
standards from <<ITU1371>> and its revisions.  There are also local
and regional extensions used on inland waterways such as the Danube
and the Thames and in British and Irish coastal waters; pointers
to some of these are included later in this document.

Note that many sources use 1-origin numbering for the bits.  We'll
use 0-origin in this document.

The first 6 bits of the payload (0-5) are the message type.  Message
types are as follows:

.Message types
[width="50%",frame="topbot"]
|======
|01 |Position Report Class A
|02 |Position Report Class A (Assigned schedule)
|03 |Position Report Class A (Response to interrogation)
|04 |Base Station Report
|05 |Static and Voyage Related Data
|06 |Binary Addressed Message
|07 |Binary Acknowledge
|08 |Binary Broadcast Message
|09 |Standard SAR Aircraft Position Report
|10 |UTC and Date Inquiry
|11 |UTC and Date Response
|12 |Addressed Safety Related Message
|13 |Safety Related Acknowledgement
|14 |Safety Related Broadcast Message
|15 |Interrogation
|16 |Assignment Mode Command
|17 |DGNSS Binary Broadcast Message
|18 |Standard Class B CS Position Report
|19 |Extended Class B Equipment Position Report
|20 |Data Link Management
|21 |Aid-to-Navigation Report
|22 |Channel Management
|23 |Group Assignment Command
|24 |Static Data Report
|25 |Single Slot Binary Message,
|26 |Multiple Slot Binary Message With Communications State
|27 |Position Report For Long-Range Applications
|======

In normal operation, an AIS transceiver will broadcast a position report
(type 1, 2, or 3) every 2 to 10 seconds depending on the vessel's
speed while underway, and every 3 minutes while the vessel is at
anchor and stationary. It will send a type 5 identification every 6
minutes. (More detail is at <<IALA>>, part 2.3)

Class 6 is used for unencrypted structured extension messages systems
conforming to the Inland AIS standard defined by <<INLAND>>, and by local
authorities such as the St. Lawrence Seaway and the U.S Coast Guard's
PAWSS.  This document describes all of the Class 6 special message
formats approved for use in <<IMO236>>, <<IMO289>>, and <<INLAND>>.

Class 8 is in common use for private encrypted messages, such as
location transmission in military exercises. It is also used for
unencrypted structured extension messages by Inland AIS, and by local
authorities such as the St. Lawrence Seaway and PAWSS. This document
describes all of the Class 8 special message formats approved for use
in <<IMO236>>, <<IMO289>>, and <<INLAND>>.

Classes 12 and 14 are used for text messaging, nominally safety-related
but also for traffic control and occasionally chatter.

In practice, message types other than 1, 3, 4, 5, 18, and 24 are
unusual or rare; many AIS transmitters never emit them.

An MMSI is a Mobile Marine Service Identifier, a unique 9-digit ID for
the ship's radio(s).  The first three digits convey information about
the country in which the ID was issued <<ITU-MID>>.  US vessels
traveling solely in U.S. waters sometimes incorrectly omit the
leading "3", the geography code for North and Central America and
Caribbean, emitting 8-digit MMSIs beginning with the U.S. country code
of 669.

According to <<MMSI>>, different formats of MMSI are used for
different classes of transmitter. In the format descriptions below, a
MID is a three-digit decimal literal ranging from 201 to 775 that
identifies a country or other maritime jurisdiction.  See <<ITU-MID>>
for a list of MIDs.

.MID formats
[frame="topbot"]
|======
|8MIDXXXXX  | Diver's radio (not used in the U.S. in 2013)
|MIDXXXXXX  | Ship
|0MIDXXXXX  | Group of ships; the U.S. Coast Guard, for example, is 03699999
|00MIDXXXX  | Coastal stations
|111MIDXXX  | SAR (Search and Rescue) aircraft
|99MIDXXXX  | Aids to Navigation
|98MIDXXXX  | Auxiliary craft associated with a parent ship
|970MIDXXX  | AIS SART (Search and Rescue Transmitter)
|972XXXXXX  | MOB (Man Overboard) device
|974XXXXXX  | EPIRB (Emergency Position Indicating Radio Beacon) AIS
|======

Detailed descriptions of message types 1-24 follow. Message types 1-22
are derived from <<IALA>>. Message type 23 was described to me by Mike
Greene based on <<IEC-62287>>.  Message type 24 was described to me by
<<Schwehr>>, whose Python toolkit decodes it. Message types 25-26 are
reported by <<Schwehr>>, who observes they were added in Version 3 of
<<ITU1371>>. Message type 27 is direct from <<ITU1371>> version 4.

The "Member" column in these tables is not derived from any of the ITU
standards or amendments.  I have invented it in order to be able to
describe a lossless textual encoding of AIS sentences in JSON.  These
names are also chosen for suitability as structure/object member names
in computer languages, so that application programming interfaces
across different languages can have a common and readily intelligible
set to use.

The "T" column declares the data type of the field, and may have
any of the values in the following table.  It is intended to be used
for generating bit field-extraction code directly from the message
type descriptions.

[frame="topbot"]
|======
|u|   Unsigned integer
|U|   Unsigned integer with scale - renders as float, suffix is decimal places
|i|   Signed integer
|I|   Signed integer with scale - renders as float, suffix is decimal places
|b|   Boolean
|e|   Enumerated type (controlled vocabulary)
|x|   Spare or reserved bit
|t|   String (packed six-bit ASCII)
|d|   Data (uninterpreted binary)
|a|   Array boundary, numeric suffix is maximum array size.
      ^ before suffix means preceding fields is the length.
      Following fields are repeated to end of message
|======

The field breakdowns in this document have been checked against live
decoded data rendered by known-good implementations for message types
1-15, 18-21, and 24.  Described but unchecked are 16-17, 22-23, and
25-27. Also, the interpretation of IMO extension subtypes of messages
6 and 8 has yet to be tested.

Bit lengths and length ranges are given because decoders should check
them against the message type. Messages with correct checksums but the
wrong payload length for their type occur with about 0.3% frequency on
AISHub; if you don't reject these, your clients will see spurious
zeros or garbage.

=== Types 1, 2 and 3: Position Report Class A

Type 1, 2 and 3 messages share a common reporting structure for
navigational information; we'll call it the Common Navigation Block
(CNB). This is the information most likely to be of interest for
decoding software. Total of 168 bits, occupying one AIVDM sentence.
More information on the CNB is at <<MSGS123>>>.

//: Type CNB
.Common Navigation Block
[frame="topbot",options="header"]
|======
|Field   |Len |Description             |Member    |T|Units
|0-5     | 6  |Message Type            |type      |u|Constant: 1-3
|6-7     | 2  |Repeat Indicator        |repeat    |u|Message repeat count
|8-37    |30  |MMSI                    |mmsi      |u|9 decimal digits
|38-41   | 4  |Navigation Status       |status    |e|See "Navigation Status"
|42-49   | 8  |Rate of Turn (ROT)      |turn      |I3|See below
|50-59   |10  |Speed Over Ground (SOG) |speed     |U1|See below
|60-60   | 1  |Position Accuracy       |accuracy  |b|See below
|61-88   |28  |Longitude               |lon       |I4|Minutes/10000 (see below)
|89-115  |27  |Latitude                |lat       |I4|Minutes/10000 (see below)
|116-127 |12  |Course Over Ground (COG)|course    |U1|Relative to true north,
                                                     to 0.1 degree precision
|128-136 | 9  |True Heading (HDG)      |heading   |u|0 to 359 degrees,
                                                      511 = not available.
|137-142 | 6  |Time Stamp              |second    |u|Second of UTC timestamp
|143-144 | 2  |Maneuver Indicator      |maneuver  |e|See "Maneuver Indicator"
|145-147 | 3  |Spare                   |          |x|Not used
|148-148 | 1  |RAIM flag               |raim      |b|See below
|149-167 |19  |Radio status            |radio     |u|See below
|======

The Repeat Indicator is a directive to an AIS transceiver that this
message should be rebroadcast.  This was intended as a way of getting
AIS messages around hills and other obstructions in coastal waters,
but is little used as base station coverage is more effective.  It is
intended that the bit be incremented on each retransmission, to a
maximum of three hops. A value of 3 indicates "Do not repeat".

.Navigation Status
[width="50%",frame="topbot"]
|======
|0 | Under way using engine
|1 | At anchor
|2 | Not under command
|3 | Restricted manoeuverability
|4 | Constrained by her draught
|5 | Moored
|6 | Aground
|7 | Engaged in Fishing
|8 | Under way sailing
|9 | Reserved for future amendment of Navigational Status for HSC
|10| Reserved for future amendment of Navigational Status for WIG
|11| Power-driven vessel towing astern (regional use)
|12| Power-driven vessel pushing ahead or towing alongside (regional use).
|13| Reserved for future use
|14| AIS-SART is active
|15| Ubdefined (default)
|======

Note, the AIS-SART value was added after <<IALA>> and designates an
AIS transmitter in an survival craft such as a lifeboat.  See
<<MSGS123>> for the field specification and <<SART>> for
background.

Turn rate is encoded as follows:

* 0 = not turning
* 1...126 = turning right at up to 708 degrees per minute or higher
* 1...-126 = turning left at up to 708 degrees per minute or higher
* 127 = turning right at more than 5deg/30s (No TI available)
* -127 = turning left at more than 5deg/30s (No TI available)
* 128 (80 hex) indicates no turn information available (default)

Values between 0 and 708 degrees/min coded by ROT~AIS~=4.733 *
SQRT(ROT~sensor~) degrees/min where ROT~sensor~ is the Rate of Turn as
input by an external Rate of Turn Indicator. ROT~AIS~ is rounded to
the nearest integer value.  Thus, to decode the field value, divide by 4.733
and then square it. Sign of the field value should be preserved when
squaring it, otherwise the left/right indication will be lost.

Speed over ground is in 0.1-knot resolution from 0 to 102 knots.
Value 1023 indicates speed is not available, value 1022 indicates
102.2 knots or higher.

The position accuracy flag indicates the accuracy of the fix.  A value of
1 indicates a DGPS-quality fix with an accuracy of < 10ms.  0, the
default, indicates an unaugmented GNSS fix with accuracy > 10m.

Longitude is given in in 1/10000 min; divide by 600000.0 to obtain
degrees. Values up to plus or minus 180 degrees, East = positive, West
\= negative.  A value of 181 degrees (0x6791AC0 hex) indicates that
longitude is not available and is the default.

Latitude is given in in 1/10000 min; divide by 600000.0 to obtain
degrees. Values up to plus or minus 90 degrees, North = positive,
South = negative. A value of 91 degrees (0x3412140 hex) indicates
latitude is not available and is the default.

Course over ground will be 3600 (0xE10) if that data is not available.

Seconds in UTC timestamp should be 0-59, except for these special values:

* 60 if time stamp is not available (default)
* 61 if positioning system is in manual input mode
* 62 if Electronic Position Fixing System operates in estimated (dead
     reckoning) mode,
* 63 if the positioning system is inoperative.

The Regional Reserved field is intended for use by local maritime
authorities.  It is not known to be in any actual use up to 2009.

The Maneuver Indicator (143-144) may have these values:

.Maneuver Indicator
[width="50%",frame="topbot"]
|======
|0 | Not available (default)
|1 | No special maneuver
|2 | Special maneuver (such as regional passing arrangement)
|======

Riverine and inland navigation systems conforming to <<INLAND>>
designate this field "Blue Sign" with the following enumerated
values:

.Blue Sign
[width="50%",frame="topbot"]
|======
|0 | Not available (default)
|1 | No
|2 | Yes
|======

The interpretation of bits 143-147 has been a bit unstable. In
<<IALA>> and therefore in the original <<ITU1371>>, they were
described like this:

[width="50%",frame="topbot",options="header"]
|======
|Field   |Len |Description
|143-145 | 3  |Regional Reserved
|146-147 | 2  |Spare
|======

The interpretation of 143-144 as a special maneuver field is new in
revision 3 of <<ITU1371>>.

The RAIM flag indicates whether Receiver Autonomous Integrity
Monitoring is being used to check the performance of the EPFD.
0 = RAIM not in use (default), 1 = RAIM in use.  See <<RAIM>> for
a detailed description of this flag.

Bits 149-167 are diagnostic information for the radio system.  Consult
<<IALA>> for detailed description of the latter.

=== Type 4: Base Station Report

This message is to be used by fixed-location base stations to
periodically report a position and time reference. Total of 168 bits,
occupying one AIVDM sentence.

The standard uses "EPFD" to designate any Electronic Position Fixing Device.

//: Type 4
[frame="topbot",options="header"]
|======
|Field   |Len  |Description      |Member   |T|Units
|0-5     |  6  |Message Type     |type     |u|Constant: 4
|6-7     |  2  |Repeat Indicator |repeat   |u|As in Common Navigation Block
|8-37    | 30  |MMSI             |mmsi     |u|9 decimal digits
|38-51   | 14  |Year (UTC)       |year     |u|UTC, 1-9999, 0 = N/A (default)
|52-55   |  4  |Month (UTC)      |month    |u|1-12; 0 = N/A (default)
|56-60   |  5  |Day (UTC)        |day      |u|1-31; 0 = N/A (default)
|61-65   |  5  |Hour (UTC)       |hour     |u|0-23; 24 = N/A (default)
|66-71   |  6  |Minute (UTC)     |minute   |u|0-59; 60 = N/A (default)
|72-77   |  6  |Second (UTC)     |second   |u|0-59; 60 = N/A (default)
|78-78   |  1  |Fix quality      |accuracy |b|As in Common Navigation Block
|79-106  | 28  |Longitude        |lon      |I4|As in Common Navigation Block
|107-133 | 27  |Latitude         |lat      |I4|As in Common Navigation Block
|134-137 |  4  |Type of EPFD     |epfd     |e|See "EPFD Fix Types"
|138-147 | 10  |Spare            |         |x|Not used
|148-148 |  1  |RAIM flag        |raim     |b|As for common navigation block
|149-167 | 19  |SOTDMA state     |radio    |u|As in same bits for Type 1
|======

.EPFD Fix Types
[width="50%",frame="topbot",options="header"]
|======
|Code |Position Fix Type
|0    |Undefined (default)
|1    |GPS
|2    |GLONASS
|3    |Combined GPS/GLONASS
|4    |Loran-C
|5    |Chayka
|6    |Integrated navigation system
|7    |Surveyed
|8    |Galileo
|9    |Reserved
|10   |Reserved
|11   |Reserved
|11   |Reserved
|13   |Reserved
|14   |Reserved
|15   |Internal GNSS
|======

Note: though values 9-15 are marked "not used" in <<IALA>>.
Decoders should be prepared to accept them as they are seen in the wild.
<<ITU1371>> specifies type value 15 (all field bits 1) as "Internal
GNSS".

=== Type 5: Static and Voyage Related Data

Message has a total of 424 bits, occupying two AIVDM sentences.

In practice, the information in these fields (especially ETA and
destination) is not reliable, as it has to be hand-updated by
humans rather than gathered automatically from sensors.

Also note that it is fairly common in the wild for this message to
have a wrong bit length (420 or 422). Robust decoders should ignore
trailing garbage and deal gracefully with a slightly truncated
destination field.

//: Type 5
[frame="topbot",options="header"]
|======
|Field   |Len |Description            |Member/Type  |T|Encoding
|0-5     |  6 |Message Type           |type         |u|Constant: 5
|6-7     |  2 |Repeat Indicator       |repeat       |u|Message repeat count
|8-37    | 30 |MMSI                   |mmsi         |u|9 digits
|38-39   |  2 |AIS Version            |ais_version  |u|0=<<ITU1371>>,
                                                       1-3 = future editions
|40-69   | 30 |IMO Number             |imo          |u|IMO ship ID number
|70-111  | 42 |Call Sign              |callsign     |t|7 six-bit characters
|112-231 |120 |Vessel Name            |shipname     |t|20 six-bit characters
|232-239 |  8 |Ship Type              |shiptype     |e|See "Codes for Ship Type"
|240-248 |  9 |Dimension to Bow       |to_bow       |u|Meters
|249-257 |  9 |Dimension to Stern     |to_stern     |u|Meters
|258-263 |  6 |Dimension to Port      |to_port      |u|Meters
|264-269 |  6 |Dimension to Starboard |to_starboard |u|Meters
|270-273 |  4 |Position Fix Type      |epfd         |e|See "EPFD Fix Types"
|274-277 |  4 |ETA month (UTC)        |month        |u|1-12, 0=N/A (default)
|278-282 |  5 |ETA day (UTC)          |day          |u|1-31, 0=N/A (default)
|283-287 |  5 |ETA hour (UTC)         |hour         |u|0-23, 24=N/A (default)
|288-293 |  6 |ETA minute (UTC)       |minute       |u|0-59, 60=N/A (default)
|294-301 |  8 |Draught                |draught      |U1|Meters/10
|302-421 |120 |Destination            |destination  |t|20 6-bit characters
|422-422 |  1 |DTE                    |dte          |b|0=Data terminal ready,
                                                       1=Not ready (default).
|423-423 |  1 |Spare                  |             |x|Not used
|======

<<INLAND>> specifies the following:

* the IMO Number field should be zeroed for inland vessels.
* ATIS code should be used for inland vessels
* ship dimensions should be set to the maximum rectangle size of the convoy
* draught information should be rounded up to nearest decimeter
* For the destination, UN/LOCODE and ERI terminal codes should be used

Ship dimensions will be 0 if not available. For the dimensions to bow
and stern, the special value 511 indicates 511 meters or greater; for
the dimensions to port and starboard, the special value 63 indicates
63 meters or greater.

.Codes for Ship Type
[width="50%",frame="topbot",options="header"]
|======
|Code   |Ship & Cargo Classification
|0      |Not available (default)
|1-19   |Reserved for future use
|20     |Wing in ground (WIG), all ships of this type
|21     |Wing in ground (WIG), Hazardous category A
|22     |Wing in ground (WIG), Hazardous category B
|23     |Wing in ground (WIG), Hazardous category C
|24     |Wing in ground (WIG), Hazardous category D
|25     |Wing in ground (WIG), Reserved for future use
|26     |Wing in ground (WIG), Reserved for future use
|27     |Wing in ground (WIG), Reserved for future use
|28     |Wing in ground (WIG), Reserved for future use
|29     |Wing in ground (WIG), Reserved for future use
|30     |Fishing
|31     |Towing
|32     |Towing: length exceeds 200m or breadth exceeds 25m
|33     |Dredging or underwater ops
|34     |Diving ops
|35     |Military ops
|36     |Sailing
|37     |Pleasure Craft
|38     |Reserved
|39     |Reserved
|40     |High speed craft (HSC), all ships of this type
|41     |High speed craft (HSC), Hazardous category A
|42     |High speed craft (HSC), Hazardous category B
|43     |High speed craft (HSC), Hazardous category C
|44     |High speed craft (HSC), Hazardous category D
|45     |High speed craft (HSC), Reserved for future use
|46     |High speed craft (HSC), Reserved for future use
|47     |High speed craft (HSC), Reserved for future use
|48     |High speed craft (HSC), Reserved for future use
|49     |High speed craft (HSC), No additional information
|50     |Pilot Vessel
|51     |Search and Rescue vessel
|52     |Tug
|53     |Port Tender
|54     |Anti-pollution equipment
|55     |Law Enforcement
|56     |Spare - Local Vessel
|57     |Spare - Local Vessel
|58     |Medical Transport
|59     |Noncombatant ship according to RR Resolution No. 18
|60     |Passenger, all ships of this type
|61     |Passenger, Hazardous category A
|62     |Passenger, Hazardous category B
|63     |Passenger, Hazardous category C
|64     |Passenger, Hazardous category D
|65     |Passenger, Reserved for future use
|66     |Passenger, Reserved for future use
|67     |Passenger, Reserved for future use
|68     |Passenger, Reserved for future use
|69     |Passenger, No additional information
|70     |Cargo, all ships of this type
|71     |Cargo, Hazardous category A
|72     |Cargo, Hazardous category B
|73     |Cargo, Hazardous category C
|74     |Cargo, Hazardous category D
|75     |Cargo, Reserved for future use
|76     |Cargo, Reserved for future use
|77     |Cargo, Reserved for future use
|78     |Cargo, Reserved for future use
|79     |Cargo, No additional information
|80     |Tanker, all ships of this type
|81     |Tanker, Hazardous category A
|82     |Tanker, Hazardous category B
|83     |Tanker, Hazardous category C
|84     |Tanker, Hazardous category D
|85     |Tanker, Reserved for future use
|86     |Tanker, Reserved for future use
|87     |Tanker, Reserved for future use
|88     |Tanker, Reserved for future use
|89     |Tanker, No additional information
|90     |Other Type, all ships of this type
|91     |Other Type, Hazardous category A
|92     |Other Type, Hazardous category B
|93     |Other Type, Hazardous category C
|94     |Other Type, Hazardous category D
|95     |Other Type, Reserved for future use
|96     |Other Type, Reserved for future use
|97     |Other Type, Reserved for future use
|98     |Other Type, Reserved for future use
|99     |Other Type, no additional information
|======

Note that garbage values greater than 99 are supposed to be unused,
but are not uncommon in the wild; AIS transmitters seem prone to put
junk in this field when it's not explicitly set.  Decoders should
treat these like value 0 rather than throwing an exception until and
unless the controlled vocabulary is extended to include the unknown
values.

=== Type 6: Binary Addressed Message

Message type 6 is an addressed point-to-point message with unspecified
binary payload.  The St. Lawrence Seaway AIS system, the USG PAWSS
system, and the Port Authority of London use this payload for local
extension messages. <<IMO236>> and <<IMO289>> describe payload use as
international extension messages.  This type is variable in length up
to a maximum of 1008 bits (up to 5 AIVDM sentence payloads).

//: Type 6
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member    |T|Units
|0-5     |  6 |Message Type         |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number      |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI     |dest_mmsi |u|9 decimal digits
|70      |  1 |Retransmit flag      |retransmit|b|0 = no retransmit (default)
                                                  1 = retransmitted
|71      |  1 |Spare                |          |x|Not used
|72-81   | 10 |Designated Area Code |dac       |u|Unsigned integer
|82-87   |  6 |Functional ID        |fid       |u|Unsigned integer
|88      |920 |Data                 |data      |d|Binary data
                                                  May be shorter than 920 bits.
|======

Interpretation of the binary payload is controlled by:

* The Designated Area Code, which is a jurisdiction code: 366 for the
  United States.  It uses the same encoding as the area designator in
  MMMSIs; see <<ITU-MID>>. 1 designates international (ITU) messages.

* The FID, which is the Functional ID for a message subtype. In some
  sources this is abbreviated FI.

The following is a non-exhaustive list of standardized DAC-FID pairs
in use for type 6. For an up-to-date list of registered application-specific messages,
see <<IALA-ASM>>.

|======
| DAC |FID | Source        | Status           | Description
|   1 | 12 | <<IMO236>>    | Deprecated       | Dangerous cargo indication
|   1 | 14 | <<IMO236>>    | Deprecated       | Tidal window
|   1 | 16 | <<IMO236>>    | Deprecated/In Use| Number of persons on board
|   1 | 16 | <<IMO289>>    | Standard         | Number of persons on board
|   1 | 18 | <<IMO289>>    | Standard         | Clearance time to enter port
|   1 | 20 | <<IMO289>>    | Standard         | Berthing data (addressed)
|   1 | 23 | <<IMO289>>    | In use           | Area notice (addressed)
|   1 | 25 | <<IMO289>>    | Standard         | Dangerous Cargo indication
|   1 | 28 | <<IMO289>>    | Standard         | Route info addressed
|   1 | 30 | <<IMO289>>    | Standard         | Text description addressed
|   1 | 32 | <<IMO289>>    | Standard         | Tidal Window
| 200 | 21 | <<INLAND>>    | Standard         | ETA at lock/bridge/terminal
| 200 | 22 | <<INLAND>>    | Standard         | RTA at lock/bridge/terminal
| 200 | 55 | <<INLAND>>    | Standard         | Number of persons on board
| 235 | 10 | <<IALA-A126>> | In use           | AtoN monitoring data (UK)
| 250 | 10 | <<IALA-A126>> | In use           | AtoN monitoring data (ROI)
|======

DAC/FID pairs are assigned separately per message type.

Note that the apparent presence of one of these DAC/FID pairs does
not guarantee that the message is structured.  Decoders should
perform range validation on the structured fields and interpret the
message as unstructured if any check fails. (As of Aug 2014 no such
collisions have been in the wild; but see the parallel note for Type 8.)

A list of binary layouts for selected subtypes of message 6 follows.

==== IMO236 Dangerous Cargo Indication

This message should be used as a response to a request for Dangerous
Cargo Indication from a competent authority. The message content is
used to identify the port where the documents for the dangerous goods
cargo can be found, e. g. last and next port of call, and to allow
the requesting authority to form a danger estimate.

A message 6 subtype. DAC = 001 FID = 12. Fixed length: 360 bits.
This is the <<IMO236>> version, now deprecated; there is a later
<<IMO289>> version.

//: Type 6(1/12)
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member    |T|Units
|0-5     |  6 |Message Type         |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number      |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI     |dest_mmsi |u|9 decimal digits
|70-70   |  1 |Retransmit flag      |retransmit|u|0 = no retransmit (default),
                                                  1 = retransmitted
|71-71   |  1 |Spare                |          |x|Not used
|72-81   | 10 |DAC                  |dac       |u|DAC = 001
|82-87   |  6 |FID                  |fid       |u|FID = 12
|88-117  | 30 |Last Port Of Call    |lastport  |t|5 6-bit characters, UN locode
|118-121 |  4 |ETA month (UTC)      |lmonth    |u|1-12, 0=N/A (default)
|122-126 |  5 |ETA day (UTC)        |lday      |u|1-31, 0=N/A (default)
|127-131 |  5 |ETA hour (UTC)       |lhour     |u|0-23, 24=N/A (default)
|132-137 |  6 |ETA minute (UTC)     |lminute   |u|0-59, 60=N/A (default)
|138-167 | 30 |Next Port Of Call    |nextport  |t|5 6-bit characters, UN locode
|168-171 |  4 |ETA month (UTC)      |nmonth    |u|1-12, 0=N/A (default)
|172-176 |  5 |ETA day (UTC)        |nday      |u|1-31, 0=N/A (default)
|177-181 |  5 |ETA hour (UTC)       |nhour     |u|0-23, 24=N/A (default)
|182-187 |  6 |ETA minute (UTC)     |nminute   |u|0-59, 60=N/A (default)
|188-307 |120 |Main Dangerous Good  |dangerous |t|20 6-bit characters
|308-331 | 24 |IMD Category         |imdcat    |t|4  6-bit characters
|332-344 | 13 |UN Number            |unid      |u|1-3363 UN Number
|345-354 | 10 |Amount of Cargo      |amount    |u|Unsigned integer
|355-356 |  2 |Unit of Quantity     |unit      |e|See "Cargo Unit Codes"
|357-359 |  3 |Spare                |          |x|Not used
|======

.Cargo Unit Codes
[width="50%",frame="topbot",options="header"]
|======
|Code |Unit
|0    |Not available (default)
|1    |kg
|2    |metric tons
|3    |metric kilotons
|======

==== IMO236 Tidal Window

This message should be used by shore stations to inform vessels about
tidal windows which allow a vessel the safe passage of a fairway.  The
message includes 1-3 predictions of current speed and current direction.
Acknowledgment is required.

A message 6 subtype. DAC = 001 FID = 14. Variable length: 190-376 bits
This is the <<IMO236>> version; there is an <<IMO289>> version with
different widths for the latitude, longitude, and current-speed fields
(also the order of lat/lon is swapped).

//: Type 6(1/14)
[frame="topbot",options="header"]
|======
|Field   |Len |Description            |Member    |T|Units
|0-5     |  6 |Message Type           |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator       |repeat    |u|See Common Navigation Block
|8-37    | 30 |Source MMSI            |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number        |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI       |dest_mmsi |u|9 decimal digits
|70-70   |  1 |Retransmit flag        |retransmit|b|0 = no retransmit (default),
                                                    1 = retransmitted
|71-71   |  1 |Spare                  |          |x|Not used
|72-81   | 10 |DAC                    |dac       |u|DAC = 001
|82-87   |  6 |FID                    |fid       |u|FID = 14
|88-91   |  4 |Month                  |month     |u|1-12; 0 = N/A (default)
|92-96   |  5 |Day                    |day       |u|1-31; 0 = N/A (default)
|97      |    |                       |tidals    |a3|Tidal information array
|0-26    | 27 |Latitude               |lat       |I4|Unit = minutes * 0.0001,
                                                    91000 = N/A (default),
                                                    N positive, S negative.
|27-54   | 28 |Longitude              |lon       |I4|Unit = minutes * 0.0001,
                                                    181000 = N/A (default),
                                                    E positive, W negative.
|55-59   |  5 |From UTC Hour          |from_hour |u|0-23, 24 = N/A (default)
|60-65   |  6 |From UTC Minute        |from_min  |u|0-59, 60 = N/A (default)
|66-70   |  5 |To UTC Hour            |to_hour   |u|0-23, 24 = N/A (default)
|71-76   |  6 |To UTC Minute          |to_min    |u|0-59, 60 = N/A (default)
|77-85   |  9 |Current Dir. Predicted |cdir      |u|0-359 deg, 360-N/A (default)
|86-92   |  7 |Current Speed Predicted|cspeed    |U1|0-126, units of 0.1 knots,
                                                   127 = N/A (default).
|======

The group of fields from longitude on may repeat twice more to convey up to
three points of tidal information.

==== IMO236 Number of persons on board

This message should be used by a ship to report the number of persons
on board, e.g. on request by a competent authority.  Acknowledgment
required.

A message 6 subtype. DAC = 001 FID = 16.

<<IMO236>> describes a fixed-length, 72-bit message with this layout:

[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member   |T|Units
|0-5     |  6 |Message Type         |type     |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat   |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi     |u|9 decimal digits
|38-39   |  2 |Spare                |         |x|Not used
|40-49   | 10 |DAC                  |dac      |u|DAC = 001
|50-55   |  6 |FID                  |fid      |u|FID = 16
|55-68   | 14 |# persons on board   |persons  |u|Unsigned integer
                                                 0 = N/A (default)
                                                 8191 = >= 8191 persons.
|69-71   |  3 |Spare                |         |x|Not used
|======

OPEN-QUESTION: Note that though this is a message 6 subtype and
described in <<IMO236>> with the attribute "addressed", there is no
destination address.  A strikeout in <<IMO236>> suggests that this was
originally a subtype of 8.  It would be good defensive implementation
for a decoder to accept either.  Bit length may be used to distinguish
them.

<<IMO289>> describes a fixed-length, 136-bit message with this layout:

//: Type 6(1/16)
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member    |T|Units
|0-5     |  6 |Message Type         |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number      |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI     |dest_mmsi |u|9 decimal digits
|70-70   |  1 |Retransmit flag      |retransmit|b|0 = no retransmit (default),
                                                  1 = retransmitted.
|71-71   |  1 |Spare                |          |x|Not used
|72-81   | 10 |DAC                  |dac       |u|DAC = 001
|82-87   |  6 |FID                  |fid       |u|FID = 16
|88-100  | 13 |# persons on board   |persons   |u|Unsigned integer,
                                                  0 = N/A (default),
                                                  8191 = >= 8191 persons.
|101-135 | 35 |Spare                |          |x|Not used
|======

==== IMO289 Clearance Time To Enter Port

This message provides specific ships with information on the port to call and
time to enter.  It should be transmitted by an authority competent to grant
use of the port.

A message 6 subtype. DAC = 001 FID = 18. Fixed length: 360 bits.

//: Type 6(1/18)
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member     |T|Units
|0-5     |  6 |Message Type         |type       |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi       |u|9 decimal digits
|38-39   |  2 |Sequence Number      |seqno      |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI     |dest_mmsi  |u|9 decimal digits
|70-70   |  1 |Retransmit flag      |retransmit |b|0 = no retransmit (default),
                                                   1 = retransmitted.
|71-71   |  1 |Spare                |           |x|Not used
|72-81   | 10 |DAC                  |dac        |u|DAC = 001
|82-87   |  6 |FID                  |fid        |u|FID = 18
|88-97   | 10 |Message Linkage ID   |linkage    |u|Unsigned integer
|98-101  |  4 |Month (UTC)          |month      |u|1-12; 0 = N/A (default)
|102-106 |  5 |Day (UTC)            |day        |u|1-31; 0 = N/A (default)
|107-111 |  5 |Hour (UTC)           |hour       |u|0-23; 24 = N/A (default)
|112-117 |  6 |Minute (UTC)         |minute     |u|0-59; 60 = N/A (default)
|118-237 |120 |Name of Port & Berth |portname   |t|20 6-bit characters
|238-267 | 30 |Destination          |destination|t|5 6-bit characters
|268-292 | 25 |Longitude            |lon        |I3|Unit = minutes * 0.001,
                                                   181000 = N/A (default).
|293-316 | 24 |Latitude             |lat        |I3|Unit = minutes * 0.001,
                                                   91000 = N/A (default).
|317-359 | 43 |Spare                |           |x|Not used
|======

==== IMO289 Berthing Data (addressed)

This message provides information on the ship's berth. If sent from a
ship it is a berthing request; if it is transmitted by a competent
authority it is a berthing assignment.

A message 6 subtype. DAC = 001 FID = 20. Fixed Length: 360 bits.

The 2-bit fields after "availability" describe services which may be
available at the berth.  They are valid only if this master availability
bit is on.

//: Type 6(1/20)
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member/Type |T|Units
|0-5     |  6 |Message Type       |type        |u|Constant: 6
|6-7     |  2 |Repeat Indicator   |repeat      |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI        |mmsi        |u|9 decimal digits
|38-39   |  2 |Sequence Number    |seqno       |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI   |dest_mmsi   |u|9 decimal digits
|70-70   |  1 |Retransmit flag    |retransmit  |b|0 = no retransmit (default)
                                                  1 = retransmitted
|71-71   |  1 |Spare              |            |x|Not used
|72-81   | 10 |DAC                |dac         |u|AC = 001
|82-87   |  6 |FID                |fid         |u|ID = 20
|88-97   | 10 |Message Linkage ID |linkage     |u|Unsigned integer
|98-106  |  9 |Berth length       |berth_length|u|In 1m steps, 1-510m,
                                                  511 = >= 511m
                                                  0 = N/A (default).
|107-114 |  8 |Berth Water Depth  |berth_depth |U1|0.1-25.4m in 0.1 steps
                                                  255 = >= 25.5m
                                                  0 = N/A (default)
|115-117 |  3 |Mooring Position   |position    |e|See "Mooring Position"
|118-121 |  4 |Month (UTC)        |month       |u|1-12; 0 = N/A (default)
|122-126 |  5 |Day (UTC)          |day         |u|1-31; 0 = N/A (default)
|127-131 |  5 |Hour (UTC)         |hour        |u|0-23; 24 = N/A (default)
|132-137 |  6 |Minute (UTC)       |minute      |u|0-59; 60 = N/A (default)
|138-138 |  1 |Services Availability|availability|b|0 = services unknown (default)
                                                    1 = services known
|139-140 |  2 |Agent              |agent       |e|See "Service Status"
|141-142 |  2 |Bunker/fuel        |fuel        |e|See "Service Status"
|143-144 |  2 |Chandler           |chandler    |e|See "Service Status"
|145-146 |  2 |Stevedore          |stevedore   |e|See "Service Status"
|147-148 |  2 |Electrical         |electrical  |e|See "Service Status"
|149-150 |  2 |Potable water      |water       |e|See "Service Status"
|151-152 |  2 |Customs house      |customs     |e|See "Service Status"
|153-154 |  2 |Cartage            |cartage     |e|See "Service Status"
|155-156 |  2 |Crane(s)           |crane       |e|See "Service Status"
|157-158 |  2 |Lift(s)            |lift        |e|See "Service Status"
|159-160 |  2 |Medical facilities |medical     |e|See "Service Status"
|161-162 |  2 |Navigation repair  |navrepair   |e|See "Service Status"
|163-164 |  2 |Provisions         |provisions  |e|See "Service Status"
|165-166 |  2 |Ship repair        |shiprepair  |e|See "Service Status"
|167-168 |  2 |Surveyor           |surveyor    |e|See "Service Status"
|169-170 |  2 |Steam              |steam       |e|See "Service Status"
|171-172 |  2 |Tugs               |tugs        |e|See "Service Status"
|173-174 |  2 |Waste disposal (solid)    |solidwaste    |e|See "Service Status"
|175-176 |  2 |Waste disposal (liquid)   |liquidwaste   |e|See "Service Status"
|177-178 |  2 |Waste disposal (hazardous)|hazardouswaste|e|See "Service Status"
|179-180 |  2 |Reserved ballast exchange |ballast       |e|See "Service Status"
|181-182 |  2 |Additional services|additional  |e|See "Service Status"
|183-184 |  2 |Regional reserved 1|regional1   |e|See "Service Status"
|185-186 |  2 |Regional reserved 2|regional2   |e|See "Service Status"
|187-188 |  2 |Reserved for future|future1     |e|See "Service Status"
|189-190 |  2 |Reserved for future|future2     |e|See "Service Status"
|191-310 |120 |Name of Berth      |berth_name  |t|20 6-bit characters
|311-335 | 25 |Longitude          |berth_lon   |I3|Minutes * 0.001,
                                                  181000 = N/A (default)
|336-359 | 24 |Latitude           |berth_lat   |I3|Minutes * 0.001,
                                                  91000 = N/A (default)
|======

The UTC timestamp refers to the time requested or granted for berthing.

The longitude and latitude refer to the center of the berth.

.Mooring Position
[width="50%",frame="topbot",options="header"]
|======
|Code |Position
|0    |Not available (default)
|1    |Port-side to
|2    |Starboard-side to
|3    |Mediterranean (end-on) mooring
|4    |Mooring buoy
|5    |Anchorage
|6-7  |Reserved for future use
|======

.Service Status
[width="50%",frame="topbot",options="header"]
|======
|Code |Meaning
|0    |Not available or requested (default)
|1    |Service available
|2    |No data or unknown
|3    |Not to be used
|======

==== IMO289 Area Notice (addressed)

This should be used to convey time- and location-dependent information about
hazards to navigation.  For information-lifetime restrictions and usage
guidance, refer to <<ITU1371>>.

A message 6 subtype. DAC = 001 FID = 23. 230 to 1013 bits.  There is a
related Message 8 subtype for broadcast use.

The message consists of a fixed-length header of 143 bits, followed by 1 to
10 sub-area indications which are fixed-length records 87 bits long. Here
is the message header format:

//: Type 6(1/23)
.Area Notice (addressed) message header
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member     |T|Units
|0-5     |  6 |Message Type         |type       |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi       |u|9 decimal digits
|38-39   |  2 |Sequence Number      |seqno      |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI     |dest_mmsi  |u|9 decimal digits
|70-70   |  1 |Retransmit flag      |retransmit |b|0 = no retransmit (default),
                                                   1 = retransmitted.
|71-71   |  1 |Spare                |           |x|Not used
|72-81   | 10 |DAC                  |dac        |u|DAC = 001
|82-87   |  6 |FID                  |fid        |u|FID = 23
|88-97   | 10 |Message Linkage ID   |linkage    |u|Unsigned integer
|98-104  |  7 |Notice Description   |notice     |e|See "Area Notice Description"
|105-108 |  4 |Month (UTC)          |month      |u|1-12; 0 = N/A (default)
|109-113 |  5 |Day (UTC)            |day        |u|1-31; 0 = N/A (default)
|114-118 |  5 |Hour (UTC)           |hour       |u|0-23; 24 = N/A (default)
|119-124 |  6 |Minute (UTC)         |minute     |u|0-59; 60 = N/A (default)
|125-142 | 18 |Duration             |duration   |u|In minutes,
                                                 262143 = N/A (default),
                                                 0 = cancel this notice.
|143-145 |  3 |Subarea shape #1     |subarea type |u|0-5 (see "Subarea Types")
|146-229 | 84 |Subarea payload #1   |shape data |array|See "Subarea Payloads"
|230-232 |  3 |Subarea shape #2     |subarea type |u|0-5 (see "Subarea Types")
|233-316 | 84 |Subarea payload #2   |shape data |array|See "Subarea Payloads"
|317-319 |  3 |Subarea shape #3     |subarea type |u|0-5 (see "Subarea Types")
|320-403 | 84 |Subarea payload #3   |shape data |array|See "Subarea Payloads"
|404-406 |  3 |Subarea shape #4     |subarea type |u|0-5 (see "Subarea Types")
|407-490 | 84 |Subarea payload #4   |shape data |array|See "Subarea Payloads"
|491-493 |  3 |Subarea shape #5     |subarea type |u|0-5 (see "Subarea Types")
|494-577 | 84 |Subarea payload #5   |shape data |array|See "Subarea Payloads"
|578-580 |  3 |Subarea shape #6     |subarea type |u|0-5 (see "Subarea Types")
|581-664 | 84 |Subarea payload #6   |shape data |array|See "Subarea Payloads"
|665-667 |  3 |Subarea shape #7     |subarea type |u|0-5 (see "Subarea Types")
|668-751 | 84 |Subarea payload #7   |shape data |array|See "Subarea Payloads"
|752-754 |  3 |Subarea shape #8     |subarea type |u|0-5 (see "Subarea Types")
|755-838 | 84 |Subarea payload #8   |shape data |array|See "Subarea Payloads"
|839-841 |  3 |Subarea shape #9     |subarea type |u|0-5 (see "Subarea Types")
|842-925 | 84 |Subarea payload #9   |shape data |array|See "Subarea Payloads"
|926-928 |  3 |Subarea shape #10    |subarea type |u|0-5 (see "Subarea Types")
|929-1012| 84 |Subarea payload #10  |shape data |array|See "Subarea Payloads"
|======

The Message Linkage field is, as usual, for linking to a textual explanatory
message sent with the same linkage ID. The standard says that in this context
it has the semantics of being an identifier of or reference to an area.

Notice description types are as follows:

.Area Notice Description
[frame="topbot"]
|======
|  0| Caution Area: Marine mammals habitat
|  1| Caution Area: Marine mammals in area - reduce speed
|  2| Caution Area: Marine mammals in area - stay clear
|  3| Caution Area: Marine mammals in area - report sightings
|  4| Caution Area: Protected habitat - reduce speed
|  5| Caution Area: Protected habitat - stay clear
|  6| Caution Area: Protected habitat - no fishing or anchoring
|  7| Caution Area: Derelicts (drifting objects)
|  8| Caution Area: Traffic congestion
|  9| Caution Area: Marine event
| 10| Caution Area: Divers down
| 11| Caution Area: Swim area
| 12| Caution Area: Dredge operations
| 13| Caution Area: Survey operations
| 14| Caution Area: Underwater operation
| 15| Caution Area: Seaplane operations
| 16| Caution Area: Fishery – nets in water
| 17| Caution Area: Cluster of fishing vessels
| 18| Caution Area: Fairway closed
| 19| Caution Area: Harbor closed
| 20| Caution Area: Risk (define in associated text field)
| 21| Caution Area: Underwater vehicle operation
| 22| (reserved for future use)
| 23| Environmental Caution Area: Storm front (line squall)
| 24| Environmental Caution Area: Hazardous sea ice
| 25| Environmental Caution Area: Storm warning (storm cell or line of storms)
| 26| Environmental Caution Area: High wind
| 27| Environmental Caution Area: High waves
| 28| Environmental Caution Area: Restricted visibility (fog, rain, etc.)
| 29| Environmental Caution Area: Strong currents
| 30| Environmental Caution Area: Heavy icing
| 31| (reserved for future use)
| 32| Restricted Area: Fishing prohibited
| 33| Restricted Area: No anchoring.
| 34| Restricted Area: Entry approval required prior to transit
| 35| Restricted Area: Entry prohibited
| 36| Restricted Area: Active military OPAREA
| 37| Restricted Area: Firing – danger area.
| 38| Restricted Area: Drifting Mines
| 39| (reserved for future use)
| 40| Anchorage Area: Anchorage open
| 41| Anchorage Area: Anchorage closed
| 42| Anchorage Area: Anchorage prohibited
| 43| Anchorage Area: Deep draft anchorage
| 44| Anchorage Area: Shallow draft anchorage
| 45| Anchorage Area: Vessel transfer operations
| 46| (reserved for future use)
| 47| (reserved for future use)
| 48| (reserved for future use)
| 49| (reserved for future use)
| 50| (reserved for future use)
| 51| (reserved for future use)
| 52| (reserved for future use)
| 53| (reserved for future use)
| 54| (reserved for future use)
| 55| (reserved for future use)
| 56| Security Alert - Level 1
| 57| Security Alert - Level 2
| 57| Security Alert - Level 3
| 59| (reserved for future use)
| 60| (reserved for future use)
| 61| (reserved for future use)
| 62| (reserved for future use)
| 63| (reserved for future use)
| 64| Distress Area: Vessel disabled and adrift
| 65| Distress Area: Vessel sinking
| 66| Distress Area: Vessel abandoning ship
| 67| Distress Area: Vessel requests medical assistance
| 68| Distress Area: Vessel flooding
| 69| Distress Area: Vessel fire/explosion
| 70| Distress Area: Vessel grounding
| 71| Distress Area: Vessel collision
| 72| Distress Area: Vessel listing/capsizing
| 73| Distress Area: Vessel under assault
| 74| Distress Area: Person overboard
| 75| Distress Area: SAR area
| 76| Distress Area: Pollution response area
| 77| (reserved for future use)
| 78| (reserved for future use)
| 79| (reserved for future use)
| 80| Instruction: Contact VTS at this point/juncture
| 81| Instruction: Contact Port Administration at this point/juncture
| 82| Instruction: Do not proceed beyond this point/juncture
| 83| Instruction: Await instructions prior to proceeding beyond this point/juncture
| 84| Proceed to this location – await instructions
| 85| Clearance granted – proceed to berth
| 86| (reserved for future use)
| 87| (reserved for future use)
| 88| Information: Pilot boarding position
| 89| Information: Icebreaker waiting area
| 90| Information: Places of refuge
| 91| Information: Position of icebreakers
| 92| Information: Location of response units
| 93| VTS active target
| 94| Rogue or suspicious vessel
| 95| Vessel requesting non-distress assistance
| 96| Chart Feature: Sunken vessel
| 97| Chart Feature: Submerged object
| 98| Chart Feature: Semi-submerged object
| 99| Chart Feature: Shoal area
|100| Chart Feature: Shoal area due north
|101| Chart Feature: Shoal area due east
|102| Chart Feature: Shoal area due south
|103| Chart Feature: Shoal area due west
|104| Chart Feature: Channel obstruction
|105| Chart Feature: Reduced vertical clearance
|106| Chart Feature: Bridge closed
|107| Chart Feature: Bridge partially open
|108| Chart Feature: Bridge fully open
|109| (reserved for future use)
|110| (reserved for future use)
|111| (reserved for future use)
|112| Report from ship: Icing info
|113| (reserved for future use)
|114| Report from ship: Miscellaneous information – define in associated text field
|115| (reserved for future use)
|116| (reserved for future use)
|117| (reserved for future use)
|118| (reserved for future use)
|119| (reserved for future use)
|120| Route: Recommended route
|121| Route: Alternative route
|122| Route: Recommended route through ice
|123| (reserved for future use)
|124| (reserved for future use)
|125| Other – Define in associated text field
|126| Cancellation – cancel area as identified by Message Linkage ID
|127| Undefined (default)
|======

Subarea types are as follows:

[frame="topbot"]
|======
|  0  | Circle or point
|  1  | Rectangle
|  2  | Sector
|  3  | Polyline
|  4  | Polygon
|  5  | Associated text
| 6-7 | Reserved
|======

Subarea payload layouts are as follows:

.Circle or Point
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member/Type |T|Units
|0-2     | 3  |Shape of area      |shape       |e|Constant: 0
|3-4     | 2  |Scale factor       |scale       |u|Power of 10 for area dimensions;
                                                  10^0 = scale factor 1 (default)
|5-29    | 25 |Longitude          |lon         |I3|Longitude of center point,
                                                   Unit = minutes * 0.001,
                                                   181000 = N/A (default).
|30-53   | 24 |Latitude           |lat         |I3|Latitude of center point,
                                                   Unit = minutes * 0.001,
                                                   91000 = N/A (default).
|54-56   |  3 |Precision          |precision   |u|Decimal places of precision
                                                  (defaults to 4)
|57-68   | 12 |Radius             |radius      |u|Radius of area
                                                  0 = point (default),
                                                  else 1-4095 * 10^scale m
|69-86   | 18 |Spare              |            |x|Not used
|======

.Rectangle
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member/Type |T|Units
|0-2     | 3  |Shape of area      |shape       |e|Constant: 1
|3-4     | 2  |Scale factor       |scale       |u|Power of 10 for area dimensions;
                                                  10^0 = scale factor 1 (default)
|5-29    | 25 |Longitude          |lon         |I3|Longitude of SW corner
                                                   Unit = minutes * 0.001,
                                                   181000 = N/A (default).
|30-53   | 24 |Latitude           |lat         |I3|Latitude of SW corner
                                                   Unit = minutes * 0.001,
                                                   91000 = N/A (default).
|54-56   |  3 |Precision          |precision   |u|Decimal places of precision
                                                  (defaults to 4)
|57-64   |  8 |E dimension        |east        |u|Box dimension east
                                                  0 = N/S line (default),
                                                  else 1-255 * 10^scale m
|65-72   |  8 |N dimension        |north       |u|Box dimension north
                                                  0 = E/W line (default),
                                                  else 1-255 * 10^scale m
|73-81   |  9 |Orientation        |orientation |u|Degrees clockwise from true N,
                                                  0 = no rotation (default),
                                                  else 1-359,
                                                  360-511 reserved.
|82-86   |  5 |Spare              |            |x|Not used
|======

.Sector
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member/Type |T|Units
|0-2     | 3  |Shape of area      |shape       |e|Constant: 2
|3-4     | 2  |Scale factor       |scale       |u|Power of 10 for area dimensions;
                                                  10^0 = scale factor 1 (default)
|5-29    | 25 |Longitude          |lon         |I3|Longitude of center point,
                                                   Unit = minutes * 0.001,
                                                   181000 = N/A (default).
|30-53   | 24 |Latitude           |lat         |I3|Latitude of center point,
                                                   Unit = minutes * 0.001,
                                                   91000 = N/A (default).
|54-56   |  3 |Precision          |precision   |u|Decimal places of precision
                                                  (defaults to 4)
|57-68   | 12 |Radius             |radius      |u|Radius of area
                                                  0 = point (default),
                                                  else 1-4095 * 10^scale m
|69-77   |  9 |Left boundary      |left       |u|Degrees clockwise from true N,
                                                  0 = no rotation (default),
                                                  else 1-359,
                                                  360-511 reserved.
|78-86   |  9 |Right boundary     |right      |u|Degrees clockwise from true N,
                                                  0 = no rotation (default),
                                                  else 1-359,
                                                  360-511 reserved.
|======

.Polyline
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member/Type |T|Units
|0-2     | 3  |Shape of area      |shape       |e|Constant: 3
|3-4     | 2  |Scale factor       |scale       |u|Power of 10 for area dimensions;
                                                  10^0 = scale factor 1 (default)
|5       |    |                   |waypoints   |a4|Waypoints array
|0-9     |10  |Bearing            |bearing     |u|True bearing in half-degree
                                                  steps from previous waypoint;
                                                  720 = N/A (default).
|10-19   |10  |Distance           |distance    |u|Distance from prev. waypoint,
                                                  0 = no point (default),
                                                  else 1-1023 * 10^scale m
|======

The last two fields are repeated 4 times; the final 2 bits of 87 are
unused.  A polyline must be preceded by either (a) a circle, in which
case the first bearing is from the center, or (b) a polyline, in which
case the first bearing is from the implied last point.

.Polygon
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member/Type |T|Units
|0-2     | 3  |Shape of area      |shape       |e|Constant: 4
|3-4     | 2  |Scale factor       |scale       |u|Power of 10 for area dimensions;
                                                  10^0 = scale factor 1 (default)
|5       |    |                   |vertices    |a4|Vertices array
|0-9     |10  |Bearing            |bearing     |u|True bearing in half-degree
                                                  steps from previous vertex;
                                                  720 = N/A (default).
|10-19   |10  |Distance           |distance    |u|Distance from prev. vertex,
|======

The last two fields are repeated 4 times; the final 2 bits of 87 are
unused.  A polygon must be preceded by a circle; the first bearing is
from the circle center, which is treated as the zero vertex.  There is
an implied boundary from the last polygon vertex to the zero vertex.

.Associated text
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member/Type |T|Units
|0-2     | 3  |Shape of area      |shape       |e|Constant: 5
|3-86    |84  |Text               |text        |t|14 chars of packed 6-bit.
|======

==== IMO289 Dangerous Cargo Indication

See the IMO236 variant for the meaning of this message.

A message 6 subtype. DAC = 001 FID = 25. Variable length: 117-576 bits.
This is the <<IMO289>> version; there is an earlier <<IMO236>> version
with a different layout, deprecated in <<IMO289>>.

//: Type 6(1/25)
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member    |T|Units
|0-5     |  6 |Message Type         |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number      |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI     |dest_mmsi |u|9 decimal digits
|70-70   |  1 |Retransmit flag      |retransmit|b|0 = no retransmit (default)
                                                  1 = retransmitted
|71-71   |  1 |Spare                |          |x|Not used
|72-81   | 10 |DAC                  |dac       |u|DAC = 001
|82-87   |  6 |FID                  |fid       |u|FID = 25
|88-89   |  2 |Unit of Quantity     |unit      |e|See "Cargo Unit Codes"
|90-99   | 10 |Amount of Cargo      |amount    |u|Unsigned integer
                                                  0 = N/A (default)
                                                  0 = N/A (default)
|100     |    |                     |cargos    |a28|Cargo types array
|0-3     |  4 |Cargo code           |code      |e|See "Cargo Codes"
|4-16    | 13 |Cargo subtype        |subtype   |u|Unsigned integer
                                                  0 = N/A (default)
|======

The last two fields may repeat to describe up to 28 subcargos. The count
of repetitions must be computed from the message payload length.

For cargo unit codes, see the description of the IMO236 variant of
this message.

.Cargo Codes
[width="50%",frame="topbot",options="header"]
|======
|Code |Code under which cargo is carried
|0    |Not available (default)
|1    |IMDG Code (in packed form)
|2    |IGC code
|3    |BC Code (from 1.1.2011 IMSBC)
|4    |MARPOL Annex I List of oils (Appendix 1)
|5    |MARPOL Annex II IBC Code
|6    |Regional use
|7-15 |Reserved for future use
|======

The subtype field may be interpreted as an IMDG class or division code
(if the cargotype is 1 = IMDG code) or as a UN number (if the
cargotype is 2 = IGC code) or as a pair of BC class and IMDG class (if
the cargotype is 3 = BC code) or as a MARPOL Annex I code (if
the cargotype is 4 = MARPOL Annex I) or as a MARPOL Annex II code (if
the cargotype is 5 = MARPOL Annex II).

.Dangerous Cargo Indication: MARPOL Annex I list of oils
[width="50%",frame="topbot"]
|======
|0    | N/A (default)
|1    | asphalt solutions
|2    | oils
|3    | distillates
|4    | gas oil
|5    | gasoline blending stocks
|6    | gasoline
|7    | jet fuels
|8    | naphtha
|9-15 | reserved for future use
|======

.Dangerous Cargo Indication: MARPOL Annex II list of oils
[width="50%",frame="topbot"]
|======
|0    | N/A (default)
|1    | Category X
|2    | Category Y
|3    | Category Z
|4    | Other substances
|5-7  | reserved for future use
|======

==== IMO289 Route Information (addressed)

The content of this message is a time and a list of waypoints describing a
course. It has a broadcast equivalent that is a message 8 subtype.

A message 6 subtype. DAC = 001 FID = 28. Variable length: 204-1029 bits.

//: Type 6(1/28)
[frame="topbot",options="header"]
|======
|Field   |Len |Description       |Member    |T|Units
|0-5     |  6 |Message Type      |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator  |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI       |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number   |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI  |dest_mmsi |u|9 decimal digits
|70-70   |  1 |Retransmit flag   |retransmit|b|0 = no retransmission (default),
                                               1 = retransmitted.
|71-71   |  1 |Spare             |          |x|Not used
|72-81   | 10 |DAC               |dac       |u|DAC = 001
|82-87   |  6 |FID               |fid       |u|FID = 28
|88-97   | 10 |Message Linkage ID|linkage   |u|Unsigned integer
|98-100  |  3 |Sender Class      |sender    |u|0 = ship (default),
                                               1 = authority,
                                               27 = reserved for future use
|101-105 |  5 |Route Type        |rtype     |e|See "Route Type Codes"
|106-109 |  4 |Start month (UTC) |month     |u|1-12, 0=N/A (default)
|110-114 |  5 |Start day (UTC)   |day       |u|1-31, 0=N/A (default)
|115-119 |  5 |Start hour (UTC)  |hour      |u|0-23, 24=N/A (default)
|120-125 |  6 |Start minute (UTC)|minute    |u|0-59, 60=N/A (default)
|126-143 | 18 |Duration          |duration  |u|Minutes from start time,
                                               0 = cancel route,
                                               262,143 = N/A (default),
|144-148 |  5 |                  |waycount  |u|Waypoint count (1-16),
                                               Values 17-31 are not used.
|149     |    |                  |waypoints |a^16|Waypoint array
|0-27    | 28 |Longitude         |lon       |I4|Minutes * 0.0001,
                                               181000 = N/A (default),
                                               E positive, W negative.
|28-54   | 27 |Latitude          |lat       |I4|Minutes * 0.0001,
                                               91000 = N/A (default),
                                               N positive, S negative.
|======

The final pair of fields in the table above is a waypoint.  The message may
end with 1 to 16 waypoints.

.Route Type Codes
[width="50%",frame="topbot"]
|======
|0    |Undefined (default)
|1    |Mandatory
|2    |Recommended
|3    |Alternative
|4    |Recommended route through ice
|5    |Ship route plan
|6-30 |Reserved for future usage
|31   |Cancel route identified by message linkage
|======

==== IMO289 Text description (addressed)

This message may be used to attach a text description to another message with
a Message Linkage ID matching this one.  It is intended that the combination
of MMSI and Message Linkage ID should be unique.

A message 6 subtype. DAC = 001 FID = 30. Variable length: 104-1028 bits.

Intended to be used to associate a text annotation with another message
via the Message Linkage ID field.

//: Type 6(1/30)
[frame="topbot",options="header"]
|======
|Field   |Len    |Description       |Member     |T|Units
|0-5     |  6    |Message Type      |type       |u|Constant: 6
|6-7     |  2    |Repeat Indicator  |repeat     |u|As in Common Navigation Block
|8-37    | 30    |Source MMSI       |mmsi       |u|9 decimal digits
|38-39   |  2    |Sequence Number   |seqno      |u|Unsigned integer 0-3
|40-69   | 30    |Destination MMSI  |dest_mmsi  |u|9 decimal digits
|70-70   |  1    |Retransmit flag   |retransmit |u|0 = no retransmit (default),
                                                   1 = retransmitted
|71-71   |  1    |Spare             |           |x|Not used
|72-81   | 10    |DAC               |dac        |u|DAC = 001
|82-87   |  6    |FID               |fid        |u|FID = 30
|88-97   | 10    |Message Linkage ID|linkage    |u|Unsigned integer
|98-?    | 6-930 |Description       |description|t|String
|======

There is an equivalent subtype of message 8 that is a broadcast description.

==== Tidal Window (IMO289)

See the <<IMO289>> version of this message for intended meaning.

A message 6 subtype. DAC = 001 FID = 32. Variable length: 186-362 bits.
This is the <<IMO289>> version; there is an <<IMO289>> version with
different bit widths for the latitude and longitude fields.

//: Type 6(1/32)
[frame="topbot",options="header"]
|======
|Field   |Len |Description            |Member    |T|Units
|0-5     |  6 |Message Type           |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator       |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI            |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number        |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI       |dest_mmsi |u|9 decimal digits
|70-70   |  1 |Retransmit flag        |retransmit|b|0 = no retransmit (default),
                                                    1 = retransmitted
|71-71   |  1 |Spare                  |          |x|Not used
|72-81   | 10 |DAC                    |dac       |u|DAC = 001
|82-87   |  6 |FID                    |fid       |u|FID = 32
|88-91   |  4 |Month                  |month     |u|1-12; 0 = N/A (default)
|92-96   |  5 |Day                    |day       |u|1-31; 0 = N/A (default)
|97      |    |                       |tidals    |a3|Tidal information array
|0-24    | 25 |Longitude              |lon       |I3|Unit = minutes * 0.001,
                                                    181000 = N/A (default),
                                                    E positive, W negative.
|25-48   | 24 |Latitude               |lat       |I3|Unit = minutes * 0.001.
                                                    91000 = N/A (default),
                                                    N positive, S negative.
|49-53   |  5 |From UTC Hour          |from_hour |u|0-23, 24 = N/A (default)
|54-59   |  6 |From UTC Minute        |from_min  |u|0-59, 60 = N/A (default)
|60-64   |  5 |To UTC Hour            |to_hour   |u|0-23, 24 = N/A (default)
|65-70   |  6 |To UTC Minute          |to_min    |u|0-59, 60 = N/A (default)
|71-79   |  9 |Current Dir. Predicted |cdir      |u|0-359 true bearing,
                                                    360 = N/A (default).
|80-87   |  8 |Current Speed Predicted|cspeed    |U1|0-250, units of 0.1 knots,
                                                    251 = speed >= 25.1 knots,
                                                    255 = N/A (default).
|======

The group of fields from longitude on may repeat twice more to convey up to
three points of tidal information.

==== ETA at lock/bridge/terminal (Inland AIS)

A message 6 subtype. DAC = 200 FID = 21. Fixed length, 248 bits.

Should be used by inland vessels only, to send an ETA report to a
lock, bridge or terminal in order to apply for a time slot in resource
planning.

An acknowledgment by Inland AIS message 22 should be received within
15 minutes. Otherwise, the Inland AIS message 21 should be repeated
once.

//: Type 6(200/21)
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member    |T|Units
|0-5     |  6 |Message Type       |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator   |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI        |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number    |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI   |dest_mmsi |u|9 decimal digits
|70      |  1 |Retransmit flag    |retransmit|u|0 = no retransmit (default),
                                                1 = retransmitted
|71      |  1 |Spare              |          |x|Not used
|72-81   | 10 |DAC                |dac       |u|DAC = 200
|82-87   |  6 |FID                |fid       |u|FID = 21
|88-99   | 12 |UN Country Code    |country   |t|2 six-bit characters
|100-117 | 18 |UN/LOCODE          |locode    |t|3 six-bit characters
|118-147 | 30 |Fairway section    |section   |t|5 six-bit characters
|148-177 | 30 |Terminal code      |terminal  |t|5 six-bit characters
|178-207 | 30 |Fairway hectometre |hectometre|t|5 six-bit characters
|208-211 |  4 |ETA month          |month     |u|1-12, 0=N/A (default)
|212-216 |  5 |ETA day            |day       |u|1-31, 0=N/A (default)
|217-221 |  5 |ETA hour           |hour      |u|0-23, 24=N/A (default)
|222-227 |  6 |ETA minute         |minute    |u|0-59, 60=N/A (default)
|228-230 |  3 |Assisting Tugs     |tugs      |u|0-6, 7 = unknown (default)
|231-242 | 12 |Air Draught        |airdraught|u|0-4000 * 0.01m,
                                                0 = Unknown (default)
|243-247 |  5 |Spare              |          |x|Not used
|======

<<INLAND>> says of the Destination MMSI field "a virtual MMSI number
should be used for each country, each national AIS network should
route messages addressed to other countries using this virtual MMSI
number".

OPEN-QUESTION: <<INLAND>> does not specify whether ETA time is UTC
or local.

==== RTA at lock/bridge/terminal (Inland AIS)

A message 6 subtype. DAC = 200 FID = 22. Fixed length, 232 bits.

This message should be sent by base stations only, to assign a RTA at
a lock, bridge or terminal to a certain vessel in response to the
preceding ETA request.

[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member    |T|Units
|0-5     |  6 |Message Type       |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator   |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI        |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number    |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI   |dest_mmsi |u|9 decimal digits
|70-70   |  1 |Retransmit flag    |retransmit|u|0 = no retransmit (default),
                                                1 = retransmitted
|71-71   |  1 |Spare              |          |x|Not used
|72-81   | 10 |DAC                |dac       |u|DAC = 200
|82-87   |  6 |FID                |fid       |u|FID = 21
|88-99   | 12 |UN Country Code    |country   |t|2 six-bit characters
|100-117 | 18 |UN/LOCODE          |locode    |t|3 six-bit characters
|118-147 | 30 |Fairway section    |section   |t|5 six-bit characters
|148-177 | 30 |Terminal code      |terminal  |t|5 six-bit characters
|178-207 | 30 |Fairway hectometre |hectometre|t|5 six-bit characters
|208-211 |  4 |RTA month          |month     |u|1-12, 0=N/A (default)
|212-216 |  5 |RTA day            |day       |u|1-31, 0=N/A (default)
|217-221 |  5 |RTA hour           |hour      |u|0-23, 24=N/A (default)
|222-227 |  6 |RTA minute         |minute    |u|0-59, 60=N/A (default)
|228-229 |  2 |Status             |status    |e|See "Status Codes" below
|230-231 |  2 |Spare              |          |x|Not used
|======

OPEN-QUESTION: <<INLAND>> does not specify whether ETA time is UTC
or local.

OPEN-QUESTION: No default is specified for the Status field.

.Lock/Bridge/Terminal status codes
[frame="topbot"]
|======
| 0 | Operational
| 1 | Limited operation
| 2 | Out of order
| 3 | N/A
|======

==== Number of persons on board (Inland AIS)

This message should be sent by inland vessels only, to inform about
the number of persons (passengers, crew, and shipboard personnel) on
board.

A message 6 subtype. DAC = 200 FID = 55.  Fixed length, 168 bits.

//: Type 6(200/55)
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member    |T|Units
|0-5     |  6 |Message Type         |type      |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi      |u|9 decimal digits
|38-39   |  2 |Sequence Number      |seqno     |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI     |dest_mmsi |u|9 decimal digits
|70-70   |  1 |Retransmit flag      |retransmit|u|0 = no retransmit (default),
                                                  1 = retransmitted
|71-71   |  1 |Spare                |          |x|Not used
|72-81   | 10 |DAC                  |dac       |u|DAC = 200
|82-87   |  6 |FID                  |fid       |u|FID = 55
|88-95   |  8 |# crew on board      |crew      |u|Unsigned integer
                                                  0-254,
                                                  255 = Unknown (default)
|96-108  | 13 |# passengers on board|passengers|u|Unsigned integer
                                                  0-8190,
                                                  8191 = Unknown (default)
|109-116 |  8 |# personnel on board |personnel |u|Unsigned integer
                                                  0-254,
                                                  255 = Unknown (default)
|117-167 | 51 |Spare                |          |x|Not used
|======

==== AtoN monitoring data (GLA)

This message provides AtoN (Aid to navigation) monitoring data for the General
Lighthouse Authorities (GLA), which consists of Trinity House (England &
Wales), Northern Lighthouse Board (Scotland) and the Commissioners of Irish
Lights (Ireland). It is described in <<IALA-A126>>.

A message 6 subtype. DAC = 235 or 250 FID = 10. Fixed length: 136 bits.

DAC and FI are user configurable, DAC=235/FI=10 is used in UK, DAC=250/FI=10
in the Republic Of Ireland.

The interval between the transmissions of these messages will be synchronized
with message 21, although not necessarily at the same reporting rate. If
Message 21 is not used at a particular site, then the reporting interval
should be selected to minimize the power requirement of the transponder,
whilst still providing enough data to enable meaningful diagnostic analysis.

OPEN-QUESTION: <<INLAND>> lists a broadcast (type 8) variant of this
message, but without indicating how the Destination MMSI field is to
be set or interpreted.  Robust implementations should accept and
process this variant.

//: Type 6(235-250:10)
[frame="topbot",options="header"]
|======
|Field   |Len |Description       |Member     |T|Units
|0-5     |  6 |Message Type      |type       |u|Constant: 6
|6-7     |  2 |Repeat Indicator  |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI       |mmsi       |u|9 decimal digits
|38-39   |  2 |Sequence Number   |seqno      |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI  |dest_mmsi  |u|9 decimal digits
|70-70   |  1 |Retransmit flag   |retransmit |u|0 = no retransmit (default),
                                                1 = retransmitted
|71-71   |  1 |Spare             |           |x|Not used
|72-81   | 10 |DAC               |dac        |u|DAC = 235 or 250
|82-87   |  6 |FID               |fid        |u|FID = 10
|88-97   | 10 |Analogue          |ana_int    |u|0.05-36V, 0.05V step
                                                Supply voltage to AIS Unit
                                                0 = Not Used
|98-107  | 10 |Analogue (ext. #1)|ana_ext1   |u|0.05-36V, 0.05V step
                                                0 = Not Used
|108-117 | 10 |Analogue (ext. #2)|ana_ext2   |u|0.05-36V, 0.05V step
                                                0 = Not Used
|118-119 |  2 |RACON status      |racon      |u|00 = no RACON installed
                                                01 = RACON not monitored
                                                10 = RACON operational
						11 = RACON ERROR
|120-121 |  2 |Light status      |light      |u|00 = no light or no monitoring
                                                01 = Light ON
                                                10 = Light OFF
                                                11 = Light ERROR
|122     |  1 |Health            |health     |b|0 = Good Health, 1 = Alarm
|123-130 |  8 |Status (external) |stat_ext   |u|7  Digital Input 0=Off, 1=On
                                                :
                                                :
                                                0  Digital Input 0=Off, 1=On
|131-131 |  1 |Position status   |off_pos    |b|0=On position, 1=Off position
|132-135 |  4 |Spare             |           |x|Not used
|======

=== Type 7: Binary Acknowledge

Message type 7 is a receipt acknowledgment to the senders of a
previous messages of type 6.  Total length varies between 72 and 168
bits by 32-bit increments, depending on the number of destination
MMSIs included.

//: Type 7
[frame="topbot",options="header"]
|======
|Field   |Len |Description         |Member   |T|Units
|0-5     |  6 |Message Type        |type     |u|Constant: 7
|6-7     |  2 |Repeat Indicator    |repeat   |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI         |mmsi     |u|9 decimal digits
|38-39   |  2 |Spare               |         |x|Not used
|40-69   | 30 |MMSI number 1       |mmsi1    |u|9 decimal digits
|70-71   |  2 |Sequence for MMSI 1 |mmsiseq1 |u|Not used
|72-101  | 30 |MMSI number 2       |mmsi2    |u|9 decimal digits
|102-103 |  2 |Sequence for MMSI 2 |mmsiseq2 |u|Not used
|104-133 | 30 |MMSI number 3       |mmsi3    |u|9 decimal digits
|134-135 |  2 |Sequence for MMSI 3 |mmsiseq3 |u|Not used
|136-165 | 30 |MMSI number 4       |mmsi4    |u|9 decimal digits
|166-167 |  2 |Sequence for MMSI 4 |mmsiseq4 |u|Not used
|======

Use of the MMSI sequence fields was introduced in ITU-1371-5 to
indicate the sequence number of the Type 6 to which this responds.
In earlier versions these were spare fields.

=== Type 8: Binary Broadcast Message

Message type 8 is a broadcast message with unspecified binary payload.
The St. Lawrence Seaway AIS system, the USG PAWSS system, and the Port
Authority of London use this payload for local extension
messages. <<IMO236>> and <<IMO289>> describe payload use as
international extension messages. This type is variable in length up
to a maximum of 1008 bits (up to 5 AIVDM sentence payloads).

//: Type 8
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member    |T|Units
|0-5     |  6 |Message Type         |type      |u|Constant: 8
|6-7     |  2 |Repeat Indicator     |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi      |u|9 decimal digits
|38-39   |  2 |Spare                |          |x|Not used
|40-49   | 10 |Designated Area Code |dac       |u|Unsigned integer
|50-55   |  6 |Functional ID        |fid       |u|Unsigned integer
|56      |952 |Data                 |data      |d|Binary data,
                                                  May be shorter than 952 bits.
|======

Interpretation of the binary payload is controlled by DAC/FID as in
message type 6. The following is a non-exhaustive list of standardized
DAC-FID pairs in use for type 8; for an up-to-date list of registered application-specific
messages, see <<IALA-ASM>>:

[frame="topbot",options="header"]
|======
| DAC     |FID | Sub | Source     | Status     | Description
|   1     | 11 |     | <<IMO236>> | Deprecated/In Use | Meteorological/Hydrological Data
|   1     | 13 |     | <<IMO236>> | Deprecated | Fairway closed
|   1     | 15 |     | <<IMO236>> | Deprecated | Extended ship and voyage
|   1     | 17 |     | <<IMO289>>  | In use     | VTS-Generated/Synthetic targets
|   1     | 19 |     | <<IMO289>> | Standard   | Marine traffic signals
|   1     | 21 |     | <<IMO289>> | Standard   | Weather observation from ship
|   1     | 22 |     | <<IMO289>> | In use     | Area notice (broadcast)
|   1     | 24 |     | <<IMO289>> | Standard   | Extended ship and voyage
|   1     | 26 |     | <<IMO289>> | Standard   | Environmental
|   1     | 27 |     | <<IMO289>> | Standard   | Route info broadcast
|   1     | 29 |     | <<IMO289>> | Standard   | Text description broadcast
|   1     | 31 |     | <<IMO289>> | In use     | Meteorological and Hydrological
|  200    | 10 |     | <<INLAND>> | Standard   | Ship static and voyage related data
|  200    | 23 |     | <<INLAND>> | Standard   | EMMA warning report
|  200    | 24 |     | <<INLAND>> | Standard   | Water levels
|  200    | 40 |     | <<INLAND>> | Standard   | Signal status
| 316/366 |  1 |   2 | <<SEAWAY>> | In use     | Wind
| 316/366 |  1 |   1 | <<SEAWAY>> | In use     | Weather station
| 316/366 |  1 |   3 | <<SEAWAY>> | In use     | Water level
| 316/366 |  1 |   6 | <<SEAWAY>> | In use     | Water flow
| 316/366 |  2 |   1 | <<SEAWAY>> | In use     | Lockage Order
| 316/366 |  2 |   2 | <<SEAWAY>> | In use     | Estimated Lock Times
| 316/366 | 32 |   1 | <<SEAWAY>> | In use     | Seaway Version Message
| 366     |  1 |   4 | <<SEAWAY>> | In use     | PAWS Hydro / Current
| 366     |  1 |   6 | <<SEAWAY>> | In use     | PAWS Hydro / Salinity Temp
| 366     |  1 |   3 | <<SEAWAY>> | In use     | PAWS Vessel Procession Order
|======

DAC/FID pairs are assigned separately per message type.  For St. Lawrence
Seaway messages, the DAC may be 316 (Canada) or 366 (U.S.) depending
on the transmitter location.

Note that the apparent presence of one of these DAC/FID pairs does
not guarantee that the message is structured.  Decoders should
perform range validation on the structured fields and interpret the
message as unstructured if any check fails. Actual false matches with
DAC/FID = 200/10 have been observed in the wild.

DAC/FID pairs 1/23, 1/28, and 1/30 have addressed versions described
under type 6.

FID types 11-15 are being phased out and are not to be used after 1
Jan 2013. The deprecated IMO236 1/11 has a different binary layout from
the IMO289 1/31.  FID type 17 is in use; there is a proposed
update for it in <<IMO289>>.

Breakdowns of Message 8 subtypes from <<IMO289>> follow.

==== Meteorological and Hydrological Data (IMO236)

A message 8 subtype. DAC = 001 FID = 11. Fixed length, 352 bits.
This is in use and described in <<IMO236>>, but has been deprecated by
<<IMO289>> in favor of a message with the same title but FID = 31 and a
different binary layout. <<IMO236>> specifies a maximum interval between
broadcast of this message of 12 minutes.

//: Type 8(1/11)
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member      |T|Units
|0-5     |  6 |Message Type       |type        |u|Constant: 8
|6-7     |  2 |Repeat Indicator   |repeat      |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI        |mmsi        |u|9 decimal digits
|38-39   |  2 |Spare              |            |x|Not used
|40-49   | 10 |DAC                |dac         |u|DAC = 001
|50-55   |  6 |FID                |fid         |u|FID = 11
|56-79   | 24 |Latitude           |lat         |I3|Unit = minutes * 0.001,
                                                  0x7FFFFF = N/A (default),
                                                  E positive, W negative.
|80-104  | 25 |Longitude          |lon         |I3|Unit = minutes * 0.001,
                                                  0xFFFFFF = N/A (default),
                                                  N positive, S negative.
|105-109 |  5 |Day (UTC)          |day         |u|1-31, 31=N/A (default)
|110-114 |  5 |Hour (UTC)         |hour        |u|0-23, 31=N/A (default)
|115-120 |  6 |Minute (UTC)       |minute      |u|0-59, 63=N/A (default)
|121-127 |  7 |Average Wind Speed |wspeed      |u|10-min avg wind speed, knots,
                                                  127 = N/A (default).
|128-134 |  7 |Gust Speed         |wgust       |u|10-min max wind speed, knots,
                                                  127 = N/A (default).
|135-143 |  9 |Wind Direction     |wdir        |u|0-359, degrees from true north
                                                   511 = N/A (default)
|144-152 |  9 |Wind Gust Direction|wgustdir    |u|0-359, degrees from true north
                                                   511 = N/A (default)
|153-163 | 11 |Air Temperature    |temperature |u|Dry bulb temp: 0.1 deg C
                                                  -60.0 to +60.0,
                                                  2047 = N/A (default),
|164-170 |  7 |Relative Humidity  |humidity    |u|0-100%, units of 1%,
                                                  127 = N/A (default).
|171-180 | 10 |Dew Point          |dewpoint    |u|-20.0 to +50.0: 0.1 deg C,
                                                  1023 = N/A (default),
|181-189 |  9 |Air Pressure       |pressure    |u|800-1200hPa: units 1hPa,
                                                  511 = N/A (default).
|190-191 |  2 |Pressure Tendency  |pressuretend|e|0 = steady,
                                                  1 = decreasing,
                                                  2 = increasing,
                                                  3 - N/A (default).
|192-199 |  8 |Horiz. Visibility  |visibility  |U1|0-25.0, units of 0.1nm
                                                  255 = N/A (default)
|200-208 |  9 |Water Level        |waterlevel  |I1|-10.0 to +30.0 in 0.1m,
                                                  511 = N/A (default).
|209-210 |  2 |Water Level Trend  |leveltrend  |e|0 = steady,
                                                  1 = decreasing,
                                                  2 = increasing,
                                                  3 - N/A (default).
|211-218 |  8 |Surface Current Speed     |cspeed  |U1|0.0-25.0 knots: units 0.1 knot
|219-227 |  9 |Surface Current Direction |cdir    |u|0-359: deg from true north,
                                                     511 = N/A (default)
|228-235 |  8 |Current Speed #2     |cspeed2   |U1|0.0-25.0 in units of 0.1 knot,
                                                  255 = N/A (default).
|236-244 |  9 |Current Direction #2 |cdir2     |u|0-359: deg. fom true north,
                                                  511 = N/A (default)
|245-249 |  5 |Measurement Depth #2 |cdepth2   |U1|0-30m down: units 0.1m,
                                                   31 = N/A (default).
|250-257 |  8 |Current Speed #3     |cspeed3   |U1|0.0-25.0: units of 0.1 knot,
                                                   255 = N/A (default).
|258-266 |  9 |Current Direction #3 |cdir3     |u|0-359: degrees fom true north,
                                                  511 = N/A (default).
|267-271 |  5 |Measurement Depth #3 |cdepth3   |U1|0-30m down: units 0.1m,
                                                  31 = N/A (default).
|272-279 |  8 |Wave height          |waveheight|U1|0-25m: units of 0.1m,
                                                  255 = N/A (default).
|280-285 |  6 |Wave period          |waveperiod|u|Seconds 0-60:
                                                  63 = N/A (default).
|286-294 |  9 |Wave direction       |wavedir   |u|0-359: deg. ffom true north,
                                                  511 = N/A (default).
|295-302 |  8 |Swell height         |swellheight|U1|0-25m: units of 0.1m
                                                   255 = N/A (default).
|303-308 |  6 |Swell period         |swellperiod|u|Seconds 0-60:
                                                   63 = N/A (default).
|309-317 |  9 |Swell direction      |swelldir   |u|0-359: deg. fom true north,
                                                   511 = N/A (default).
|318-321 |  4 |Sea state            |seastate   |e|See "Beaufort Scale"
|322-331 | 10 |Water Temperature    |watertemp  |U1|-10.0 to 50.0: units 0.1 C,
                                                   1023 = N/A (default).
|332-334 |  3 |Precipitation        |preciptype |e|See "Precipitation Types"
|335-343 |  9 |Salinity             |salinity   |U1|0.0-50.0%: units 0.1%,
                                                   511 = N/A (default)
|344-345 |  2 |Ice                  |ice        |e| 0 = No
                                                    1 = Yes
                                                    2 = (reserved for future use)
                                                    3 = not available = default
|346-351 |  6 |Spare                |           |x|Not used
|======

<<IMO236>> says "If there is no data available, default value to be
transmitted is the highest available binary value for that particular
data field.", the above table reflects that.  The day, hour and minute
have to be considered not available when all three are set to their
individual "N/A" value.  For the latitude and the longitude, the
highest positive value is used, as the highest available binary value
for a signed integer is -1, which would forbid the -0.001/-0.001
position.  The replacement FID=31 message has different default values
that remove any ambiguities.

<<IMO236>> gives the length of this message as 352, but lists only 336
payload bits.

Water level is deviation from local chart datum and includes tide.

The waveheight field is labeled as "Significant" in <<IMO236>>,
for whatever that means.

The seastate field has a note in <<IMO236>> reading "(manual input?)"?

WMO 306 Code table 4.201 specifies the following precipitation type values:

.Precipitation Types
[width="50%",frame="topbot",options="header"]
|======
|Code |Precipitation Type
|0    |Reserved
|1    |Rain
|2    |Thunderstorm
|3    |Freezing rain
|4    |Mixed/ice
|5    |Snow
|6    |Reserved
|7    |N/A (default)
|======

.Beaufort Scale
[width="50%",frame="topbot",options="header"]
|======
|Scale|Description    | Sea Conditions
|0    |Calm           |Flat.
|1    |Light air      |Ripples without crests.
|2    |Light breeze   |Small wavelets.
                       Crests of glassy appearance, not breaking.
|3    |Gentle breeze  |Large wavelets.
                       Crests begin to break; scattered whitecaps.
|4    |Moderate breeze|Small waves.
|5    |Fresh breeze   |Moderate (1.2 m) longer waves. Some foam and spray.
|6    |Strong breeze  |Large waves with foam crests and some spray.
|7    |High wind      |Sea heaps up and foam begins to streak.
|8    |Gale           |Moderately high waves with breaking crests
                       forming spindrift. Streaks of foam.
|9    |Strong gale    |High waves (6-7 m) with dense foam.
                       Wave crests start to roll over. Considerable spray.
|10   |Storm          |Very high waves. The sea surface is white and there
                       is considerable tumbling. Visibility is reduced.
|11   |Violent storm  |Exceptionally high waves.
|12   |Hurricane force|Huge waves. Air filled with foam and spray. Sea
                       completely white with driving spray. Visibility
                       greatly reduced.
|13   |               |N/A (default)
|14-15|               |Reserved
|======

==== Fairway Closed

This message should be broadcast from shore stations to inform ships,
in particular to give guidance to large vessels about temporary closed
fairways or sections in ports.

A message 8 subtype. DAC = 001 FID = 13. Fixed length, 472 bits.
Described in <<IMO236>> but deprecated by <<IMO289>>.

//: Type 8(1/13)
[frame="topbot",options="header"]
|======
|Field   |Len |Description             |Member   |T|Units
|0-5     |  6 |Message Type            |type     |u|Constant: 8
|6-7     |  2 |Repeat Indicator        |repeat   |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI             |mmsi     |u|9 decimal digits
|38-39   |  2 |Spare                   |         |x|Not used
|40-49   | 10 |DAC                     |dac      |u|DAC = 001
|50-55   |  6 |FID                     |fid      |u|FID = 13
|56-175  |120 |Reason For Closing      |reason   |t|20 6-bit characters
|176-295 |120 |Location Of Closing From|closefrom|t|20 6-bit characters
|296-415 |120 |Location of Closing To  |closeto  |t|20 6-bit characters
|416-425 | 10 |Radius extension        |radius   |u|0-1000, 10001 = N/A (default)
|426-427 |  2 |Unit of extension       |extunit  |u|0=m, 1=km, 2=nm, 3=cables
|428-432 |  5 |From day (UTC)          |fday     |u|1-31, 0=N/A (default)
|433-436 |  4 |From month (UTC)        |fmonth   |u|1-12, 0=N/A (default)
|437-441 |  5 |From hour (UTC)         |fhour    |u|0-23, 24=N/A (default)
|442-447 |  6 |From minute (UTC)       |fminute  |u|0-59, 60=N/A (default)
|448-452 |  5 |To day (UTC)            |tday     |u|1-31, 0=N/A (default)
|453-456 |  4 |To month (UTC)          |tmonth   |u|1-12, 0=N/A (default)
|457-461 |  5 |To hour (UTC)           |thour    |u|0-23, 24=N/A (default)
|462-467 |  6 |To minute (UTC)         |tminute  |u|0-59, 60=N/A (default)
|468-471 |  4 |Spare                   |         |x|Not used
|======

No default is specified for the radius field in the standard.

==== IMO236 Extended Ship Static and Voyage Related Data

This message should be used by a ship to report the height over keel.

A message 8 subtype. DAC = 001 FID = 15 in <<IMO236>>.  Fixed length, 72 bits.
Deprecated in <<IMO289>>.

//: Type 8(1/15)
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member    |T|Units
|0-5     |  6 |Message Type         |type      |u|Constant: 8
|6-7     |  2 |Repeat Indicator     |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi      |u|9 decimal digits
|38-39   |  2 |Spare                |          |x|Not used
|40-49   | 10 |DAC                  |dac       |u|DAC = 001
|50-55   |  6 |FID                  |fid       |u|FID = 15
|55-66   | 11 |Air Draught          |airdraught|u|Height in 0.1m steps
                                                  0 = N/A (default),
                                                  204.7 = >= 204.7 m,
|67-71   |  5 |Spare                |          |x|Not used
|======

==== VTS-Generated/Synthetic targets

A message 8 subtype. DAC = 001 FID = 17. Variable length: 176-536
bits.  This message is laid out identically in <<IMO236>> and
<<IMO289>>.  In <<IMO236>> it is titled "Pseudo-AIS Targets".

//: Type 8(1/17)
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member     |T|Units
|0-5     |  6 |Message Type       |type       |u|Constant: 8
|6-7     |  2 |Repeat Indicator   |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI        |mmsi       |u|9 decimal digits
|38-39   |  2 |Spare              |           |x|Not used
|40-49   | 10 |DAC                |dac        |u|DAC = 001
|50-55   |  6 |FID                |fid        |u|FID = 17
|56      |    |                   |targets    |a4|Synthetic targets array
|0-1     |  2 |Identifier type    |idtype     |e|0 = id is the MMSI number,
                                                 1 = id is the IMO number,
                                                 2 = id is the call sign,
                                                 3 = Other (default).
|2-43    | 42 |Target identifier  |id         |u|Target ID data.
|44-47   |  4 |Spare              |           |x|Not used
|48-71   | 24 |Latitude           |lat        |I3|Minutes * 0.001,
                                                 91000 = N/A (default),
                                                 N positive, S negative.
|72-96   | 25 |Longitude          |lon        |I3|Minutes * 0.001,
                                                 181000 = N/A (default),
                                                 E positive, W negative.
|97-105  |  9 |Course Over Ground |course     |u|0-359 deg from true north,
                                                 360 = N/A (default).
|106-111 |  6 |Time Stamp         |second     |u|Second of UTC timestamp.
|112-121 | 10 |Speed Over Ground  |speed      |u|0-254 in knots, 255 = N/A.
|======

The interpretation of the target identifier field depends on the
preceding type key.  For 0 and 1 it is a big-endian unsigned binary
integer (as shown above).  For type 2 and 3 it is 6-bit ASCII text. An
unknown target is expressed by type 3 and the string "@@@@@@@".

OPEN-QUESTION: <<IMO289>> says: "When MMSI or IMO number is used, the
least significant bit should equal bit zero of the Target Identifier."
It is unclear how "bit zero" is to be interpreted, but it is not
possible to reconcile interpreting it as the leading bit of the field
with AIS big-endian encoding. Settling this awaits live testing.

The trailing eight fields may be repeated up to 3 times (for a total of 1 to 4
field groups) to represent up to 4 targets.

==== IMO289 Marine Traffic Signal

This message provides information on a signal station and status of
the control signal at the entrance of a harbor or channel where the
shipping direction controlled so that the traffic flow be kept in
order.

A message 8 subtype described in <<IMO289>>. DAC = 001 FID = 19. Fixed
length: 360 bits.

//: Type 8(1/19)
[frame="topbot",options="header"]
|======
|Field   |Len  |Description           |Member  |T|Units
|0-5     |   6 |Message Type          |type    |u|Constant: 8
|6-7     |   2 |Repeat Indicator      |repeat  |u|As in Common Navigation Block
|8-37    |  30 |Source MMSI           |mmsi    |u|9 decimal digits
|38-39   |   2 |Spare                 |        |x|Not used
|40-49   | 10  |DAC                   |dac     |u|DAC = 001
|50-55   |  6  |FID                   |fid     |u|FID = 19
|56-65   |  10 |Message Linkage ID    |linkage |u|Unsigned integer
|66-185  | 120 |Name of Signal Station|station |t|20 6-bit chars
|186-210 |  25 |Longitude             |lon     |I3|Unit = minutes * 0.001
                                                  181000 = N/A (default)
                                                  E positive, W negative.
|211-234 |  24 |Latitude              |lat     |I3|Unit = minutes * 0.001
                                                  91000 = N/A (default)
                                                  N positive, S negative.
|235-236 |   2 |Status of Signal      |status  |u|0=N/A (default
                                                  1 = In regular service
                                                  2 = Irregular service
                                                  3 = Reserved for future use
|237-241 |   5 |Signal In Service     |signal  |e|See "Marine Traffic Signals"
|242-246 |   5 |UTC hour              |hour    |u|0-23, 24=N/A (default)
|247-252 |   6 |UTC minute            |minute  |u|0-59, 60=N/A (default)
|253-257 |   5 |Expected Next Signal  |nextsignal|e|See "Marine Traffic Signals"
|258-359 | 102 |Spare                 |        |x|Not used
|======

.Marine Traffic Signals
[width="50%",frame="topbot",options="header"]
|======
|Code |Position Fix Type
|  0  |N/A (default)
|  1  |IALA port traffic signal 1: Serious emergency – all vessels to stop
       or divert according to instructions.
|  2  |IALA port traffic signal 2: Vessels shall not proceed.
|  3  |IALA port traffic signal 3: Vessels may proceed. One way traffic.
|  4  |IALA port traffic signal 4: Vessels may proceed. Two way traffic.
|  5  |IALA port traffic signal 5: A vessel may proceed only when it has
       received specific orders to do so.
|  6  |IALA port traffic signal 2a: Vessels shall not proceed, except that
       vessels which navigate outside the main channel need not comply with
       the main message.
|  7  |IALA port traffic signal 5a: A vessel may proceed only when it has
       received specific orders to do so; except that vessels which navigate
       outside the main channel need not comply with the main message.
|  8  |Japan Traffic Signal - I = "in-bound" only acceptable.
|  9  |Japan Traffic Signal - O = "out-bound" only acceptable.
| 10  |Japan Traffic Signal - F = both "in- and out-bound" acceptable.
| 11  |Japan Traffic Signal - XI = Code will shift to "I" in due time.
| 12  |Japan Traffic Signal - XO = Code will shift to "O" in due time.
| 13  |Japan Traffic Signal - X = Vessels shall not proceed, except a vessel
       which receives the direction from the competent authority.
|14-31|Reserved
|======

==== IMO289 Weather observation report from ship

There are two variants of this message.  They are distinguished by bit 56,
the WMO bit.  Field layouts after that bit vary depending on it.

A message 8 subtype described in <<IMO289>>. DAC = 001 FID = 21. Fixed
length: 360 bits.

//: Type 8(1/21)
.Weather observation report from ship: Non-WMO variant
[frame="topbot",options="header"]
|======
|Field   |Len  |Description        |Member      |T|Units
|0-5     |   6 |Message Type       |type        |u|Constant: 8
|6-7     |   2 |Repeat Indicator   |repeat      |u|As in Common Navigation Block
|8-37    |  30 |Source MMSI        |mmsi        |u|9 decimal digits
|38-39   |   2 |Spare              |            |x|Not used
|40-49   | 10  |DAC                |dac         |u|DAC = 001
|50-55   |  6  |FID                |fid         |u|FID = 21
|56-56   |   1 |Variant            |wmo         |b|Constant: 0 in this variant
|57-176  | 120 |Location           |location    |t|20 6-bit characters
|177-201 |  25 |Longitude          |lon         |I3|Unit = minutes * 0.001,
                                                   181000 = N/A (default),
                                                   E positive, W negative.
|202-225 |  24 |Latitude           |lat         |I3|Unit = minutes * 0.001,
                                                   91000 = N/A (default),
                                                   N positive, S negative.
|226-230 |   5 |UTC Day            |day         |u|1-31, 0=N/A (default)
|231-235 |   5 |UTC hour           |hour        |u|0-23, 24=N/A (default)
|236-241 |   6 |UTC minute         |minute      |u|0-59, 60=N/A (default)
|242-245 |   4 |Present Weather    |weather     |u|wmocode: 0-15
|246-246 |   1 |Visibility Limit   |vislimit    |b|See below
|247-253 |   7 |Horiz. Visibility  |visibility  |U1|0.0-12.6nm, units = nm*0.1,
                                                   127 = N/A (default).
|254-260 |   7 |Relative Humidity  |humidity    |u|0-100%: units of 1%,
                                                   127 = N/A (default).
|261-267 |   7 |Average Wind Speed |wspeed      |u|10-min avg wind speed knots,
                                                   127 = N/A (default).
|268-276 |   9 |Wind Direction     |wdir        |u|0-359, deg. fom true north,
                                                   360 = N/A (default).
|277-285 |   9 |Air Pressure       |pressure    |u|800-1200hPa: units 1hPa,
                                                   402 = pressure >= 1201 hPa,
                                                   403 - N/A (default),
                                                   else add 400 to value,
|286-289 |   4 |Pressure Tendency  |pressuretend|u|WMO FM13 code
|290-300 |  11 |Air Temperature    |airtemp     |I1|Dry bulb temp: units 0.1C,
                                                   -60.0 to +60.0,
                                                   -1024 = N/A (default).
|301-310 |  10 |Water Temperature  |watertemp   |I1|-10.0 to 50.0 in 0.1 C,
                                                   601 = N/A (default),
                                                   else -10C after scaling.
|311-316 |  6  |Wave period        |waveperiod  |u|Seconds: 0-60,
                                                   63 = N/A (default).
|317-324 |  8  |Wave height        |waveheight  |U1|0-25m in units of 0.1m,
                                                   255 = N/A (default).
|325-333 |  9  |Wave direction     |wavedir     |u|0-359: deg. from true north,
                                                   360 = N/A (default).
|334-341 |  8  |Swell height       |swellheight |U1|0-25m: units 0.1m,
                                                   255 = N/A (default)
|342-350 |  9  |Swell direction    |swelldir    |u|0-359: deg, fom true north,
                                                   360 = N/A (default).
|351-356 |  6  |Swell period       |swellperiod |u|Seconds: 0-60,
                                                   63 = N/A (default).
|357-359 |  3  |Spare              |            |x|Not used
|======

The vislimit bit, when on, indicates that the maximum range of the
visibility equipment was reached and the visibility reading shall be
regarded as > x.x NM.

The standard (<<IMO289>>) does not list the WMO FM13 codes.
The following table applies:

.Weather observation report from ship: WMO Code 45501
[width="50%",frame="topbot",options="header"]
|======
|Code |Precipitation Type
|  0  |Clear (no clouds at any level)
|  1  |Cloudy
|  2  |Rain
|  3  |Fog
|  4  |Snow
|  5  |Typhoon/hurricane
|  6  |Monsoon
|  7  |Thunderstorm
|  8  |N/A (default)
| 9-15|Reserved for future use
|======

.Weather observation report from ship: WMO variant
[frame="topbot",options="header"]
|======
|Field   |Len  |Description            |Member  |T|Units
|0-5     |   6 |Message Type           |type    |u|Constant: 8
|6-7     |   2 |Repeat Indicator       |repeat  |u|As in Common Navigation Block
|8-37    |  30 |Source MMSI            |mmsi    |u|9 decimal digits
|38-39   |   2 |Spare                  |        |x|Not used
|40-49   |  10 |DAC                    |dac     |u|DAC = 001
|50-55   |   6 |FID                    |fid     |u|FID = 21
|56-56   |   1 |Variant                |wmo     |u|Constant: 1 in this variant
|57-72   |  16 |Longitude              |lon     |I3|Unsigned: minutes * 0.01,
                                                   E positive, W negative,
                                                   Lon = (value / 100) – 180,
                                                   65536 = N/A (default).
|73-87   |  15 |Latitude               |lat     |I3|Unsigned: minutes * 0.01,
                                                   N positive, S negative,
                                                   Lat = (value / 100) – 90,
                                                   32767 = N/A (default).
|88-91   |   4 |UTC Month              |month   |u|1-12, 15=N/A (default)
|92-97   |   6 |UTC Day                |day     |u|1-31, 63=N/A (default)
|98-102  |   5 |UTC hour               |hour    |u|0-23, 31=N/A (default)
|103-105 |   3 |UTC minute             |minute  |u|0-50, 7=N/A (default),
                                                   Minute = (value * 10).
|106-112 |   7 |Course Over Ground     |course  |u|0-359, unit = 1 degree,
                                                   average over last 10 minutes.
|113-117 |   5 |Speed Over Ground      |speed   |U1|0-14.5m/s:
                                                   SOG = (value * 0.5) for 0-29,
                                                   30 = 15 m/s and more,
                                                   average over last 10 minutes.
                                                   31 = N/A (default)
|118-124 |   7 |Heading of the ship    |heading |u|5-360: units of 5 degrees,
                                                   HDT = (value * 5) for 1-72,
                                                   average over last 10 minutes.
                                                   127 = N/A (default)
|125-135 |  11 |Pressure at sea level  |pressure|U1|90-1100 hPa:
                                                   P = (value/10)+900 for 0-2000
|136-145 |  10 |Pressure Change        |pdelta  |U1|-50-+50hPa: units of 0.1hPa,
                                                   d = (value/10)-50 for 0-100,
                                                   averaged over last 3 hours.
                                                   1023 = N/A (default)
|146-149 |   4 |Pressure Tendency      |ptend   |u|WMO BUFR table 010063:
                                                   Averaged over last 3 hours,
                                                   0-8, 15 = N/A.
|150-156 |   7 |True Wind Direction    |twinddir|u|005-360: deg:
                                                   average over last 10 minutes,
                                                   dir = (value*5), value 1-72,
                                                   0 = calm,
                                                   127 = N/A (default).
|157-164 |   8 |True Wind Speed        |twindspeed|u|0-127 m/s:
                                                     average over last 10 mins,
                                                     speed = value * 0.5,
                                                     255 = N/A (default).
|165-171 |   7 |Relative Wind Direction|rwinddir  |u|005-360 deg:
                                                     average over last 10 mins,
                                                     dir = (value*5) for 1-72,
                                                     0 = calm,
                                                     127 = N/A (default).
|172-179 |   8 |Relative Wind Speed    |rwindspeed|U1|0-127 m/s:
                                                    average over last 10 mins,
                                                     speed = val*0.5 for 0-254,
                                                     255 = N/A (default).
|180-187 |   8 |Maximum Gust Speed     |mgustspeed|U1|0-127 m/s:
                                                     speed = val*0.5 for 0-254,
                                                     255 = N/A (default),
|188-194 |   7 |Maximum Gust Direction |mgustdir  |u|05-360 deg:
                                                     dir = (value*5) for 1-72,
                                                     0 = calm,
                                                     127 = N/A (default).
|195-204 |  10 |Air Temperature        |airtemp   |U1|Dry bulb temp: units 0.1C,
                                                     223-323K (c.-50C - +50C).
                                                     T = (val/10)+223 for 0-1000,
                                                     1023 = N/A (default).
|205-211 |   7 |Relative Humidity      |humidity  |u|0-100%: units of 1%,
                                                    127 = N/A (default).
|212-220 |   9 |Sea Surface Temperature|surftemp  |U1|268-318K (c.-5C - +45C):
                                                     T = (val/10)+268 for 0-500,
                                                     511 = N/A (default).
|221-226 |   6 |Horiz. Visibility      |visibility|U2|0-50000m,
                                                     Vis = (val**2)*13.073 for 0-62,
                                                     63 = N/A (default).
|227-235 |   9 |Present Weather        |weather   |u|BUFR table 020003:
                                                     Codes 0-510,
                                                     511 = N/A (default).
|236-240 |   5 |Past Weather 1         |pweather1 |u|BUFR table 020005:
                                                     Codes 0-30,
                                                     31 = N/A (default).
|241-245 |   5 |Past Weather 2         |pweather2 |u|BUFR table 020004:
                                                     Codes 0-30,
                                                     31 = N/A (default).
|246-249 |   4 |Total Cloud Cover      |totalcloud|u|0-100%:
                                                     Cover = val * 10% for 0-10,
                                                     15 = N/A (default.)
|250-253 |   4 |Cloud amount (low)     |lowclouda |u|0-14: BUFR table 020011:
                                                     15 = N/A (default).
|254-259 |   6 |Cloud type (low)       |lowcloudt |u|0-62: BUFR table 020012:
                                                     63 = N/A (default).
|260-265 |   6 |Cloud type (middle)    |midcloudt |u|0-62: BUFR table 020012:
                                                     63 = N/A (default).
|266-271 |   6 |Cloud type (high)      |highcloudt|u|0-62: BUFR table 020012:
                                                     63 = N/A (default).
|272-278 |   7 |Height of cloud base   |cloudbase |U2|0-2500m:
                                                     h = (value**2)*.0.16 for 0-125,
                                                   126 = more than 2500m,
                                                   127 = N/A (default).
|279-283 |   5 |Period of Wind Waves   |wwperiod  |u|0-30s:
                                                     31 = N/A (default).
|284-289 |   6 |Height of Wind Waves   |wwheight  |u|Height in meters: 0-30,
                                                     h = (value * 0.5) for 0-60,
                                                     63 = N/A (default).
|290-295 |   6 |First Swell Direction  |swelldir1 |u|10-360 deg:
                                                     dir = (value*10) for 1-36,
                                                     0 = calm,
                                                     63 = N/A (default).
|296-300 |   5 |First Swell Period     |swperiod1 |u|Period in seconds: 0-30,
                                                     31 = N/A (default).
|301-306 |   6 |First Swell Height     |swheight1 |U1|Height in meters: 0-30,
                                                     h = (value * 0.5) for 0-60,
                                                     63 = N/A (default).
|307-312 |   6 |Second Swell Direction |swelldir2 |u|10-360 deg:
                                                     dir = (value*10) for 1-36,
                                                     0 = calm,
                                                     63 = N/A (default).
|313-317 |   5 |Second Swell Period    |swperiod2 |u|Period in seconds: 0-30,
                                                     31 = N/A (default).
|318-323 |   6 |Second Swell Height    |swheight2 |U1|Height in meters: 0-30,
                                                     h = (value * 0.5) for 0-60,
                                                     63 = N/A (default).
|324-330 |   7 |Ice deposit (thickness)|icedeposit|u|Thickness: 0-126cm,
                                                     127 = N/A (default).
|331-333 |   3 |Rate of Ice Accretion  |icerate   |u|0-6: BUFR table 020032:
                                                     7 = N/A (default).
|334-336 |   3 |Cause of Ice Accretion |icecause  |u|0-6: BUFR table 020033:
                                                     7 = N/A (default).
|337-341 |   5 |Sea Ice Concentration  |seaice    |u|0-30: BUFR table 020034:
                                                     31 = N/A (default).
|342-345 |   4 |Amount and Type of Ice |icetype   |u|0-14: BUFR table 020035:
                                                     15 = N/A (default).
|346-350 |   5 |Ice Situation          |icestate  |u|0-30: BUFR table 020036:
                                                     31 = N/A (default).
|351-355 |   5 |Ice Development        |icedevel  |u|0-30: BUFR table 020037:
                                                     31 = N/A (default).
|356-359 |   4 |Bearing of Ice Edge    |icebearing|u|Bearing: 45-360 deg,
                                                     dir = (value*45) for 1-8,
                                                     15 = N/A (default).
|======

The "minute" entry actually only identifies the end of a 10-minute interval.

In <<IMO289>>, the Latitude formula is given as "Lat = (value / 100) – 9000".
This is incorrect; the decrement needs to be 90 for the range to be -90..+90.

Swell directions are arrival directions.

==== IMO289 Area Notice (broadcast)

This should be used to broadcast time- and location-dependent information about
hazards to navigation.  For information-lifetime restrictions and usage
guidance, refer to <<ITU1371>>.

A message 8 subtype. DAC = 001 FID = 22. 196 to 981 bits.  There is a
related Message 6 subtype for addressed use.

The message consists of a fixed-length header of 111 bits, followed by 1 to
10 sub-area indications which are fixed-length records 87 bits long. Here
is the message header format:

//: Type 8(1/22)
.Area Notice (addressed) message header
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member     |T|Units
|0-5     |  6 |Message Type         |type       |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi       |u|9 decimal digits
|38-39   |  2 |Spare                |           |x|Not used
|40-49   | 10 |DAC                  |dac        |u|DAC = 001
|50-55   |  6 |FID                  |fid        |u|FID = 22
|56-65   | 10 |Message Linkage ID   |linkage    |u|Unsigned integer
|66-72   |  7 |Notice Description   |notice     |u|See table below
|73-76   |  4 |Month (UTC)          |month      |u|1-12; 0 = N/A (default)
|77-81   |  5 |Day (UTC)            |day        |u|1-31; 0 = N/A (default)
|82-86   |  5 |Hour (UTC)           |hour       |u|0-23; 24 = N/A (default)
|87-92   |  6 |Minute (UTC)         |minute     |u|0-59; 60 = N/A (default)
|93-110  | 18 |Duration             |duration   |u|In minutes,
                                                 262143 = N/A (default),
                                                 0 = cancel this notice.
|111-113 |  3 |Subarea shape #1     |subarea type |u|0-5 (see "Subarea Types")
|114-197 | 84 |Subarea payload #1   |shape data |array|See "Subarea Payloads"
|198-200 |  3 |Subarea shape #2     |subarea type |u|0-5 (see "Subarea Types")
|201-284 | 84 |Subarea payload #2   |shape data |array|See "Subarea Payloads"
|285-287 |  3 |Subarea shape #3     |subarea type |u|0-5 (see "Subarea Types")
|288-371 | 84 |Subarea payload #3   |shape data |array|See "Subarea Payloads"
|372-374 |  3 |Subarea shape #4     |subarea type |u|0-5 (see "Subarea Types")
|375-458 | 84 |Subarea payload #4   |shape data |array|See "Subarea Payloads"
|459-461 |  3 |Subarea shape #5     |subarea type |u|0-5 (see "Subarea Types")
|462-545 | 84 |Subarea payload #5   |shape data |array|See "Subarea Payloads"
|546-548 |  3 |Subarea shape #6     |subarea type |u|0-5 (see "Subarea Types")
|549-632 | 84 |Subarea payload #6   |shape data |array|See "Subarea Payloads"
|633-635 |  3 |Subarea shape #7     |subarea type |u|0-5 (see "Subarea Types")
|636-719 | 84 |Subarea payload #7   |shape data |array|See "Subarea Payloads"
|720-722 |  3 |Subarea shape #8     |subarea type |u|0-5 (see "Subarea Types")
|723-806 | 84 |Subarea payload #8   |shape data |array|See "Subarea Payloads"
|807-809 |  3 |Subarea shape #9     |subarea type |u|0-5 (see "Subarea Types")
|810-893 | 84 |Subarea payload #9   |shape data |array|See "Subarea Payloads"
|894-896 |  3 |Subarea shape #10    |subarea type |u|0-5 (see "Subarea Types")
|897-980 | 84 |Subarea payload #10  |shape data |array|See "Subarea Payloads"
|======

The sub-area indications are as described under the addressed form,
message type 6 with DAC = 1 and FID = 23.

==== IMO289 Extended Ship Static and Voyage Related Data

This message should be used by a ship to report the height over keel.

A message 8 subtype. DAC = 001 FID = 24 in <<IMO289>>.  Fixed length, 360 bits.
Replaces a deprecated trial message from <<IMO236>>.

//: Type 8(1/24)
[frame="topbot",options="header"]
|======
|Field   |Len |Description            |Member        |T|Units
|0-5     |  6 |Message Type           |type          |u|Constant: 8
|6-7     |  2 |Repeat Indicator       |repeat        |u|As in CNB
|8-37    | 30 |Source MMSI            |mmsi          |u|9 digits
|38-39   |  2 |Spare                  |              |x|Not used
|40-49   | 10 |DAC                    |dac           |u|DAC = 001
|50-55   |  6 |FID                    |fid           |u|FID = 24
|56-65   | 10 |Message Linkage ID     |linkage       |u|Unsigned integer
|66-78   | 13 |Air Draught            |airdraught    |u|Unsigned int, in 0.01m
                                                        1-81.9 m.
                                                        0 = N/A (default)
                                                        81.91 = >= 81.91 m
|79-108  | 30 |Last Port Of Call      |lastport      |t|5 6-bit chars, UN locode
|109-138 | 30 |Next Port Of Call      |nextport      |t|5 6-bit chars, UN locode
|139-168 | 30 |Second Port Of Call    |secondport    |t|5 6-bit chars, UN locode
|169-170 |  2 |AIS Class A            |ais_state     |e|See "SOLAS Status"
|171-172 |  2 |Automatic Tracking Aid |ata_state     |e|See "SOLAS Status"
|173-174 |  2 |BNWAS                  |bnwas_state   |e|See "SOLAS Status"
|175-176 |  2 |ECDIS Back-up          |ecdisb_state  |e|See "SOLAS Status"
|177-178 |  2 |Paper Nautical Chart   |chart_state   |e|See "SOLAS Status"
|179-180 |  2 |Echo sounder           |sounder_state |e|See "SOLAS Status"
|181-182 |  2 |Electronic plotting aid|epaid_state   |e|See "SOLAS Status"
|183-184 |  2 |Emergency steering gear|steer_state   |e|See "SOLAS Status"
|185-186 |  2 |GNSS                   |gnss_state    |e|See "SOLAS Status"
|187-188 |  2 |Gyro compass           |gyro_state    |e|See "SOLAS Status"
|189-190 |  2 |LRIT                   |lrit_state    |e|See "SOLAS Status"
|191-192 |  2 |Magnetic compass       |magcomp_state |e|See "SOLAS Status"
|193-194 |  2 |NAVTEX                 |navtex_state  |e|See "SOLAS Status"
|195-196 |  2 |Radar (ARPA)           |arpa_state    |e|See "SOLAS Status"
|197-198 |  2 |Radar (S-band)         |sband_state   |e|See "SOLAS Status"
|199-200 |  2 |Radar (X-band)         |xband_state   |e|See "SOLAS Status"
|201-202 |  2 |Radio HF               |hfradio_state |e|See "SOLAS Status"
|203-204 |  2 |Radio INMARSAT         |inmarsat_state|e|See "SOLAS Status"
|205-206 |  2 |Radio MF               |mfradio_state |e|See "SOLAS Status"
|207-208 |  2 |Radio VHF              |vhfradio_state|e|See "SOLAS Status"
|209-210 |  2 |Speed Log over ground  |grndlog_state |e|See "SOLAS Status"
|211-212 |  2 |Speed Log through water|waterlog_state|e|See "SOLAS Status"
|213-214 |  2 |THD                    |thd_state     |e|See "SOLAS Status"
|215-216 |  2 |Track control system   |tcs_state     |e|See "SOLAS Status"
|217-218 |  2 |VDR/S-VDR              |vdr_state     |e|See "SOLAS Status"
|219-220 |  2 |Reserved               |              |x|Not used
|221-224 |  4 |Ice Class              |iceclass      |e|See "Ice Class"
|225-242 | 18 |Shaft Horsepower       |horsepower    |u|Total ship HP: 1hp units,
                                                        262,142 = >= 262,142hp,
                                                        262,143 = N/A (default).
|243-254 | 12 |VHF Working Channel    |vhfchan       |u|Channel number,
                                                        0 = N/A (default).
|255-296 | 42 |Lloyd's Ship Type      |lshiptype     |t|7 six-bit characters
|297-314 | 18 |Gross Tonnage          |tonnage       |u|0-262,141,
                                                        262,142 = >= 262,142hp,
                                                        262,143 = N/A (default).
|315-316 |  2 |Laden or Ballast       |lading        |e|0 = N/A (default),
                                                        1 = Laden,
                                                        2 = Ballast,
                                                        3 = Not in use.
|317-318 |  2 |Heavy Fuel Oil Bunkered|heavyoil      |e|0 = N/A (default),
                                                        1 = No,
                                                        2 = Yes,
                                                        3 = Not in use.
|319-320 |  2 |Light Fuel Oil Bunkered|lightoil      |e|0 = N/A (default),
                                                        1 = No,
                                                        2 = Yes,
                                                        3 = Not in use.
|321-322 |  2 |Diesel Oil Bunkered    |dieseloil     |e|0 = N/A (default),
                                                        1 = No,
                                                        2 = Yes,
                                                        3 = Not in use.
|323-336 | 14 |Total Bunker Oil       |totaloil      |u|0-16381 in tonnes,
                                                        16382 = >= 16382 tonnes,
                                                        16382 = N/A (default).
|337-349 | 13 |Number of persons      |persons       |u|0 = N/A (default),
                                                        1-8190,
                                                        8191 = >= 8191.
|350-359 | 10 |Spare                  |              |x|Not used
|======

IMO289 says air draught is in 0.1m steps, but this is incompatible with the
two digits of precision in the range.

The special value of 81.91 for air draught in <<IMO289>>, also implies
the step size is really 0.01m.

The 2-bit _state fields describe the operational state of various sorts of
SOLAS-required navigational equipment.  GNSS systems may include GPS, Loran-C, or GLONASS. BNWAS is the Bridge Navigational Watch Alarm System.  THD is a
Transmitting Heading Device.  Paper Nautical Chart state is officially
"ECDIS/Paper Nautical Chart" state in <<IMO289>>.  Status codes should be
interpreted according to the following table:

.SOLAS Status
[width="50%",frame="topbot",options="header"]
|======
|Code |Meaning
|0    |Not available or requested (default)
|1    |Equipment operational
|2    |Equipment not operational
|3    |No data (equipment may or may not be on board/or its status is unknown)
|======

.Ice Class
[width="50%",frame="topbot",options="header"]
|======
|Code |Meaning
|0    |Not classified
|1    |IACS PC 1
|2    |IACS PC 2
|3    |IACS PC 3
|4    |IACS PC 4
|5    |IACS PC 5
|6    |IACS PC 6 / FSICR IA Super / RS Arc5
|7    |IACS PC 7 / FSICR IA / RS Arc4
|8    |FSICR IB / RS Ice3
|9    |FSICR IC / RS Ice2
|10   |RS Ice1
|11-14|Reserved for future use
|15   |Not available = default
|======

ACS = International Association of Classification Societies

PC = Polar Class. For further details, see IACS Req. 2007 Requirements
concerning POLAR CLASS and MSC/Circ.1056 and MEPC/Circ.399 on
Guidelines for ships operating in Arctic ice-covered waters.

FSICR = Finnish-Swedish Ice Class Rules. For further details, see
Finnish Maritime Administration's Bulletin No.10/10.12.2008 Ice class
regulations 2008 (Finnish-Swedish ice class rules). Note: Authorized
classification society equivalents for the Finnish-Swedish Ice Class
Rules should also be recognized, as issued in the Finnish Maritime
Administration's Bulletin No.4/2.4.2007 (as amended). Both bulletins
can be found at www.fma.fi.

RS = Russian Maritime Register of Shipping. For further details see
Rules for the classification and construction of seagoing ships,
Edition 2008.

VHF channel number is encoded according to Recommendation ITU-R M.1084.

The lshiptype field uses Lloyd's Register STATCODE 5 encoding.

==== IMO289 Environmental

A message 8 subtype. DAC = 001 FID = 26. Variable length: 168-1008 bits.

//: Type 8(1/26)
.Environmental message header
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member     |T|Units
|0-5     |  6 |Message Type         |type       |u|Constant: 6
|6-7     |  2 |Repeat Indicator     |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI          |mmsi       |u|9 decimal digits
|38-39   |  2 |Spare                |           |x|Not used
|40-49   | 10 |DAC                  |dac        |u|DAC = 001
|50-55   |  6 |FID                  |fid        |u|FID = 26
|56      |    |                     |reports    |a5|Sensor records array
|0-3     |  4 |Sensor Report Type   |sensor     |u|See table below
|4-8     |  5 |Day (UTC)            |day        |u|1-31; 0 = N/A (default)
|9-13    |  5 |Hour (UTC)           |hour       |u|0-23; 24 = N/A (default)
|14-19   |  6 |Minute (UTC)         |minute     |u|0-59; 60 = N/A (default)
|20-26   |  7 |Site ID              |site       |u|Binary ID of sensor site
|27-111  | 85 |Sensor payload       |payload    |d|Sensor payload data
|======

The fixed header is followed by 1-5 sensor records, each 112 bits
long.  The Sensor Report Type is interpreted as follows, and controls
the interpretation of the sensor payload data.

.Sensor report types
[frame="topbot"]
|======
| 0 | Site location
| 1 | Station ID
| 2 | Wind
| 3 | Water level
| 4 | Current flow (2D)
| 5 | Current flow (3D)
| 6 | Horizontal current flow
| 7 | Sea state
| 8 | Salinity
| 9 | Weather
| 10| Air gap/Air draft
| 11| (reserved for future use)
|======

Here are the payload types for each variant:

.Site location payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description      |Member   |T|Units
|0-27    | 28 |Longitude        |lon      |I4|As in Common Navigation Block
|28-54   | 27 |Latitude         |lat      |I4|As in Common Navigation Block
|55-65   | 11 |Altitude         |alt      |u|Sensor altitude above MSL,
                                             0-200 in units of 0.1m,
                                             2001 = 200.1 m or higher,
                                             2002 = N/A (default),
                                             2003-2046 reserved.
|66-69   |  4 |Sensor owner     |owner    |e|See "Sensor Owner Codes"
|70-72   |  3 |Data timeout     |timeout  |e|See "Data Timeout Codes"
|73-84   | 12 |Spare            |         |x|Not used
|======

.Sensor Owner Codes
[frame="topbot"]
|======
|   0  | Unknown (default)
|   1  | Hydrographic office
|   2  | Inland waterway authority
|   3  | Coastal directorate
|   4  | Meteorological service
|   5  | Port Authority
|   6  | Coast guard
| 7-13 | (reserved for future use)
|  14  | (reserved for regional use)
|======

.Data Timeout Codes
[frame="topbot"]
|======
|   0  | No time period (default)
|   1  | 10 minutes
|   2  | 1 hour
|   3  | 6 hours
|   4  | 12 hours
|   5  | 24 hours
|  6-7 | (reserved for future use)
|======

.Station ID payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member     |T|Units
|  0     | 84 |Name                 |name       |t|14 chars of six-bit ASCII.
|  84    |  1 |Spare                |           |x|Not used
|======

.Wind report payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member      |T|Units
|0-6     |  7 |Average Wind Speed |wspeed      |u|10-min avg wind speed,
                                                  0-120 in 1-knot units,
                                                  121 = 121 knots or greater,
                                                  122 = N/A (default),
                                                  123-126 = reserved.
|7-13   |  7 |Wind Gust          |wgust       |u|10-min max wind speed,
                                                  0-120 in 1-knot units,
                                                  121 = 121 knots or greater,
                                                  122 = N/A (default),
                                                  123-126 = reserved.
|14-22  |  9 |Wind Direction     |wdir        |u|0-359, degrees fom true north
                                                   >=360 = N/A (default)
|23-31  |  9 |Wind Gust Direction|wgustdir    |u|0-359, degrees fom true north
                                                   >=360 = N/A (default)
|32-34  |  3 |Sensor Description |sensortype  |e|See "Sensor Types"
|35-41  |  7 |Forecast Wind Speed|fwspeed     |u|Predicted average wind speed,
                                                  0-120 in 1-knot units,
                                                  121 = 121 knots or greater,
                                                  122 = N/A (default),
                                                  123-126 = reserved.
|42-48  |  7 |Forecast Wind Gust |fwgust      |u|Predicted max wind speed,
                                                  0-120 in 1-knot units,
                                                  121 = 121 knots or greater,
                                                  122 = N/A (default),
                                                  123-126 = reserved.
|49-57  |  9 |Forecast Wind Direction|fwdir   |u|0-359, degrees fom true north
                                                   >=360 = N/A (default)
|58-62  |  5 |Day (UTC)            |day       |u|1-31; 0 = N/A (default)
|63-67  |  5 |Hour (UTC)           |hour      |u|0-23; 24 = N/A (default)
|68-73  |  6 |Minute (UTC)         |minute    |u|0-59; 60 = N/A (default)
|74-81  |  8 |Duration             |duration  |u|Forecast duration in minutes,
                                                  255 = N/A (default),
                                                  0 = cancel forecast.
|82-84  |  3 |Spare                |          |x|Not used
|======

The timestamp group is intended as a valid time of forecast.

.Sensor Types
[frame="topbot"]
|======
|   0  | No data (default)
|   1  | Raw real time
|   2  | Real time with quality control
|   3  | Predicted (based on historical statistics)
|   4  | Forecast (predicted, refined with real-time information)
|   5  | Nowcast (a continuous forecast)
|   6  | (reserved for future use)
|   7  | Sensor not available
|======

.Water level report payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description              |Member     |T|Units
|0-0     | 1  |Water Level Type         |absolute   |b|False if relative to
                                                       reference datum.
|1-16    | 16 |Water Level              |level      |i|In 0.01 meter steps,
                                                      -327.67 to 327.67,
                                                      -32767 = -327.67m or less,
                                                      32767 = 327.67m or more,
                                                      -32768 = N/A (default).
|17-18   | 2  |Water Level Trend        |leveltrend |u| 0 = increasing,
                                                       1 = decreasing,
                                                       2 = steady,
                                                       3 = N/A (default).
|19-23   | 5  |Vertical Reference Datum |datum      |u|See table below
|24-26   | 3  |Sensor Description       |sensortype |e|See "Sensor Types"
|27-27   | 1  |Forecast Water Level Type|absolute   |b|False if relative to
                                                       reference datum.
|28-43   | 16 |Forecast Water Level     |level      |i|In 0.001 meter steps,
                                                      -327.67 to 327.67,
                                                      -32767 = -327.67m or less,
                                                      32767 = 327.67m or more,
                                                      -32768 = N/A (default).
|44-48  |  5 |Day (UTC)            |day             |u|1-31; 0 = N/A (default)
|49-53  |  5 |Hour (UTC)           |hour            |u|0-23; 24 = N/A (default)
|54-59  |  6 |Minute (UTC)         |minute          |u|0-59; 60 = N/A (default)
|60-67  |  8 |Duration             |duration        |u|Forecast duration in minutes,
                                                       255 = N/A (default),
                                                       0 = cancel forecast.
|68-84  | 17 |Spare                |                |x|Not used
|======

IMO289 says water level is in 0.1m steps, but this is incompatible with the
two digits of precision in the range.

The timestamp group is intended as a valid time of forecast.

.Vertical Reference Datum
[frame="topbot"]
|======
|   0  | Mean Lower Low Water (MLLW)
|   1  | International Great Lakes Datum (IGLD-85)
|   2  | Local river datum
|   3  | Station Datum (STND)
|   4  | Mean Higher High Water (MHHW)
|   5  | Mean High Water (MHW)
|   6  | Mean Sea Level (MSL)
|   7  | Mean Low Water (MLW)
|   8  | National Geodetic Vertical Datum (NGVD-29)
|   9  | North American Vertical Datum (NAVD-88)
|   10 | World Geodetic System (WGS-84)
|   11 | Lowest Astronomical Tide (LAT)
|   12 | pool
|   13 | gauge
|   14 | Unknown/not available (default)
| 15-30| Reserved for future use
|======

.Current flow (2D) report payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member    |T|Units
|0-7     |  8 |Current Speed #1     |cspeed1   |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|8-16    |  9 |Current Direction #1 |cdir1     |u|0-359: deg from true north,
                                                  >=360 = N/A (default).
|17-25   |  9 |Measurement Depth #1 |cdepth1   |u|0-360m down: units 1m,
                                                   361 = 361m or greater,
                                                   362 = N/A (default),
                                                   363-511 (reserved).
|26-33   |  8 |Current Speed #2     |cspeed2   |U1|0.0-24.5 knots,
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|34-42   |  9 |Current Direction #2 |cdir2     |u|0-359: deg. fom true north,
                                                  >=360 = N/A (default)
|43-51   |  9 |Measurement Depth #2 |cdepth2   |u|0-360m down: units 1m,
                                                  361 = 361m or greater,
                                                  362 = N/A (default),
                                                  363-511 (reserved).
|52-59   |  8 |Current Speed #3     |cspeed3   |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|60-68   |  9 |Current Direction #3 |cdir3     |u|0-359: degrees fom true north,
                                                  >=360 = N/A (default).
|69-77   |  9 |Measurement Depth #3 |cdepth3   |u|0-360m down: units 1m,
                                                  361 = 361m or greater,
                                                  362 = N/A (default),
                                                  363-511 (reserved).
|78-80   | 3  |Sensor Description   |sensortype|e|See "Sensor Types"
|81-84   | 4  |Spare                |          |x|Not used
|======

.Current flow (3D) payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description                          |Member  |T|Units
|0-7     |  8 |Current Vector component North (u) #1|cnorth1 |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|8-15    |  8 |Current Vector component East (v) #1|ceast1   |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|16-23   |  8 |Current Vector component Up (z) #1  |cup1     |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|24-32   |  9 |Measurement Depth #1                |cdepth1 |u|0-360m down:
                                                   units 1m,
                                                   361 = 361m or greater,
                                                   362 = N/A (default),
                                                   363-511 (reserved).
|33-40   |  8 |Current Vector component North (u) #2|cnorth2 |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|41-48   |  8 |Current Vector component East (v) #2|ceast2   |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|49-56   |  8 |Current Vector component Up (z) #2  |cup2     |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|57-65   |  9 |Measurement Depth #2                |cdepth2 |u|0-360m down:
                                                   units 1m,
                                                   361 = 361m or greater,
                                                   362 = N/A (default),
                                                   363-511 (reserved).
|66-68   | 3  |Sensor Description   |sensortype|e|See "Sensor Types"
|69-84   | 16 |Spare                |          |x|Not used
|======

.Horizontal current report payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member     |T|Units
|0-8     |  9 |Current Bearing #1   |bearing1   |u|0-359: deg from true north,
                                                  >=360 = N/A (default).
|9-15    |  7 |Current Distance #1  |distance1  |u|0-120m:
                                                  121 = 121m or greater,
                                                  122 = N/A (default),
                                                  123-127 (reserved).
|16-23   |  8 |Current Speed #1     |speed1    |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|24-32   |  9 |Current Direction #1 |direction1|u|0-359: deg from true north,
                                                  >=360 = N/A (default).
|33-41   |  9 |Measurement Depth #1 |depth1    |u|0-360m down: units 1m,
                                                   361 = 361m or greater,
                                                   362 = N/A (default),
                                                   363-511 (reserved).
|42-50   |  9 |Current Bearing #2   |bearing1   |u|0-359: deg from true north,
                                                  >=360 = N/A (default).
|51-57   |  7 |Current Distance #2  |distance1  |u|0-120m:
                                                  121 = 121m or greater,
                                                  122 = N/A (default),
                                                  123-127 (reserved).
|58-65   |  8 |Current Speed #2     |speed1    |U1|0.0-24.5 knots:
                                                   units 0.1 knots,
                                                   246 = speed >= 24.6 knots,
                                                   247 = N/A (default),
                                                   248-255 (reserved).
|66-74   |  9 |Current Direction #2 |direction1|u|0-359: deg from true north,
                                                  >=360 = N/A (default).
|75-83   |  9 |Measurement Depth #2 |depth1    |u|0-360m down: units 1m,
                                                   361 = 361m or greater,
                                                   362 = N/A (default),
                                                   363-511 (reserved).
|84-84   |  1 |Spare                |          |x|Not used
|======

.Sea state report payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description                |Member     |T|Units
|0-7     |  8 |Swell Height               |swheight   |U1|Units 0.1m: 0.0-24.5m,
                                                         246 = height >= 24,6m,
                                                         247 = N/A (default),
                                                         248-255 reserved,
                                                         else h = (value * 0.5).
|8-13    |  6 |Swell Period               |swperiod   |u|Period in seconds:
                                                         0-60,
                                                         61 = N/A (default),
                                                         62-63 (reserved).
|14-22   |  9 |Swell Direction            |swelldir   |u|0-359 deg:
                                                         0-359 true bearing,
                                                         360 = N/A (default),
                                                         361-511 reserved,
|23-26   |  4 |Sea State                  |seastate   |u|Beaufort scale: 0-12
                                                       >= 13 = N/A (default)
|27-29   |  3 |Swell Sensor Description   |swelltype  |e|See "Sensor Types"
|30-39   | 10 |Water Temperature          |watertemp  |U1|-10.0 to 50.0:
                                                      units 0.1 C,
                                                      >=601 = N/A (default).
                                                      602-1023 reserved,
                                                      else -10.0m after scaling.
|40-46   |  7 |Water Temperature Depth    |distance1  |U1|0.0-12.0m: 0.1m units,
                                                      121 = 12.1m or greater,
                                                      122 = N/A (default),
                                                      123-126 (reserved).
|47-49   |  3 |Depth Sensor Description   |depthtype  |e|See "Sensor Types"

|50-57   |  8 |Wave Height                |waveheight|U1|Height 0.0-24.5m:
                                                         units 0.1m,
                                                         246 = height >= 24.6m,
                                                         247 N/A (default),
                                                         248-255 reserved.
|58-63   |  6 |Wave Period                |waveperiod|u|0-60: units of seconds,
                                                        61 = N/A (default),
                                                        62-63 reserved.
|64-72   |  9 |Wave Direction             |wavedir   |u|0-359: true bearing,
                                                        360 = N/A (default),
                                                        361-511 reserved.
|73-75   |  3 |Wave Sensor Description    |wavetype  |e|See "Sensor Types"
|76-84   |  9 |Salinity                   |salinity  |U1|0.0-50.0%:
                                                         units of 0.1%
                                                         501 = salinity >= 50.1%
                                                         502 = data N/A (default)
                                                         503 - sensor N/A,
                                                         504-511 reserved.
|======

The standard does not fix the meaning of a water temperature depth of 127.

.Salinity report payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member       |T|Units
|0-9     | 10 |Water Temperature  |watertemp    |U1|-10.0 to 50.0:
                                                    units 0.1 C,
                                                    >=601 = N/A (default).
                                                    602-1023 reserved,
                                                    else -10.0m after scaling.
|10-19   | 10 |Conductivity       |conductivity |U1|In Siemens/m, 0.0-7.0,
                                                    0.1 S/m steps,
                                                    701 = not less than 7.01,
                                                    702 = data N/A,
                                                    703 = sensor N/A (default),
                                                    704-1023 (reserved).
|20-35   | 16 |Water Pressure     |pressure    |U1|Water pressure: 0.0-6000.0,
                                                   0.1 decibar steps,
                                                   60001 = pressure >= 6000.1,
                                                   60002 = data N/A,
                                                   60003 = sensor N/A (default),
                                                   60004-65536 reserved.
|36-44   |  9 |Salinity           |salinity    |U1|0.0-50.0%:
                                                   units of 0.1%
                                                   501 = salinity >= 50.1%
                                                   502 = data N/A (default)
                                                   503 - sensor N/A,
                                                   504-511 reserved.
|45-46   |  2 |Salinity Type      |salinitytype|e|0 = measured,
                                                  1 = calculated using PSS-78,
                                                  2 = calculated using other method,
                                                  3 = reserved.
|47-49   |  3 |Sensor Description |sensortype  |e|See "Sensor Types"
|50-84   | 35 |Spare              |            |x|Not used
|======

No default is specified for salinity type.

.Weather report payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description          |Member     |T|Units
|0-10    | 11 |Air Temperature      |temperature|i|Dry bulb temp: 0.1 deg C
                                                    -60.0 to +60.0,
                                                    -1024 = data N/A (default),
                                                    601-1023 reserved.
|11-13   |  3 |Temp. Sensor Type    |sensortype |e|See "Sensor Types"
|14-15   |  2 |Precipitation Type   |preciptype |e|0 = rain,
                                                     1 = rain and snow,
                                                     2 = rain and snow,
                                                     3 = other.
|16-23   |  8 |Horiz. Visibility    |visibility |U1|Units of 0.1 nautical miles,
                                                  0.0-24.0,
                                                  241 = visibility >= 24.1nm,
                                                  242 = data N/A,
                                                  243 = sensor N/A (default),
                                                  244-255 reserved.
|24-33   | 10 |Dew Point           |dewpoint    |i|-20.0 to +50.0: 0.1 deg C,
                                                  501 = N/A (default),
                                                  502-511 reserved,
                                                  -511--201 reserved.
|34-36   |  3 |Dewpoint Sensor Type|dewtype     |e|See "Sensor Types".



|37-45   |  9 |Air Pressure        |pressure    |u|0 = pressure <= 800hpA,
                                                   1-401 = 800-1200hPa,
                                                   402 = pressure >= 1201 hPa,
                                                   403 - data N/A (default),
                                                   404-511 reserved.
|46-47   |  2 |Pressure Tendency  |pressuretend |e|0 = steady,
                                                  1 = decreasing,
                                                  2 = increasing,
                                                  3 - N/A (default).
|48-50   |  3 |Pressure Sensor Type|pressuretype|e|See "Sensor Types"
|51-59   |  9 |Salinity            |salinity    |U1|0.0-50.0%:
                                                     units of 0.1%
                                                     501 = salinity >= 50.1%
                                                     502 = data N/A (default)
                                                     503 - sensor N/A,
                                                     504-511 reserved.
|60-84   | 25 |Spare                |           |x|Not used
|======

The standard does not specify how to code 'Precipitation Type' when
there is none.

.Air Gap/Air Draft report payload
[frame="topbot",options="header"]
|======
|Field   |Len |Description         |Member     |T|Units
|0-12    | 13 |Air Draught         |airdraught |U1|1-81.9m in 0.01m steps,
                                                   8191 = distance >= 81.91m,
                                                   0 = N/A (default).
|13-25   | 13 |Air Gap             |airgap     |U1|1-81.9m in 0.01m steps,
                                                   8191 = distance >= 81.91m,
                                                   0 = N/A (default).
|26-27   |  2 |Air Gap Trend       |gaptrend   |e|0 = steady,
                                                   1 = rising,
                                                   2 = falling,
                                                   3 = N/A (default).
|28-40   | 13 |Forecast Air Gap    |fairgap    |U1|1-81.9m in 0.01m steps,
                                                   8191 = distance >= 81.91m,
                                                   0 = N/A (default).
|41-45  |  5 |Day (UTC)            |day             |u|1-31; 0 = N/A (default)
|46-50  |  5 |Hour (UTC)           |hour            |u|0-23; 24 = N/A (default)
|51-56  |  6 |Minute (UTC)         |minute          |u|0-59; 60 = N/A (default)
|57-84  | 28 |Spare                |                |x|Not used
|======

Air draught is the vertical distance measured from the ship's
waterline to the highest point on the ship. Air gap is the vertical
distance measured from the surface of the water to the sensor.

IMO289 says steps are in 0.1m steps, but this is incompatible with the
two digits of precision in the range.

The timestamp is for the forecast air gap.

==== IMO289 Route Information (broadcast)

The content of this message is a time and a list of waypoints describing a
course.  It has an addressed equivalent that is a message 6 subtype.

A message 8 subtype. DAC = 001 FID = 27. Variable length: 172-997 bits.

//: Type 8(1/27)
[frame="topbot",options="header"]
|======
|Field   |Len |Description       |Member   |T|Units
|0-5     |  6 |Message Type      |type     |u|Constant: 8
|6-7     |  2 |Repeat Indicator  |repeat   |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI       |mmsi     |u|9 decimal digits
|38-39   |  2 |Spare             |         |x|Not used
|40-49   | 10 |DAC               |dac      |u|DAC = 001
|50-55   |  6 |FID               |fid      |u|FID = 27
|56-65   | 10 |Message Linkage ID|linkage  |u|Unsigned integer
|66-68   |  3 |Sender Class      |sender   |u|0 = ship (default), 1 = authority,
                                              27 = reserved for future use.
|69-73   |  5 |Route Type        |rtype    |e|See below
|74-77   |  4 |Start month       |month    |u|1-12, 0=N/A (default)
|78-82   |  5 |Start day         |day      |u|1-31, 0=N/A (default)
|83-87   |  5 |Start hour        |hour     |u|0-23, 24=N/A (default)
|88-93   |  6 |Start minute      |minute   |u|0-59, 60=N/A (default)
|94-111  | 18 |Duration          |duration |u|Minutes from start time,
                                              0 = cancel route,
                                              262,143 = not available (default).
|112-116 |  5 |Waypoint count    |waycount |u|1-16, values 17-31 are not used.
|117     |    |                  |waypoints|a^16|Waypoint array
|0-27    | 28 |Longitude         |lon      |I4|Unit = minutes * 0.0001,
                                              181000 = N/A (default),
                                              E positive, W negative.
|28-54   | 27 |Latitude          |lat      |I4|Unit = minutes * 0.001,
                                              91000 = N/A (default),
                                              N positive, S negative.
|======

The final pair of fields in the table above is a waypoint.  The message may
end with 1 to 16 waypoints.

For interpretation of the Route Type field, see the table under the "Route
Information (addressed)" message (DAC=1, FID=28).

==== IMO289 Text description (broadcast)

A message 8 subtype. DAC = 001 FID = 29. Variable length: 72-1032 bits.

Intended to be used to associate a text annotation with another message
via the Message Linkage ID field.

//: Type 8(1/29)
[frame="topbot",options="header"]
|======
|Field   |Len    |Description       |Member     |T|Units
|0-5     |  6    |Message Type      |type       |u|Constant: 8
|6-7     |  2    |Repeat Indicator  |repeat     |u|As in Common Navigation Block
|8-37    | 30    |Source MMSI       |mmsi       |u|9 decimal digits
|38-39   |  2    |Spare             |           |x|Not used
|40-49   | 10    |DAC               |dac        |u|DAC = 001
|50-55   |  6    |FID               |fid        |u|FID = 29
|56-65   | 10    |Message Linkage ID|linkage    |u|Unsigned integer
|66-?    | 6-966 |Description       |description|t|String
|======

There is an equivalent subtype of message 6 that is an addressed description.

==== Meteorological and Hydrological Data (IMO289)

A message 8 subtype. DAC = 001 FID = 31. Fixed length, 360 bits.
Supersedes an <<IMO236>> message with the same title but FID = 11 and
a different binary layout.  The exact differences are: (a) The
addition of the Position Accuracy field, (b) water level has 12 bits
of precision rather than 9 (units of centimeters rather than
decimeters), and (c) end padding changes from 6 to 10 bits.

//: Type 8(1/31)
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member      |T|Units
|0-5     |  6 |Message Type       |type        |u|Constant: 8
|6-7     |  2 |Repeat Indicator   |repeat      |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI        |mmsi        |u|9 decimal digits
|38-39   |  2 |Spare              |            |x|Not used
|40-49   | 10 |DAC                |dac         |u|DAC = 001
|50-55   |  6 |FID                |fid         |u|FID = 31
|56-80   | 25 |Longitude          |lon         |I3|Unit = minutes * 0.001,
                                                  E positive, We negative,
                                                  181000 = N/A (default).
|81-104  | 24 |Latitude           |lat         |I3|Unit = minutes * 0.001,
                                                  N positive, S negative,
                                                  91000 = N/A (default).
|105-105 |  1 |Fix quality        |accuracy    |b|As in Common Navigation Block
|106-110 |  5 |Day                |day         |u|1-31, 0=N/A (default)
|111-115 |  5 |Hour               |hour        |u|0-23, 24=N/A (default)
|116-121 |  6 |Minute             |minute      |u|0-59, 60=N/A (default)
|122-128 |  7 |Average Wind Speed |wspeed      |u|10-min avg wind speed: knots,
                                                  126 = wind >= 126 knots,
                                                  127 = N/A (default).
|129-135 |  7 |Gust Speed         |wgust       |u|10-min max wind speed: knots,
                                                  126 = wind >= 126 knots,
                                                  127 = N/A (default).
|136-144 |  9 |Wind Direction     |wdir        |u|0-359, true bearing,
                                                  360 = N/A (default).
|145-153 |  9 |Wind Gust Direction|wgustdir    |u|0-359, true bearing,
                                                  360 = N/A (default).
|154-164 | 11 |Air Temperature    |airtemp     |I1|Dry bulb temp: units 0.1C,
                                                -60.0 to +60.0,
                                                -1024 = N/A (default).
|165-171 |  7 |Relative Humidity  |humidity    |u|0-100%: units of 1%,
                                                  101 = N/A (default).
|172-181 | 10 |Dew Point          |dewpoint    |I1|-20.0 to +50.0: units 0.1C,
                                                  501 = N/A (default).
|182-190 |  9 |Air Pressure       |pressure    |u|800-1200hPa, 1hPa,
                                                  0 = pressure <= 799hPa,
                                                  402 = pressure >= 1201 hPa,
                                                  511 = N/A (default).
|191-192 |  2 |Pressure Tendency  |pressuretend|e|0 = steady,
                                                  1 = decreasing,
                                                  2 = increasing,
                                                  3 = N/A (default).
|193-193 |  7 |Max. visibility    |visgreater  |b|Visibility greater than.
|194-200 |  8 |Horiz. Visibility  |visibility  |U1|Units are 0.1 nautical miles,
                                                  127 = N/A (default).
|201-212 | 12 |Water Level        |waterlevel  |I2|-10.0 to +30.0 in 0.01m,
                                                  -10.0m after scaling,
                                                  4001 = N/A (default).
|213-214 |  2 |Water Level Trend  |leveltrend  |e|0 = steady,
                                                  1 = decreasing,
                                                  2 = increasing,
                                                  3 = N/A (default).
|215-222 |  8 |Surface Current Speed  |cspeed  |U1|0.0-25.0: units 0.1 knot,
                                                   251 = speed >= 25.1 knots,
                                                   255 = N/A (default).
|223-231 |  9 |Surface Current Direction |cdir |u|0-359: deg. from true north,
                                                       360 = N/A (default).
|232-239 |  8 |Current Speed #2     |cspeed2   |U1|0.0-25.0 knots:
                                                   units 0.1 knot,
                                                   251 = speed >= 25.1 knots,
                                                   255 = N/A (default).
|240-248 |  9 |Current Direction #2 |cdir2     |u|0-359: true bearing,
                                                 360 = N/A (default).
|249-253 |  5 |Measurement Depth #2 |cdepth2   |U1|0-30m down: units 0.1m,
                                                 31 = N/A (default).
|254-261 |  8 |Current Speed #3     |cspeed3   |U1|0.0-25.0 knots: units 0.1 knot,
                                                 251 = speed >= 25.1 knots,
                                                 255 = N/A (default).
|262-270 |  9 |Current Direction #3 |cdir3     |u|0-359: true bearing,
                                                 360 = N/A (default).
|271-275 |  5 |Measurement Depth #3 |cdepth3   |u|0-30m down: units 0.1m,
                                                 31 = N/A (default).
|276-283 |  8 |Wave Height          |waveheight|U1|Height 0-25m: units 0.1m,
                                                  251 = height >= 25.1m,
                                                  255 = N/A (default).
|284-289 |  6 |Wave Period          |waveperiod|u|Seconds, 0-60, units 1s,
                                                  63 = N/A (default).
|290-298 |  9 |Wave Direction       |wavedir   |u|0-359: true bearing,
                                                  360 = N/A (default).
|299-306 |  8 |Swell Height         |swellheight|U1|0-25m: units 0.1m,
                                                   251 = height >= 25.1m,
                                                   255 = N/A (default).
|307-312 |  6 |Swell Period         |swellperiod|u|0-60, units 1s
                                                   360 = N/A (default)
|313-321 |  9 |Swell Direction      |swelldir   |u|0-359: true bearing,
                                                   360 = N/A (default)
|322-325 |  4 |Sea State            |seastate   |e|See "Beaufort scale"
|326-335 | 10 |Water Temperature    |watertemp  |I1|-10.0 to 50.0 C:
                                                   units 0.1 deg,
                                                   501 = N/A (default)
|336-338 |  3 |Precipitation        |preciptype  |e|See "Precipitation type"
|339-347 |  9 |Salinity             |salinity   |U1|0.0-50.0%: units of 0.1%
                                                   501 = salinity >= 50.1%
                                                   510 = N/A (default)
                                                   511 = sensor not available
|348-349 |  2 |Ice                  |ice        |u| 0 = No
                                                    1 = Yes
                                                    2 = (reserved for future use)
                                                    3 = not available = default
|350-359 | 10 |Spare                |           |x|Not used
|======

Precipitation types and Beaufort scale are as for the <<IMO236>> version.

==== Inland ship static and voyage related data (Inland AIS)

A message 8 subtype. DAC = 200 FID = 10. Fixed length, 168 bits.

This message should be used by inland vessels only to broadcast ship
static and voyage related data in addition to message 5. The message
should be sent as soon as possible (from the AIS point of view) after
message 5.

//: Type 8(200/10)
[frame="topbot",options="header"]
|======
|Field   |Len |Description            |Member   |T|Units
|0-5     |  6 |Message Type           |type     |u|Constant: 8
|6-7     |  2 |Repeat Indicator       |repeat   |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI            |mmsi     |u|9 decimal digits
|38-39   |  2 |Spare                  |         |x|Not used
|40-49   | 10 |Designated Area Code   |dac      |u|Constant: 200
|50-55   |  6 |Functional ID          |fid      |u|Constant: 10
|56-103  | 48 |European Vessel ID     |vin      |t|8 six-bit characters
|104-116 | 13 |Length of ship         |length   |u|1-8000 * 0.1m, default 0
|117-126 | 10 |Beam of ship           |beam     |u|1-1000 * 0.1m, default 0
|127-140 | 14 |Ship/combination type  |shiptype |e|ERI Classification
|141-143 |  3 |Hazardous cargo        |hazard   |e|See "Hazard Codes" below
|144-154 | 11 |Draught                |draught  |u|1-200 * 0.01m, default 0
|155-156 |  2 |Loaded/Unloaded        |loaded   |e|See "Load Status" below
|157-157 |  1 |Speed inf. quality     |speed_q  |b|0 = low/GNSS (default)
                                                   1 = high
|158-158 |  1 |Course inf. quality    |course_q |b|0 = low/GNSS (default)
                                                   1 = high
|159-159 |  1 |Heading inf. quality   |heading_q|b|0 = low/GNSS (default)
                                                   1 = high
|160-167 |  8 |Spare                  |         |x|Not used
|======

OPEN-QUESTION: <<INLAND>> is not explicit whether the
"Ship/combination type" field is to contain full ERI codes with range
8000-8370 or ERI SOLAS codes in the range 1-99.  The tables below
expand both.  Full ERI codes have been observed in the wild.

.ERI Classification
[frame="topbot",options="header"]
|======
|Code | SOLAS | Description
|8000 |  99   | Vessel, type unknown
|8010 |  79   | Motor freighter
|8020 |  89   | Motor tanker
|8021 |  80   | Motor tanker, liquid cargo, type N
|8022 |  80   | Motor tanker, liquid cargo, type C
|8023 |  89   | Motor tanker, dry cargo as if liquid (e.g. cement)
|8030 |  79   | Container vessel
|8040 |  80   | Gas tanker
|8050 |  79   | Motor freighter, tug
|8060 |  89   | Motor tanker, tug
|8070 |  79   | Motor freighter with one or more ships alongside
|8080 |  89   | Motor freighter with tanker
|8090 |  79   | Motor freighter pushing one or more freighters
|8100 |  89   | Motor freighter pushing at least one tank-ship
|8110 |  79   | Tug, freighter
|8120 |  89   | Tug, tanker
|8130 |  31   | Tug freighter, coupled
|8140 |  31   | Tug, freighter/tanker, coupled
|8150 |  99   | Freightbarge
|8160 |  99   | Tankbarge
|8161 |  90   | Tankbarge, liquid cargo, type N
|8162 |  90   | Tankbarge, liquid cargo, type C
|8163 |  99   | Tankbarge, dry cargo as if liquid (e.g. cement)
|8170 |  99   | Freightbarge with containers
|8180 |  90   | Tankbarge, gas
|8210 |  79   | Pushtow, one cargo barge
|8220 |  79   | Pushtow, two cargo barges
|8230 |  79   | Pushtow, three cargo barges
|8240 |  79   | Pushtow, four cargo barges
|8250 |  79   | Pushtow, five cargo barges
|8260 |  79   | Pushtow, six cargo barges
|8270 |  79   | Pushtow, seven cargo barges
|8280 |  79   | Pushtow, eight cargo barges
|8290 |  79   | Pushtow, nine or more barges
|8310 |  80   | Pushtow, one tank/gas barge
|8320 |  80   | Pushtow, two barges at least one tanker or gas barge
|8330 |  80   | Pushtow, three barges at least one tanker or gas barge
|8340 |  80   | Pushtow, four barges at least one tanker or gas barge
|8350 |  80   | Pushtow, five barges at least one tanker or gas barge
|8360 |  80   | Pushtow, six barges at least one tanker or gas barge
|8370 |  80   | Pushtow, seven barges at least one tanker or gas barge
|======

.SOLAS ship type, first digit
[frame="topbot"]
|======
| 3 | Vessel
| 7 | Cargo ship
| 8 | Tanker
| 9 | Other types of ship
|======

.SOLAS ship type, second digit
[frame="topbot"]
|======
| 0 | All ships of this type
| 1 | Towing
| 8 | Tanker
| 9 | No additional information
|======

.Hazard code
[frame="topbot"]
|======
| 0 | 0 blue cones/lights
| 1 | 1 blue cone/light
| 2 | 2 blue cones/lights
| 3 | 3 blue cones/lights
| 4 | 4 B-Flag
| 5 | Unknown (default)
|======

.Load status
[frame="topbot"]
|======
| 0 | N/A (default)
| 1 | Unloaded
| 2 | Loaded
|======

==== EMMA Warning Report (Inland AIS)

A message 8 subtype. DAC = 200 FID = 23. Fixed length, 256 bits.

The EMMA warning is sent by base stations to drive shipboard ECDIS
displays of heavy weather conditions. The following message is capable
of transmitting the EMMA data using the AIS channel. It does not
replace the Notices to Skippers warnings.

//: Type 8(200/23)
[frame="topbot",options="header"]
|======
|Field   |Len |Description         |Member      |T|Units
|0-5     |  6 |Message Type        |type        |u|Constant: 8
|6-7     |  2 |Repeat Indicator    |repeat      |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI         |mmsi        |u|9 decimal digits
|38-39   |  2 |Spare               |            |x|Not used
|40-49   | 10 |Designated Area Code|dac         |u|Constant: 200
|50-55   |  6 |Functional ID       |fid         |u|Constant: 23
|56-63   |  8 |Start Year          |start_year  |u|1-55, year since 2000
                                                   0 = N/A (default)
|64-67   |  4 |Start Month         |start_month |u|1-12; 0 = N/A (default)
|68-72   |  5 |Start Day           |start_day   |u|1-31; 0 = N/A (default)
|73-80   |  8 |End Year            |end_year    |u|1-55, year since 2000
                                                  0 = N/A (default)
|81-84   |  4 |End Month           |end_month   |u|1-12; 0 = N/A (default)
|85-89   |  5 |End Day             |end_day     |u|1-31; 0 = N/A (default)
|90-94   |  5 |Start Hour          |start_hour  |u|0-23; 24 = N/A (default)
|95-100  |  6 |Start Minute        |start_minute|u|0-59; 60 = N/A (default)
|101-105 |  5 |End Hour            |end_hour    |u|0-23; 24 = N/A (default)
|106-111 |  6 |End Minute          |end_minute  |u|0-59; 60 = N/A (default)
|112-139 | 28 |Start Longitude     |start_lon   |I4|Minutes/10000 (as in CNB)
|140-166 | 27 |Start Latitude      |start_lat   |I4|Minutes/10000 (as in CNB)
|167-194 | 28 |End Longitude       |end_lon     |I4|Minutes/10000 (as in CNB)
|195-221 | 27 |End Latitude        |end_lat     |I4|Minutes/10000 (as in CNB)
|222-225 |  4 |Type                |type        |e|See "EMMA Type Codes" below
|226-234 |  9 |Min value           |min         |i|Signed Integer, see below
|235-243 |  9 |Max value           |max         |i|Signed Integer, see below
|244-245 |  2 |Classification      |intensity   |e|1 = Slight,
                                                   2 = Medium,
                                                   3 = Strong
|246-249 |  4 |Wind Direction      |wind        |e|See "EMMA Winds" below
|250-255 |  6 |Spare               |            |x|Not used
|======

OPEN-QUESTION: <<INLAND>> is not explicit about the interpretation of
the longitude and latitude fields; these semantics are assumed here
from 28 and 27-bit fields in other messages.

<<INLAND>> specifies an 8-bit format with 9th leading sign bit for the
min and max fields. Values 0-253 are interpreted as integer data,
modified by the leading sign bit. The values +254 and -254 are
interpreted as "greater that +253" and "less than -253" respectively.
Both values +255 and -255 are interpreted as "unknown" (default).

OPEN-QUESTION: What are the semantics of the min and max values?  To
what parameters do they apply?

OPEN-QUESTION: <<INLAND>> does not specify whether start and end times
are UTC or local.

.EMMA Type Codes
[frame="topbot"]
|======
|  0 | NA | Not Available
|  1 | WI | Wind
|  2 | RA | Rain
|  3 | SN | Snow and ice
|  4 | TH | Thunderstorm
|  5 | FO | Fog
|  6 | LT | Low temperature
|  7 | HT | High temperature
|  8 | FL | Flood
|  9 | FI | Forest Fire
|======

.EMMA Winds
[frame="topbot"]
|======
|  1 | N  | North
|  2 | NE | North East
|  3 | E  | East
|  4 | SE | South East
|  5 | S  | South
|  6 | SW | South West
|  7 | W  | West
|  8 | NW | North West
|======

==== Water Levels (Inland AIS)

A message 6 subtype. DAC = 200 FID = 24. Fixed length, 168 bits.

This message should be used to inform skippers about actual water
levels in their area.  It is additional short-term information to the
water levels distributed via Notices to Skippers. It is possible to
transmit the water levels of more than 4 gauges using multiple
messages.

//: Type 8(200/24)
[frame="topbot",options="header"]
|======
|Field   |Len |Description         |Member     |T|Units
|0-5     |  6 |Message Type        |type       |u|Constant: 6
|6-7     |  2 |Repeat Indicator    |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI         |mmsi       |u|9 decimal digits
|38-39   |  2 |Spare               |           |x|Not used
|40-49   | 10 |Designated Area Code|dac        |u|Constant: 200
|50-55   |  6 |Functional ID       |fid        |u|Constant: 24
|56-67   | 12 |UN Country Code     |country    |t|2 six-bit characters
|68      |    |                    |gauges     |a4|Gauge measurement array
|0-10    | 11 |Gauge ID            |id         |u|0=unknown (default)
|11-24   | 14 |Water Level         |level      |i|cm, 0=unknown (default)
|======

Water levels are relative to the local standard, e.g. GIW in Germany
and RNW on the Danube.

==== Signal Strength (Inland AIS)

A message 8 subtype. DAC = 200 FID = 40. Fixed length, 168 bits.

This message should be sent by base stations only, to inform about the
status of different light signals to all vessels in a certain
area. The information should be displayed on an external Inland ECDIS
display as dynamic symbols. The message should be sent with binary
message 8 at regular intervals.

//: Type 8(200/40)
[frame="topbot",options="header"]
|======
|Field   |Len |Description         |Member     |T|Units
|0-5     |  6 |Message Type        |type       |u|Constant: 8
|6-7     |  2 |Repeat Indicator    |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI         |mmsi       |u|9 decimal digits
|38-39   |  2 |Spare               |           |x|Not used
|40-49   | 10 |Designated Area Code|dac        |u|Constant: 200
|50-55   |  6 |Functional ID       |fid        |u|Constant: 40
|56-83   | 28 |Signal Longitude    |lon        |I4|Minutes/10000 (as in CNB)
|84-110  | 27 |Signal Latitude     |lat        |I4|Minutes/10000 (as in CNB)
|111-114 |  4 |Signal form         |form       |u|Consult <<INLAND>> Annex C
|115-123 |  9 |Signal orientation  |facing     |u|0-39 deg, 511= N/A (default)
|124-126 |  3 |Direction of impact |direction  |e|See "Signal Impact" below
|127-156 | 30 |Light Status        |status     |e|See "Signal Status" below
|157-167 | 11 |Spare               |           |x|Not used
|======

OPEN-QUESTION: <<INLAND>> is not explicit about the interpretation of
the longitude and latitude fields; these semantics are assumed here
from 28 and 27-bit fields in other messages.

The Signal Form field describes the physical arrangement of the
signal lights.  Values 0 and 15 indicate the shape is unknown or
unspecified.

//: direction vocabulary
.Direction of Signal Impact
[frame="topbot"]
|======
| 0 | Unknown (default)
| 1 | Upstream
| 2 | Downstream
| 3 | To left bank
| 4 | To right bank
|======

The Signal Status field is interpreted as 9 decimal digits describing
the lights as numbered in their Signal Form diagram  - typically left
to right and then top to bottom - with each digit interpreted in the
following way:

//: status vocabulary
.Signal Status
[frame="topbot"]
|======
| 0 | Unknown (default)
| 1 | No light
| 2 | White
| 3 | Yellow
| 4 | Green
| 5 | Red
| 6 | White flashing
| 7 | Yellow flashing.
|======

=== Type 9: Standard SAR Aircraft Position Report

Tracking information for search-and-rescue aircraft.  Total number of
bits is 168.

//: Type 9
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member     |T|Encoding
|0-5     | 6  |Message Type       |type       |u|Constant: 9
|6-7     | 2  |Repeat Indicator   |repeat     |u|As in Common Navigation Block
|8-37    |30  |MMSI               |mmsi       |u|9 decimal digits
|38-49   |12  |Altitude           |alt        |u|See below
|50-59   |10  |SOG                |speed      |u|See below
|60-60   | 1  |Position Accuracy  |accuracy   |u|See below
|61-88   |28  |Longitude          |lon        |I4|Minutes/10000 (as in CNB)
|89-115  |27  |Latitude           |lat        |I4|Minutes/10000 (as in CNB)
|116-127 |12  |Course Over Ground |course     |U1|True bearing, 0.1 degree units
|128-133 | 6  |Time Stamp         |second     |u|UTC second.
|134-141 | 8  |Regional reserved  |regional   |x|Reserved
|142-142 | 1  |DTE                |dte        |b|0=Data terminal ready,
                                                 1=Data terminal not ready (default)
|143-145 | 3  |Spare              |           |x|Not used
|146-146 | 1  |Assigned           |assigned   |b|Assigned-mode flag
|147-147 | 1  |RAIM flag          |raim       |b|As for common navigation block
|148-167 |20  |Radio status       |radio      |u|See <<IALA>> for details.
|======

Altitude is in meters.  The special value 4095 indicates altitude is
not available; 4094 indicates 4094 meters or higher.

Speed over ground is in knots, not deciknots as in the common
navigation block; planes go faster. The special value 1023 indicates
speed not available, 1022 indicates 1022 knots or higher.

Position Accuracy, Longitude, Latitude, and Course over Ground
are encoded identically as in the common navigation block and
are even at the same bit offsets.  Time stamp has the same special
values as in the common navigation block, but is at a different offset.

=== Type 10: UTC/Date Inquiry

Request for UTC/Date information from an AIS base station. Total
number of bits is 72.

//: Type 10
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member     |T|Encoding
|0-5     | 6  |Message Type       |type       |u|Constant: 10
|6-7     | 2  |Repeat Indicator   |repeat     |u|As in Common Navigation Block
|8-37    |30  |Source MMSI        |mmsi       |u|9 decimal digits
|38-39   | 2  |Spare              |           |x|Not used
|40-69   |30  |Destination MMSI   |dest_mmsi  |u|9 decimal digits
|70-71   | 2  |Spare              |           |x|Not used
|======

=== Type 11: UTC/Date Response

Identical to message 4, with the semantics of a response to inquiry.

=== Type 12: Addressed Safety-Related Message

This is a point-to-point text message.  The payload is interpreted as
six-bit text.  This message is variable in length up to a maximum of
1008 bits (up to 5 AIVDM sentence payloads).

//: Type 12
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member      |T|Units
|0-5     |  6 |Message Type       |type        |u|Constant: 12
|6-7     |  2 |Repeat Indicator   |repeat      |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI        |mmsi        |u|9 decimal digits
|38-39   |  2 |Sequence Number    |seqno       |u|Unsigned integer 0-3
|40-69   | 30 |Destination MMSI   |dest_mmsi   |u|9 decimal digits
|70      |  1 |Retransmit flag    |retransmit  |b|0 = no retransmit (default),
                                                  1 = retransmitted
|71      |  1 |Spare              |            |x|Not used
|72      |936 |Text               |text        |t|1-156 chars of six-bit text.
                                                  May be shorter than 936 bits.
|======

Pragmatic note: On <<AISHUB>> the actual content of these messages is
highly variable, ranging from fairly plain English ("PLEASE REPORT TO
JOBOURG TRAFFIC CHANNEL 13") through snippets of tabular data ("PAX
589 FG 36 IX 74 MOTO 10 CREW 108+1" through what look like opaque
commercial codes ("EP285 IX46 FG3 DK8 PL56") to empty strings and
content that looks like line noise ("]XFD5D/\7`>PA!Q DX0??K?8?>D").

Such apparently garbled content does *not* mean there is an error in
your decoder. It may indicate faulty encoders, operator error, or even
the use of private encodings for non-ASCII character sets.

=== Type 13: Safety-Related Acknowledgment

Message type 13 is a receipt acknowledgement to senders of
previous messages of type 12.  The message layout is identical to
a type 7 Binary Acknowledge.

=== Type 14: Safety-Related Broadcast Message

This is a broadcast text message.  The payload is interpreted as
six-bit text.  This message is variable in length up to a maximum of
1008 bits (up to 5 AIVDM sentence payloads).

//: Type 14
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member    |T|Units
|0-5     |  6 |Message Type       |type      |u|Constant: 14
|6-7     |  2 |Repeat Indicator   |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI        |mmsi      |u|9 decimal digits
|38-39   |  2 |Spare              |          |x|Not used
|40      |968 |Text               |text      |t|1-161 chars of six-bit text.
                                                May be shorter than 968 bits.
|======

Note: 161 * 6 = 966. <<IALA>> specifies 968 because over-the-air
messages are required to be padded to an 8-bit byte boundary by
<<ITU1371>>.

Also see the pragmatic note on message content attached to type 12; it
applies to type 14 messages as well.

=== Type 15: Interrogation

Message type 15 is used by a base station to query one or two other AIS
transceivers for status messages of specified types. "Source MMSI" is
the interrogating station.  88-160 bits depending on the number of queries.

This message is probably not interesting unless you are doing traffic
analysis of information flow in an AIS station network.  The "slot offset"
members are a request for the response to interrogation to occupy
a particular time division in the TDMA packet layer.

//: Type 15
[frame="topbot",options="header"]
|======
|Field   |Len |Description         |Member    |T|Units
|0-5     |  6 |Message Type        |type      |u|Constant: 15
|6-7     |  2 |Repeat Indicator    |repeat    |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI         |mmsi      |u|9 decimal digits
|38-39   |  2 |Spare               |          |x|Not used
|40-69   | 30 |Interrogated MMSI   |mmsi1     |u|9 decimal digits
|70-75   |  6 |First message type  |type1_1   |u|Unsigned integer
|76-87   | 12 |First slot offset   |offset1_1 |u|Unsigned integer
|88-89   |  2 |Spare               |          |x|Not used
|90-95   |  6 |Second message type |type1_2   |u|Unsigned integer
|96-107  | 12 |Second slot offset  |offset1_2 |u|Unsigned integer
|108-109 |  2 |Spare               |          |x|Not used
|110-139 | 30 |Interrogated MMSI   |mmsi2     |u|9 decimal digits
|140-145 |  6 |First message type  |type2_1   |u|Unsigned integer
|146-157 | 12 |First slot offset   |offset2_1 |u|Unsigned integer
|158-159 |  2 |Spare               |          |x|Not used
|======

There are four use cases for this message.  A decoder must dispatch on
the length of the data packet to determine which it is seeing:

1. One station is interrogated for one message type.  Length is 88 bits.

2. One station is interrogated for two message types, Length is 110
bits. There is a design error in the standard here; according to the
<<ITU1371>> requirement for padding to 8 bits, this should have been
112 with a 4-bit trailing spare field, and decoders should be prepared
to handle that length as well. See the discussion of byte alignment
elsewhere in this document for context.

3. Two stations are interrogated for one message type each. Length is
160 bits.  The second message type and second slot offset associated
with the first queried MMSI should be zeroed.

4. One station is interrogated for two message types, and a second for
one message type.  Length is 160 bits.

=== Type 16: Assignment Mode Command

Message type 16 is used by a base station with control authority to
configure the scheduling of AIS informational messages from
subordinate stations, either as a frequency per 10-minute interval or
by specifying the TDMA slot(s) offset on which those messages should
be transmitted.  It is probably not of interest unless you are
studying the internal operation of an AIS base station network.
Length may be 96 or 144 bits.

//: Type 16
[frame="topbot",options="header"]
|======
|Field   |Len |Description         |Member     |T|Units
|0-5     |  6 |Message Type        |type       |u|Constant: 16
|6-7     |  2 |Repeat Indicator    |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI         |mmsi       |u|9 decimal digits
|38-39   |  2 |Spare               |           |x|Not used
|40-69   | 30 |Destination A MMSI  |mmsi1      |u|9 decimal digits
|70-81   | 12 |Offset A            |offset1    |u|See <<IALA>>
|82-91   | 10 |Increment A         |increment1 |u|See <<IALA>>
|92-121  | 30 |Destination B MMSI  |mmsi2      |u|9 decimal digits
|122-133 | 12 |Offset B            |offset2    |u|See <<IALA>>
|134-143 | 10 |Increment B         |increment2 |u|See <<IALA>>
|======

If the message is 96 bits long, it should be interpreted as an
assignment for a single station (92 bits) followed by 4 bits of
padding reserved for future use. If the message is 144 bits long it
should be interpreted as a channel assignment for two stations; no
padding follows.

When increment is zero, the offset field is interpreted as the
frequency with which the subordinate station should report per
10-minute interval. When increment is nonzero, reporting interval
is specified at the level of TDMA slot numbers; see <<IALA>> for the
detailed specification.

Note: While the 96-bit form of Type 16 is not uncommon, the 144-bit
form is extremely rare. As of March 2010 it has not been observed even
in long-duration samples from AISHub.

=== Type 17: DGNSS Broadcast Binary Message ===

Message type 17 is used to broadcast differential corrections for GPS.
The data in the payload is intended to be passed directly to GPS
receivers capable of accepting such corrections. 80 to 816 bits
depending on payload size.

//: Type 17
[frame="topbot",options="header"]
|======
|Field   |Len |Description         |Member     |T|Units
|0-5     |  6 |Message Type        |type       |u|Constant: 17
|6-7     |  2 |Repeat Indicator    |repeat     |u|As in Common Navigation Block
|8-37    | 30 |Source MMSI         |mmsi       |u|9 decimal digits
|38-39   |  2 |Spare               |           |x|Not used
|40-57   | 18 |Longitude           |lon        |I1|Signed: minutes/10
|58-74   | 17 |Latitude            |lat        |I1|Signed: minutes/10
|75-79   |  5 |Spare               |           |x|Not used - reserved
|80-815  |736 |Payload             |data       |d|DGNSS correction data
|======

Note that latitude and longitude are in units of a tenth of a minute;
sign interpretation and out-of-band values are as in the Common
Navigation Clock.  (Note, however, that the hex representation of
the out-of-band values differs; it is 181 \* 60 \* 10 = 0x1a838 for
longitude, 91 \* 60 \* 10 = 0xd548 for latitude.)

The <<IALA>> description of the payload portion sub-fields has been
omitted, as it appears to be tied to the now obsolete RTCM2 protocol.

=== Type 18: Standard Class B CS Position Report

A less detailed report than types 1-3 for vessels using Class B
transmitters.  Omits navigational status and rate of turn.  Fields are
encoded as in the common navigation block.  168 bits total.

In <<IALA>> (and <<ITU1371>>) bits 141-145 were designated "Spare"; the
bit-flag semantics given here are from ITU-1371-3 and were
communicated by Kurt Schwehr.  Kurt warns that "the spec does not do a
good job of explaining these fields... I don't think that I totally
understand these fields."

//: Type 18
[frame="topbot",options="header"]
|======
|Field   |Len |Description        |Member   |T|Units
|0-5     | 6  |Message Type       |type     |u|Constant: 18
|6-7     | 2  |Repeat Indicator   |repeat   |u|As in Common Navigation Block
|8-37    |30  |MMSI               |mmsi     |u|9 decimal digits
|38-45   | 8  |Regional Reserved  |reserved |x|Not used
|46-55   |10  |Speed Over Ground  |speed    |U1|As in common navigation block
|56-56   | 1  |Position Accuracy  |accuracy |b|See below
|57-84   |28  |Longitude          |lon      |I4|Minutes/10000 (as in CNB)
|85-111  |27  |Latitude           |lat      |I4|Minutes/10000 (as in CNB)
|112-123 |12  |Course Over Ground |course   |U1|0.1 degrees from true north
|124-132 | 9  |True Heading       |heading  |u|0 to 359 degrees, 511 = N/A
|133-138 | 6  |Time Stamp         |second   |u|Second of UTC timestamp.
|139-140 | 2  |Regional reserved  |regional |u|Uninterpreted
|141-141 | 1  |CS Unit            |cs       |b|0=Class B SOTDMA unit
                                               1=Class B CS (Carrier Sense) unit
|142-142 | 1  |Display flag       |display  |b|0=No visual display,
                                               1=Has display,
                                               (Probably not reliable).
|143-143 | 1  |DSC Flag           |dsc      |b|If 1, unit is attached to a VHF
                                               voice radio with DSC capability.
|144-144 | 1  |Band flag          |band     |b|Base stations can command units
                                               to switch frequency. If this flag
                                               is 1, the unit can use any part
                                               of the marine channel.
|145-145 | 1  |Message 22 flag    |msg22    |b|If 1, unit can accept a channel
                                               assignment via Message Type 22.
|146-146 | 1  |Assigned           |assigned |b|Assigned-mode flag:
                                               0 = autonomous mode (default),
                                               1 = assigned mode.
|147-147 | 1  |RAIM flag          |raim     |b|As for common navigation block
|148-167 |20  |Radio status       |radio    |u|See <<IALA>> for details.
|======

The radio status is 20 bits rather than 19 because an extra first bit
selects whether it should be interpreted as a SOTDMA or ITDMA state.

=== Type 19: Extended Class B CS Position Report

A slightly more detailed report than type 18 for vessels using Class B
transmitters.  Omits navigational status and rate of turn.  Fields are
encoded as in the common navigation block and the Type 5 message.
Note that until just before the reserved field at bit 139 this is
identical to message 18.  312 bits total.

In practice, the information in the ship name and dimension fields is
not reliable, as it has to be hand-entered by humans rather than
gathered automatically from sensors.

//: Type 19
[frame="topbot",options="header"]
|======
|Field    |Len |Description            |Member       |T|Units
|0-5      |  6 |Message Type           |type         |u|Constant: 19
|6-7      |  2 |Repeat Indicator       |repeat       |u|As in CNN
|8-37     | 30 |MMSI                   |mmsi         |u|9 digits
|38-45    |  8 |Regional Reserved      |reserved     |u|
|46-55    | 10 |Speed Over Ground      |speed        |U1|As in CNB.
|56-56    |  1 |Position Accuracy      |accuracy     |b|As in CNB.
|57-84    | 28 |Longitude              |lon          |I4|Minutes/10000 (as in CNB)
|85-111   | 27 |Latitude               |lat          |I4|Minutes/10000 (as in CNB)
|112-123  | 12 |Course Over Ground     |course       |U1|Relative to true north,
                                                        units of 0.1 degrees
|124-132  |  9 |True Heading           |heading      |u|0 to 359 degrees,
                                                        511 = N/A
|133-138  |  6 |Time Stamp             |second       |u|Second of UTC timestamp.
|139-142  |  4 |Regional reserved      |regional     |u|Uninterpreted
|143-262  |120 |Name                   |shipname     |s|20 6-bit characters
|263-270  |  8 |Type of ship and cargo |shiptype     |e|As in Message 5
|271-279  |  9 |Dimension to Bow       |to_bow       |u|Meters
|280-288  |  9 |Dimension to Stern     |to_stern     |u|Meters
|289-294  |  6 |Dimension to Port      |to_port      |u|Meters
|295-300  |  6 |Dimension to Starboard |to_starboard |u|Meters
|301-304  |  4 |Position Fix Type      |epfd         |e|See "EPFD Fix Types"
|305-305  |  1 |RAIM flag              |raim         |b|As in CNB.
|306-306  |  1 |DTE                    |dte          |b|0=Data terminal ready,
                                                       1=Not ready (default).
|307-307  |  1 |Assigned mode flag     |assigned     |u|See <<IALA>> for details
|308-311  |  4 |Spare                  |             |x|Unused, should be zero
|======

=== Type 20 Data Link Management Message

This message is used to pre-allocate TDMA slots within an AIS base
station network.  It contains no navigational information, and is
unlikely to be of interest unless you are implementing or studying
an AIS base station network. Length varies from 72-160 depending on
the number of slot reservations (1 to 4) in the message.

//: Type 20
[frame="topbot",options="header"]
|======
|Field    |Len |Description       |Member      |T|Units
|0-5      | 6  |Message Type      |type        |u|Constant: 20
|6-7      | 2  |Repeat Indicator  |repeat      |u|As in CNB
|8-37     |30  |MMSI              |mmsi        |u|9 decimal digits
|38-39    | 2  |Spare             |            |x|Not used
|40-51    |12  |Offset number 1   |offset1     |u|Reserved offset number
|52-55    | 4  |Reserved slots    |number1     |u|Consecutive slots
|56-58    | 3  |Time-out          |timeout1    |u|Allocation timeout in minutes
|59-69    |11  |Increment         |increment1  |u|Repeat increment
|70-81    |12  |Offset number 2   |offset2     |u|Reserved offset number
|82-85    | 4  |Reserved slots    |number2     |u|Consecutive slots
|86-88    | 3  |Time-out          |timeout2    |u|Allocation timeout in minutes
|89-99    |11  |Increment         |increment2  |u|Repeat increment
|100-111  |12  |Offset number 3   |offset3     |u|Reserved offset number
|112-115  | 4  |Reserved slots    |number3     |u|Consecutive slots
|116-118  | 3  |Time-out          |timeout3    |u|Allocation timeout in minutes
|119-129  |11  |Increment         |increment3  |u|Repeat increment
|130-141  |12  |Offset number 4   |offset4     |u|Reserved offset number
|142-145  | 4  |Reserved slots    |number4     |u|Consecutive slots
|146-148  | 3  |Time-out          |timeout4    |u|Allocation timeout in minutes
|149-159  |11  |Increment         |increment4  |u|Repeat increment
|======

See <<IALA>> for details on the meaning of these fields.

=== Type 21: Aid-to-Navigation Report

Identification and location message to be emitted by aids to
navigation such as buoys and lighthouses.

This message is unusual in that it varies in length depending on the
presence and size of the Name Extension field. May vary between 272
and 360 bits.

//: Type 21
[frame="topbot",options="header"]
|======
|Field    |Len |Description            |Member      |T|Units
|0-5      | 6  |Message Type           |type        |u|Constant: 21
|6-7      | 2  |Repeat Indicator       |repeat      |u|As in CNB
|8-37     |30  |MMSI                   |mmsi        |u|9 digits
|38-42    | 5  |Aid type               |aid_type    |e|See "Navaid Types"
|43-162  1|120 |Name                   |name        |t|Name in sixbit chars
|163-163  | 1  |Position Accuracy      |accuracy    |b|As in CNB
|164-191  |28  |Longitude              |lon         |I4|Minutes/10000 (as in CNB)
|192-218  |27  |Latitude               |lat         |I4|Minutes/10000 (as in CNB)
|219-227  | 9  |Dimension to Bow       |to_bow      |u|Meters
|228-236  | 9  |Dimension to Stern     |to_stern    |u|Meters
|237-242  | 6  |Dimension to Port      |to_port     |u|Meters
|243-248  | 6  |Dimension to Starboard |to_starboard|u|Meters
|249-252  | 4  |Type of EPFD           |epfd        |e|As in Message Type 4
|253-258  | 6  |UTC second             |second      |u|As in Message Types 1-3
|259-259  | 1  |Off-Position Indicator |off_position|b|See Below
|260-267  | 8  |Regional reserved      |regional    |u|Uninterpreted
|268-268  | 1  |RAIM flag              |raim        |b|As in CNB
|269-269  | 1  |Virtual-aid flag       |virtual_aid |b|See Below
|270-270  | 1  |Assigned-mode flag     |assigned    |b|See <<IALA>> for details
|271-271  | 1  |Spare                  |            |x|Not used
|272-360  |88  |Name Extension         |            |t|See Below
|======

According to <<IALA>>, the aid type field has values 1-15 for fixed
and 16-31 for floating aids to navigation.  The detailed list is as follows:

.Navaid Types
[frame="topbot",options="header"]
|======
|Code |Definition
|0    |Default, Type of Aid to Navigation not specified
|1    |Reference point
|2    |RACON (radar transponder marking a navigation hazard)
|3    |Fixed structure off shore, such as oil platforms, wind farms,
       rigs. (Note: This code should identify an obstruction that is
       fitted with an Aid-to-Navigation AIS station.)
|4    |Spare, Reserved for future use.
|5    |Light, without sectors
|6    |Light, with sectors
|7    |Leading Light Front
|8    |Leading Light Rear
|9    |Beacon, Cardinal N
|10   |Beacon, Cardinal E
|11   |Beacon, Cardinal S
|12   |Beacon, Cardinal W
|13   |Beacon, Port hand
|14   |Beacon, Starboard hand
|15   |Beacon, Preferred Channel port hand
|16   |Beacon, Preferred Channel starboard hand
|17   |Beacon, Isolated danger
|18   |Beacon, Safe water
|19   |Beacon, Special mark
|20   |Cardinal Mark N
|21   |Cardinal Mark E
|22   |Cardinal Mark S
|23   |Cardinal Mark W
|24   |Port hand Mark
|25   |Starboard hand Mark
|26   |Preferred Channel Port hand
|27   |Preferred Channel Starboard hand
|28   |Isolated danger
|29   |Safe Water
|30   |Special Mark
|31   |Light Vessel / LANBY / Rigs
|======

The name field is up to 20 characters of 6-bit ASCII.  If this field
is full (has no trailing @ characters) the decoder should interpret
the Name Extension field later in the message (no more than 14 6-bit
characters) and concatenate it to this one to obtain the full name.

<<IALA>> describes bits 219-248 As "Dimension/Reference for Position",
implying that it is vessel dimensions as in message type 5.

The Off-Position Indicator is for floating Aids-to-Navigation only: 0
means on position; 1 means off position. Only valid if UTC second is equal to
or below 59.

The Virtual Aid flag is interpreted as follows: 0 = default = real Aid
to Navigation at indicated position; 1 = virtual Aid to
Navigation simulated by nearby AIS station.

If present, the Name Extension consists of packed six-bit ASCII
characters followed by 0-6 bits of padding to an 8-bit boundary. The
<<IALA>> description says "This parameter should be omitted when no
more than 20 characters for the name of the A-to-N are needed in
total. Only the required number of characters should be transmitted,
i.e. no @-character should be used."  A decoder can deduce the bit length
of the name extension field by subtracting 272 from the total message
bit length.

=== Type 22: Channel Management

This message is broadcast by a competent authority (an AIS network
control base station) to set VHF parameters for an AIS coverage
region.  Length is 168 bits.

This message contains no navigational information, and is unlikely to be of
interest unless you are implementing or studying an AIS base station
network.

//: Type 22
[frame="topbot",options="header"]
|======
|Field    |Len |Description      |Member    |T|Units
|0-5      | 6  |Message Type     |type      |u|Constant: 22
|6-7      | 2  |Repeat Indicator |repeat    |u|As in Common Navigation Block
|8-37     |30  |MMSI             |mmsi      |u|9 decimal digits
|38-39    | 2  |Spare            |          |x|Not used
|40-51    |12  |Channel A        |channel_a |u|Channel number
|52-63    |12  |Channel B        |channel_b |u|Channel number
|64-67    | 4  |Tx/Rx mode       |txrx      |u|Transmit/receive mode
|68-68    | 1  |Power            |power     |b|Low=0, high=1
|69-86    |18  |NE Longitude     |ne_lon    |I1|NE longitude to 0.1 minutes
|87-103   |17  |NE Latitude      |ne_lat    |I1|NE latitude to 0.1 minutes
|104-121  |18  |SW Longitude     |sw_lon    |I1|SW longitude to 0.1 minutes
|122-138  |17  |SW Latitude      |sw_lat    |I1|SW latitude to 0.1 minutes
|69-98    |30  |MMSI1            |dest1     |u|MMSI of destination 1
|104-133  |30  |MMSI2            |dest2     |u|MMSI of destination 2
|139-139  | 1  |Addressed        |addressed |b|0=Broadcast, 1=Addressed
|140-140  | 1  |Channel A Band   |band_a    |b|0=Default, 1=12.5kHz
|141-141  | 1  |Channel B Band   |band_b    |b|0=Default, 1=12.5kHz
|142-144  | 3  |Zone size        |zonesize  |u|Size of transitional zone
|145-167  |23  |Spare            |          |x|Reserved for future use
|======

The values of the channel_a and channel_b fields are ITU frequency
designators for channels A and B. Normally these will be 2087 and
2088, the AIS 1 and AIS 2 frequencies of 87B (161.975 MHz) and 88B
(162.025 MHz) respectively. Regional authorities may set different
frequencies.

The txrx field encodes the same information as the 2-bit field txrx
field in message type 23; only the two low bits are used.

The power bit instructs designated receivers which power level to use.

If the message is broadcast (addressed field is 0), the ne_lon,
ne_lat, sw_lon, and sw_lat fields are the corners of a rectangular
jurisdiction area over which control parameters are to be set.  If it
is addressed (addressed field is 1), the same span of data is
interpreted as two 30-bit MMSIs beginning at at bit offsets 69 and 104
respectively.

Yes, the addressed bit is *after* the fields it controls the
interpretation of.

Note that the 'not available' values for longitude and latitudes
match the short ones used in message 17, not the long ones used in the
common navigation block and elsewhere.

The band fields control channel bandwidth for channels A and B, and
the zonesize field describes the size of the transition zone around the
control jurisdiction. The semantics of these fields are complicated,
controlling transmitter behavior as it moves between jurisdictions;
see <<IALA>> for full details.

=== Type 23: Group Assignment Command

This message is intended to be broadcast by a competent authority (an
AIS network-control base station) to set operational parameters for
all mobile stations in an AIS coverage region. Length is 160 bits.

This message contains no navigational information, and is unlikely to
be of interest unless you are implementing or studying an AIS base
station network.

//: Type 23
[frame="topbot",options="header"]
|======
|Field    |Len |Description      |Member       |T|Units
|0-5      | 6  |Message Type     |type         |u|Unsigned Integer: 23
|6-7      | 2  |Repeat Indicator |repeat       |u|As in Common Navigation Block
|8-37     |30  |MMSI             |mmsi         |u|Unsigned Integer: 9 digits
|38-39    | 2  |Spare            |             |x|Not used
|40-57    |18  |NE Longitude     |ne_lon       |u|Same as broadcast type 22
|58-74    |17  |NE Latitude      |ne_lat       |u|Same as broadcast type 22
|75-92    |18  |SW Longitude     |sw_lon       |u|Same as broadcast type 22
|93-109   |17  |SW Latitude      |sw_lat       |u|Same as broadcast type 22
|110-113  | 4  |Station Type     |station_type |e|See "Station Types"
|114-121  | 8  |Ship Type        |ship_type    |e|See "Ship Types"
|122-143  |22  |Spare            |             |x|Not used
|144-145  | 2  |Tx/Rx Mode       |txrx         |u|See "Transmit/Receive Modes"
|146-149  | 4  |Report Interval  |interval     |e|See "Station Intervals"
|150-153  | 4  |Quiet Time       |quiet        |u|0 = none, 1-15 quiet time in minutes
|154-159  | 6  |Spare            |             |x|Not used
|======

The target set of mobile stations is specified by the station-type and
ship-type fields. An addressed (non-broadcast) message 22 overrides a
message 23, but a message 23 overrides a broadcast message 22.

Note that the 'not available' values for longitude and latitudes
match the short ones used in messages 17 and 22, not the long ones
used in the common navigation block and elsewhere.

The txrx field tells the affected stations which channel or
channels they may transmit on.  The options refer to the same A and B
VHF channels as in Message Type 22. The field is interpreted as follows:

.Transmit Modes
[width="25%",frame="topbot"]
|======
|0 |TxA/TxB, RxA/RxB (default)
|1 |TxA, RxA/RxB
|2 |TxB, RxA/RxB
|3 |Reserved for Future Use
|======

.Station Types
[width="50%",frame="topbot"]
|======
|0     |All types of mobiles (default)
|1     |Reserved for future use
|2     |All types of Class B mobile stations
|3     |SAR airborne mobile station
|4     |Aid to Navigation station
|5     |Class B shipborne mobile station (IEC62287 only)
|6-9   |Regional use and inland waterways
|10-15 |Reserved for future use
|======

<<INLAND>> specifies 6 (only) as the station type value for inland
waterways, reserving 7-9 for (other) regional uses.

Reporting Interval is a 4 bit unsigned integer, how often to report
while within the area specified by this message. When the dual-channel
operation is suspended by Tx/Rx mode command 1 or 2, the reporting
interval is twice the interval given in the table.

.Station Intervals
[width="50%",frame="topbot"]
|======
|0     |As given by the autonomous mode
|1     |10 Minutes
|2     |6 Minutes
|3     |3 Minutes
|4     |1 Minute
|5     |30 Seconds
|6     |15 Seconds
|7     |10 Seconds
|8     |5 Seconds
|9     |Next Shorter Reporting Interval
|10    |Next Longer Reporting Interval
|11-15 |Reserved for future use
|======

Quiet Time is a 4 bit unsigned integer specifying how many minutes
affected stations are to remain silent. If a class B station receives
a quiet time command, it will continue to schedule nominal
transmission time periods, but is not to transmit message 18 or 24
during the quiet period.

=== Type 24: Static Data Report

Equivalent of a Type 5 message for ships using Class B equipment.  Also
used to associate an MMSI with a name on either class A or class B equipment.

A "Type 24" may be in part A or part B format; According to the
standard, parts A and B are expected to be broadcast in adjacent
pairs; in the real world they may (due to quirks in various
aggregation methods) be separated by other sentences or even
interleaved with different Type 24 pairs; decoders must cope with
this.  The interpretation of some fields in Type B format changes
depending on the range of the Type B MMSI field. 160 bits for part A,
168 bits for part B.

According to the standard, both the A and B parts are supposed to
be 168 bits. However, in the wild, A parts are often transmitted
with only 160 bits, omitting the 'spare' 7 bits at the end.
Implementers should be permissive about this.

<<IALA>> does not describe this message type; format information
is thanks to Kurt Schwehr.

//: Type 24
[frame="topbot",options="header"]
|======
|Field   |Len |Description            | Member         |T|Units
|0-5     |  6 | Message Type          | type           |u|Constant: 24
|6-7     |  2 | Repeat Indicator      | repeat         |u|As in CNB
|8-37    | 30 | MMSI                  | mmsi           |u|9 digits
|38-39   |  2 | Part Number           | partno         |u|0-1
|40-159  |120 | Vessel Name           | shipname       |t|(Part A) 20 sixbit chars
|160-167 |  8 | Spare                 |                |x|(Part A) Not used
|40-47   |  8 | Ship Type             | shiptype       |e|(Part B) See "Ship Types"
|48-65   | 18 | Vendor ID             | vendorid       |t|(Part B) 3 six-bit chars
|66-69   |  4 | Unit Model Code       | model          |u|(Part B)
|70-89   | 20 | Serial Number         | serial         |u|(Part B)
|90-131  | 42 | Call Sign             | callsign       |t|(Part B) As in Message Type 5
|132-140 |  9 | Dimension to Bow      | to_bow         |u|(Part B) Meters
|141-149 |  9 | Dimension to Stern    | to_stern       |u|(Part B) Meters
|150-155 |  6 | Dimension to Port     | to_port        |u|(Part B) Meters
|156-161 |  6 | Dimension to Starboard| to_starboard   |u|(Part B) Meters
|132-161 | 30 | Mothership MMSI       | mothership_mmsi|u|(Part B) See below
|162-167 |  6 | Spare                 |                |x|(Part B) Not used
|======

If the Part Number field is 0, the rest of the message is interpreted
as a Part A; if it is 1, the rest of the message is interpreted
as a Part B; values 2 and 3 are not allowed.

Bits 48-89 are as described in ITU-R 1371-4.  In earlier versions to
1371-3 this was one sixbit-encoded 42-bit (7-character) string field,
the name of the AIS equipment vendor.  The last 4 characters of
the string are reinterpreted as a model/serial numeric pair. It is not
clear that field practice has caught up with this incompatible change.
Implementations would be wise to decode that but span in both ways and
trust human eyes to detect when the final 4 characters of the string
or the model and serial fields are garbage.

Interpretation of the 30 bits 132-162 in Part B is variable. If the
MMSI at 8-37 is that of an auxiliary craft, the entry is taken to
refer to a small attached auxiliary vessel and these 30 bits are read
as the MMSI of the mother ship.  Otherwise the 30 bits describe vessel
dimensions as in Message Type 5.

According to <<MMSI>>, an MMSI is associated with an auxiliary craft
when it is of the form 98XXXYYYY, where (1) the '98' in positions 1
and 2 is required to designate an auxiliary craft, (2) the digits XXX
in the 3, 4 and 5 positions are the MID (the three-digit country code
as described in <<ITU-MID>>) and (3) YYYY is any decimal literal from
0000 to 9999.

=== Type 25: Single Slot Binary Message

Maximum of 168 bits (a single slot). Fields after the Destination
MMSI are at variable offsets depending on that flag and the Destination
Indicator; they always occur in the same order but some may be omitted.

//: Type 25
[frame="topbot",options="header"]
|======
|Field   |Len   |Description            | Member      |T|Units
|0-5     |  6   | Message Type          | type        |u|Constant: 25
|6-7     |  2   | Repeat Indicator      | repeat      |u|As in CNB
|8-37    | 30   | MMSI                  | mmsi        |u|9 digits
|38      |  1   | Destination indicator | addressed   |b|0=broadcast,
                                                         1=addressed.
|39      |  1   | Binary data flag      | structured  |b|See below
|40      |0/30  | Destination MMSI      | dest_mmsi   |u|Message destination
|?       |0/16  | Application ID        | app_id      |u|Unsigned integer
|?       |0-128 | Data                  | data        |d|Binary data
|======

If the 'addressed' flag is on, 30 bits of data at offset 40 are
interpreted as a destination MMSI.  Otherwise that field span becomes
part of the message payload, with the first 16 bits used as an
Application ID if the 'structured' flag is on.

If the 'structured' flag is on, a 16-bit application identifier is
extracted; this field is to be interpreted as a 10 bit DAC and 6-bit
FID as in message types 6 and 8.  Otherwise that field span becomes
part of the message payload.

The data fields are not, in contrast to message type 26, followed by a
radio status block.

Note: Type 25 is extremely rare. As of April 2011 it has not been
observed even in long-duration samples from AISHub.

=== Type 26: Multiple Slot Binary Message

Takes up 60-1064 bits (up to 5 slots).

//: Type 26
[frame="topbot",options="header"]
|======
|Field   |Len   |Description           | Member     |T|Units
|0-5     |  6   | Message Type         | type       |u|Constant: 26
|6-7     |  2   | Repeat Indicator     | repeat     |u|As in CNB
|8-37    | 30   | MMSI                 | mmsi       |u|9 digits
|38      |  1   | Destination indicator| addressed  |b|0=broadcast,
                                                       1=addressed.
|39      |  1   | Binary data flag     | structured |b|See below
|40      |0/30  | Destination MMSI     | dest_mmsi  |u|Message destination
|?       |0/16  | Application ID       | app_id     |u|Unsigned integer
|?       |0-1004| Data                 | data       |d|Binary data
|?       |20    | Radio status         | radio      |u|See <<IALA>> for details.
|======

The data field may span up to 5 256-bit slots in addition to the tail
end of the base slot.  The application_ID field, if present, is to be
interpreted as a 10 bit DAC and 6-bit FID as in message types 6 and 8.
Documentation says the data length of each slot is 224 and adds the
note "Allows for 32 bits of bit-stuffing."

The 20 radio status bits are always present after end-of-data in the
last slot and are in the format specified by <<IALA>>.  The radio
status is 20 bits rather than 19 because an extra first bit selects
whether it should be interpreted as a SOTDMA or ITDMA state.

Note: Type 26 is extremely rare. As of April 2011 it has not been
observed even in long-duration samples from AISHub.

=== Type 27: Long Range AIS Broadcast message

ITU-1371-4 says this message is primarily intended for long-range
detection of AIS Class A equipped vessels (typically by
satellite). This message has a similar content to Messages 1, 2 and 3,
but the total number of bits has been compressed to allow for
increased propagation delays associated with long-range detection

Length according to ITU-1374 is 96 bits.  However, in the wild these
are sometimes transmitted with 168 bits (a full slot). Robust decoders
should warn when this occurs but decode the first 96 bits.

//: Type 27
[frame="topbot",options="header"]
|======
|Field   |Len   |Description         |Member   |T|Units
|0-5     |  6   |Message Type        |type     |u|Constant: 27
|6-7     |  2   |Repeat Indicator    |repeat   |u|As in CNB
|8-37    | 30   |MMSI                |mmsi     |u|9 decimal digits
|38-38   | 1    |Position Accuracy   |accuracy |u|See Common Navigation Block
|39-39   | 1    |RAIM flag           |raim     |u|See Common Navigation Block
|40-43   | 4    |Navigation Status   |status   |u|See Common Navigation Block
|44-61   | 18   |Longitude           |lon      |I1|minutes/10
                                                  East positive, West negative
                                                  181000 = N/A (default)
|62-78   | 17   |Latitude            |lat      |I1|minutes/10
                                                  North positive, South negative
                                                  91000 = N/A (default)
|79-84   | 6    |Speed Over Ground   |speed    |u|Knots (0-62);
                                                  63 = N/A (default)
|85-93   | 9    |Course Over Ground  |course   |u|0 to 359 degrees,
                                                  511 = not available.
|94-94   | 1    |GNSS Position status|gnss     |u|0 = current GNSS position
                                                  1 = not GNSS position (default)
|95-95   | 1    |Spare               |         |x|Not used
|======

== Local extensions

Some regional authorities extend the AIS message set.

The St. Lawrence Seaway broadcasts hydrological and lock-scheduling
messages using special encodings of the binary data of message types 6
and 8 (described in <<SEAWAY>>, freely available), and safety
information using types 12 and 14.  These message types are listed
under the description of type 8.

The U.S. Coast Guard has a system called PAWSS (Port and Water Safety
System) which uses extended AIS binary formats. <<SEAWAY>> says it's
compatible with the St. Lawrence Seaway system and describes three
PAWSS-specific messages in its Appendix A.

Since 2007 the Port Authority of London has operated a Thames AIS
system covering the Thames River; it uses extensions of message types
6 and 8.  It is describes in <<THAMES>>.

Since 2006 there has been some effort to standardize inland-waterway
extensions, described in <<INLAND>>.  It uses different message formats
and identifiers than the PAWS/St. Lawrence systems, which do not conform.
There is an AIS extension called "RIS" (River Information System) that
covers portions of the Danube River and Black Sea which does conform.

<<IMO289>> standardizes some subtypes of messages 6 and 8 similar to
PAWSS messages for DAC 1, the international jurisdiction code. However,
in some cases identically named subtypes are assigned different FIDs.

=== U.S. Coast Guard Extended AIVDM

You may occasionally see AIVDM packets with additional comma-separated
fields following the checksum.  This is a semi-obsolescent
logging format used by the USCG, which has never documented it well
and plans to replace it with a new one based on NMEA 4.0.

Here's a sample sentence and field breakdown:

-------------------------------------------------------------------------------
!AIVDM,1,1,,B,15Cjtd0Oj;Jp7ilG7=UkKBoB0<06,0*63,s1234,d-119,T12.34567123,r003669958,1085889680
-------------------------------------------------------------------------------

Following the "*63" checksum are additional fields delimited by commas.
These fields provide additional metadata about the reception of each AIS
broadcast.

The field beginning with the lower case "s" is a Relative Signal
Strength Indicator (RSSI) measurement from the receiver. This
measurement has a range of 0-65535. This is one of the parameters used
internally by the AIS receiver to determine the signal strength value as
reported in the field beginning with the lower case "d". This field only
exists when the AIS receiver provides this data.

The field beginning with a lower case "d" is the signal strength
measurement for this broadcast in dBm. This field only exists when the
AIS receiver provides this data.

The field beginning with the upper case "T" is the Time of Arrival
of the received broadcast in seconds from UTC 0. This field only exists
when the AIS receiver provides this data.

Another optional field not shown is one that begins with an upper
case "S" and represents the slot number in which the reception occurred.
The field would appear after the checksum and before the station
identifier field. This field only exists when the AIS receiver provides
this data. Example: S0042

The field beginning with the lower case "r" is a station identifier
field. This field is always provided, regardless of the type of AIS
equipment.  (Occasionally a base station identifier will be prefixed with
"b" instead.)

The last field is a time tag based on the standard C programming
language time function. Both date and time to the nearest second can be
derived from this field. This field is always provided, regardless of
the type of AIS equipment.

== NMEA Tag Blocks

Beginning with NMEA 4.10, the standard describes a way to intersperse
"tag blocks" with AIS sentences in order to supply additional
metadata, usually a Unix timestamp to be associated with a sentence
(or contiguous group of sentences such as an armored AIS Type 5).

The tag block facility is complex, in some respects poorly specified,
and there has not yet been much public discussion of it.  This section
should be considered provisional and in need of improvement.

The general format of a tag block is: an opening backslash, followed
by multiple comma-separated fields none of which may contain
backslashes, followed by an asterisk and NMEA checksum, followed by a
closing backslash.

The following is an example of an AIS Type 1 sentence preceded by a
tag block:

-------------------------------------------------------------------------------------------------
\g:1-2-73874,n:157036,s:r003669945,c:1241544035*4A\!AIVDM,1,1,,B,15N4cJ`005Jrek0H@9n`DW5608EP,0*13
-------------------------------------------------------------------------------------------------

Each comma-separated field is expected to be a type key, followed by a
colon, followed by content.  The semantics of the type keys are as
follows:

Confusingly, there is a different standard introduced with NMEA 4.00,
IEC 62320-1, that uses the same tag block format but a slightly
different (overlapping) set of field keys

.NMEA 4.00 Field Types
[frame="topbot",options="header"]
|======
| IEC   | NMEA	| Type           | Description
|  c    | c	| int>0          | UNIX time in seconds or milliseconds
|  d    | d	| string         | Destination (at most 15 chars)
| xGy   | g	| int-int-int	 | Sentence grouping
|  x    | n	| int>0          | line count
|       | r	| int>0          | relative time
|  s    | s	| string         | Source / station
|  i    | t	| string         | Text string (at most 15 chars)
|======

The IEC 62320-1 and NMEA 4.10 c field is an emission time for the
sentence it precedes. We're not yet sure what the time unit is.

The NMEA 4.10 g value describes a sentence or sentence group to
associate the tag block to. It is a triple of ints separated by
dashes.  The first number is the sentence number, the second is total
number of sentences to make up one group.  The third number is an
identifier for that particular group.  If there is no g value, the tag
block simply applies to the following sentence.

As of May 2014 no NMEA 4.10 relative time fields have been observed in the
wild. It is unknown whether the unit is seconds or milliseconds.

The d, i/t, and s fields are intended to be used for filtering by
station IDs included in them.

Part of the NMEA 4.10 standard describes a configuration message facility by
which AIS message receivers can send queries to AIS senders, and under
some circumstances change the semantics if tag blocks (in particular,
by specifying time units and epoch).

We do not yet have more definite information on the meaning of these
fields or how they are related to nearby AIVDM/AIVDO sentences.  We
hope to add this in a future revision.

== AIS Payload Byte Alignment, Padding, and Bit Stuffing

Warning: Here there be dragons. Read with care. Once you get through
it, you will at least not encounter anything more confusing in the
rest of this document.

=== Byte Alignment

AIS is a bit-sync protocol.  While some fields within AIS payloads are
8-bit-byte-aligned with preceding padding, most are not. Furthermore,
while most message variants have bit lengths that are a multiple of 8,
some do not.

<<ITU1371>> includes a single sentence, easy to miss, requiring
over-the-air messages to have trailing padding to a 8-bit boundary.
In most cases message lengths are a multiple of 8 with trailing spare
fields added to ensure this; thus, the requirement will not change the
transmitted bitlength of the message from what's described in the
standard. There are, however, two exceptions to this rule.

One is an apparent error in the format design.  The type 15 message
has a variant with 108 data bits and a trailing 2-bit spare field, for
110. This spare should have been 4 bits to guarantee a byte boundary
at 112 bits. Decoders need to be prepared to encounter this length
in case the transmitter has implemented the padding requirement
properly.

The other is messages containing variable-length text packed into
6-bit nibbles: types 6, 12, and 14.  They may have trailing padding
after the last nibble to get to an 8-bit boundary.  Decoders should
be prepared to encounter and ignore this.

The variable-length binary message types 8, 17, 25, and 26 are
constrained to have data payloads of a size such that the payload ends
on a byte boundary, but should not require special handling on this
account. The binary data in message types 8 and 17 is also guaranteed
to *begin* on a byte boundary, but this is not true of the addressed
variants of type 25 and 26.

=== Interaction with AIVDM padding

AIVDM armoring introduces a second layer of padding, with confusing
consequences.  The real payload, already padded to a bit length that
is a multiple of 8 by the AIS radio layer, gets armored as a sequence
of ASCII characters encoding 6-bit nibbles.  To capture all of it, the
payload must in effect be padded to the next multiple of 6.

Consider a type 12 message with 5 sixbit characters in it. These will
become bits 72-102 in the over-the-air message. The AIS radio layer will
pad that to 104 bits at transmission to get to an 8-bit boundary.  The
receiver, reporting the data in AIVDM armoring, will pad that to 108
bits to get to a 6-bit boundary, encode the result, and issue a pad
character of '4' to indicate that the low 4 bits of the last 6-bit
nibble should be ignored.

Because these requirements are tricky and poorly documented in the
official standards, receivers not uncommonly get them wrong.  The most
common way to get them wrong seems to be by computing the pad
character incorrectly.

The most common error observed in the wild on AISHub is reporting a
pad 2 bits too small, making the message look like it is 2 bits longer
than it actually is.  This seems for some reason to be most common
on Type 5 messages, which then decode as 426 bits rather than 424.

Accordingly, we recommend that when validating fixed-size messages by
type and bit-length, decoders should accept messages that are up to 5
bits over their theoretically correct length.

For messages with a variable-length trailing payload (6, 8, 12, 14,
17, 25, 26) there is no way to detect that the pad character might
be wrong.  If it is, this will manifest as truncation of the last
nibble or extra trailing zero data.

=== Bit Stuffing

The following probably will not affect decoders. Nevertheless, we
document it here because it is just the sort of thing that is (a)
likely to confuse implementors reading the public portions of the
standards, and (b) all too likely to become visible if there are
firmware or software errors in the transmission chain.

There are references to "bit-stuffing" in the <<IALA>> clarifications
describing certain payload fields.  <<C2>> reveals the following in
3.2.2.1: "The bitstream is subject to bit stuffing. This means that if
more than 5 consecutive 1s are found in the output bit stream, a zero
is inserted. This applies to all bits except the databits of HDLC
flags."  <<IALA>> clarifies as follows: "On the transmitting side,
this means that if five (5) consecutive ones (1s) are found in the
output bit stream, a zero should be inserted after the five (5)
consecutive ones (1s). This applies to all bits between the HDLC flags
[...] On the receiving side, the first zero after five (5) consecutive
ones (1s) should be removed."

It appears that this bit stuffing is meant to be performed by the AIS
radio link layer at transmission time and undone at reception time,
and should not be visible in AIVDM payloads reported by the
receiver.

== AIS feed sites

Most sites that advertise "live" AIS feeds actually give you a map
display through a browser.  Here are a few from which you can get raw
sentence data over a TCP/IP port for testing.  Coverage on these is
not yet very comprehensive; these sites tend to have good coverage in
Europe, the U.S. and a few ports in Asia but to be spotty elsewhere.
See their siting maps for details.

- http://aishub.net[AIS Hub]:
Share alike. You contribute a feed, you get back all feeds.

- http://maritime.ihs.com[AIS Live]:
Subscription access to real-time data. No longer has free access even
to delayed data.

== JSON-AIS encoding

Here is an application of the JSON metaformat to present AIS data in a
form more convenient for application use than AIVDM/AIVDO sentences.
This encoding is implemented by GPSD and its client libraries.  It is
described here because (a) the specification is closely tied to the
field encodings, and (b) the author wishes to offer it as an
interoperability standard for other applications.

One previous effort, JSON-AIS, has been made to define a JSON-based
standard for exchange of unpacked, human-readable AIS data. The latest
version at time of writing, from December 2008,covers only a small
subset of the most common AIS messages, and many data fields in the
messages it does dump are omitted.  Online links to the project have
disappeared as of June 2023.  The member names given in the bit-field
tables match the attributes used in HAM-JSON-AIS when HAM-JSON-AIS
includes that field.

The general ground rules for JSON-AIS encoding are as follows:

. Each sentence decodes to a JSON object.

. When multiple kinds of JSON objects may occur in a data stream, AIS
objects have the attribute "class":"AIS".

. Some collections of fields aggregating to a timestamp are dumped in
ISO8601 format.

.Timestamp fields
[frame="topbot",options="header"]
|======
|Message| ITU/IMO fields                   | JSON ISO8601 | Format
|   4   | year,month,day,hour,minute,second| timestamp    | %4u-%02u-%02uT%02u:%02u:%02uZ
|   5   | month,day,hour,minute            | eta          | %02u-%02uT%02u:%02uZ
|6(1/12)| lmonth,lday,lhour,lminute        | departure    | %02u-%02uT%02u:%02uZ
|       | nmonth,nday,nhour,nminute        | eta          | %02u-%02uT%02u:%02uZ
|6(1/18)| month,day,hour,minute            | arrival      | %02u-%02uT%02u:%02uZ
|6(1/20)| month,day,hour,minute            | arrival      | %02u-%02uT%02u:%02uZ
|6(1/23)| month,day,hour,minute            | timestamp    | %02u-%02uT%02u:%02uZ
|6(1/28)| month,day,hour,minute            | start        | %02u-%02uT%02u:%02uZ
|8(1/11)| day,hour,minute                  | timestamp    | %02uT%02u:%02uZ
|8(1/11)| day,hour,minute                  | timestamp    | %02uT%02u:%02uZ
|8(1/13)| fmonth,fday,fhour,fminute        | from         | %02u-%02uT%02u:%02uZ
|8(1/13)| tmonth,tday,thour,tminute        | to           | %02u-%02uT%02u:%02uZ
|8(1/22)| month,day,hour,minute            | timestamp    | %02u-%02uT%02u:%02uZ
|8(1/27)| month,day,hour,minute            | start        | %02u-%02uT%02u:%02uZ
|8(200/23)| year,month,day,hour,minute     | start        |%4u-%02u-%02uT%02u:%02u
|8(200/23)| year,month,day,hour,minute     | end          |%4u-%02u-%02uT%02u:%02u
|======

. There are two variants of the encoding, one scaled and one
unscaled, which differ in the treatment of float and
controlled-vocabulary fields. An AIS-JSON object may have the optional
attribute "scaled":true to signify that the rest of its fields are
scaled; if this attribute has the value 'false' or is omitted, no
scaling has been performed.  Message types for which the unscaled and
scaled dumps will differ are 1-5, 9, 11, 17-19, 21-24, and 27.

. In unscaled mode, float-valued fields are dumped in their unscaled
integer form. In scaled mode, division or other specified scaling is
applied and the value dumped as a float, *except* that certain extreme
or data-unavailable value as may be dumped as fixed strings; see the
table below.

. Each field in the Controlled Vocabularies list in the following
table is dumped twice: once with its base name as an integer, once
with "_text" appended to the name and the vocabulary item as value.
(This behavior is new in GPSD protocol version 3.9; older versions
made integer or string display dependent on the "scaled" flag.)

.Special fields
[frame="topbot",options="header"]
|======
|Message | Float Fields                         | Controlled Vocabularies
| 1-3    | turn, speed, lon, lat                | status
| 4, 11  | lon, lat                             | epfd
| 5      | draught                              | shiptype, epfd
| 6(1/14)| lon, lat, cspeed                     | -
| 6(1/18)| lon, lat                             | -
| 6(1/20)| berth_lon, berth_lat, berth_depth    | position
| 6(1/22)| lon, lat                             | -
| 6(1/25)| code                                 | code*
| 6(1/28)| lon, lat                             | rtype
| 6(1/32)| lon, lat, cspeed                     | -
| 6(235/100) | -                                | racon, light
| 8(1/11)| lon, lat, airtemp, dewpoint,         | preciptype, ice
|        | visibility, waterlevel, cspeed,      |
|        | cspeed2, cspeed3, waveheight,        |
|        | swellheight, watertemp salinity      |
| 8(1/17)| lon, lat                             | idtype*
| 8(1/18)| lon, lat                             | -
| 8(1/19)| -                                    | signal, nextsignal
| 8(1/21)| lon, lat, visibility, airtemp,       | -
|        | watertemp, waveheight,               |
|        | swellheight, speed, pressure, pdelta,|
|        | rwindspeed, mgustspeed, airtemp,     |
|        | surftemp, cloudbase,                 |
|        | swheight1, swheight2                 |
| 8(1/22)| lon, lat                             | -
| 8(1/25)| -                                    | iceclass*
| 8(1/27)| lon, lat                             | rtype
| 8(1/31)| lon, lat, airtemp, dewpoint,         | preciptype
|        | visibility, waterlevel, cspeed,      |
|        | cspeed2, cdepth2, cspeed3,           |
|        | cdepth3, waveheight,                 |
|        | swellheight, watertemp, salinity     |
| 9      | lon, lat, course                     | -
| 17     | lon, lat                             | -
| 18     | lon, lat, course                     | -
| 19     | lon, lat, course                     | shiptype, epfd
| 21     | lon, lat                             | aid_type, epfd
| 22-23  | ne_lon,ne_lat, sw_lon, sw_lat        | shiptype,stationtype
| 24     | -                                    | shiptype
| 27     | lon, lat                             | -
|======

This table does not include fields that are scaled by multipliers to integers.
These are reported in the multiplied form in both scaled and unscaled modes.

Asterisked fields are not yet decoded by GPSD, but probably will be in
a future release.

As the Beaufort scale is usually quoted numerically, conforming implementations
should do so rather than expanding to its controlled vocabulary.

.String special values in scaled mode
[frame="topbot",options="header"]
|======
|Message | Fieldname | Special values
| 1-3    | turn      |
"nan" = not available, "fastright" = fast right turn (above 5deg/30sec degrees),
"fastleft" = fast left turn (above 5deg/30sec degrees).
| 1-3    | speed     | "nan" = not available, "fast" = speed >= 102.2 knots
|  9     | alt       | "nan" = not available, "high" = alt >= 4094 meters
|  9     | speed     | "nan" = not available, "fast" = speed >= 1023.0 knots
|======

. Trailing arrays are dumped as JSON subobject arrays.  The name of the
array item is the name given in the 'a' table entry.  Explicit array count
fields (presently "waycount" in Route Info messages") are not dumped.

. In the VTS-Generated/Synthetic Targets message, the id field is
dumped as the value of an attribute the name of which is specified by
the idtype field.

== Open Questions

The AIS standards are not marvels of clear and unambiguous drafting.
We list here some open questions which could usefully be addressed
by governing authorities.

Some of these duplicate material in paragraphs tagged with OPEN-QUESTION.
They are collected here for convenience.

The message type summary table in <<INLAND>> indicates the existence
of a broadcast (unaddressed) variant of Inland Number of Persons On
Board. But no field breakdown is given for this variant. If it
supposed to be identical to the Type 6 layout, what is to be done
with the Destination MMSI field?

Are <<INLAND>> time-of-day fields UTC or local?

In <<INLAND>> RTA at lock/bridge/terminal message, no default is
specified for the Status field.

In Inland Ship Static and Voyage Related Data, which of two possible
ERI numeric codesets are used for the Type field - the 4-digit codes
in the 8000-8073 range, or the AIS codes in the 1-99 range?  Full ERI
codes have been observed in the wild.

In the <<INLAND>> description of Message 5 extensions, footnote 6 is
incomprehensible and not actually referenced in the table.

In the <<INLAND>> EMMA Warning and Signal Strength messages, the
description is not explicit about the interpretation of the longitude
and latitude fields; these semantics are assumed here from 28 and
27-bit fields in other messages.

In the <<INLAND>> EMMA Warning, what are the semantics of the min and
max values?  To what parameters do they apply?

Is the <<IMO236>> versions of Persons On Board, which is supposed to
be Type 6 and thus addressed, erroneous?  There is no destination
address field in the layout.

<<IMO289>> says of the "VTS Generated/Synthetic Targets" message:
"When MMSI or IMO number is used, the least significant bit should
equal bit zero of the Target Identifier."  It is unclear how "bit
zero" is to be interpreted, but it is not possible to reconcile
interpreting it as the leading bit of the field with AIS big-endian
encoding. Settling this awaits live testing.

== References

[bibliography]
- [[[AIS]]] https://en.wikipedia.org/wiki/Automatic_Identification_System[Automatic
Identification System]

- [[[SOLAS]]]
  http://www.navcen.uscg.gov/enav/ais/AIS_Regs_SOLAS_MTSA_FR.pdf[SOLAS
  AIS Regulations]

- [[[US-REQUIREMENTS]]]
  http://www.navcen.uscg.gov/enav/AIS/AIS_carriage_reqmts.htm[AIS
  Carriage Requirements]

- [[[ITU1371]]]
  http://www.itu.int/rec/R-REC-M.1371/en
  ITU Recommendation on the Technical Characteristics for a Universal
  Shipborne Automatic Identification System (AIS) using Time Division
  Multiple Access in the Maritime Mobile Band]. All versions are
  available here.

- [[[IALA]]]  http://www.ialathree.org/iala/pages/AIS/IALATech1.5.pdf[IALA
  Technical Clarifications on Recommendation ITU-R M.1371-1]

- [[[IALA-ASM]]] https://www.iala-aism.org/asm/[Collection of regional applications for AIS Application Specific Messages of regional applications for AIS Binary Messages]

- [[[NAVCEN]]] http://www.navcen.uscg.gov/enav/ais/[NAVCEN AIS pages]

- [[[NMEA]]] "NMEA Revealed" This document collects information from
  public sources on the proprietary NMEA 0183 standard.  It is widely
  distributed, but due to its hosting site having folded in late 2011,
  its home location is unstable. It is probably best found by typing
  the title into a search engine.

- [[[INLAND]]] http://www.unece.org/fileadmin/DAM/trans/doc/finaldocs/sc3/ECE-TRANS-SC3-176e.pdf[International Standard for tracking and tracing on Inland Waterways]

- [[[SEAWAY]]] https://greatlakes-seaway.com/wp-content/uploads/2020/05/Seaway-AIS-message-specifications-R41.pdf[St. Lawrence
  Seaway AIS Data Messaging Formats and Specifications]

- [[[THAMES]]] https://www.pla.co.uk/assets/APPENDIX_I__II_amendment_for_2012_byelaws_FINAL.pdf[Thames AIS - Technical Requirements Specification]

- [[[IMO236]]] https://www.navcen.uscg.gov/sites/default/files/pdf/IMO_SN1_Circ290_Guidance_on_presentation%20of%20AIS_ASM.pdf[IMO
-  Circular 236: Guidance on the Application of AIS Binary Messages (June 2010)]

- [[[IMO289]]]
  http://vislab-ccom.unh.edu/~schwehr/papers/2010-IMO-SN.1-Circ.289.pdf[IMO
  289: Guidance on the Use of AIS Application-Specific Messages
  (June 2010)]

- [[IALA-A126]]
  http://www.ialathree.org/chapo/publications/documentspdf/doc_235_eng.pdf[IALA
  Recommendation A-126 On The Use of the Automatic Identification System (AIS)
  in Marine Aids to Navigation Services

- [[[IEC-PAS]]] IEC-PAS 61162-100, "Maritime navigation and
  radiocommunication equipment and systems"  The ASCII armoring
  is described on page 26 of Annex C, Table C-1. Communicated by
  Kurt Schwehr; I have not looked at it.

- [[[IEC-62287]]] "Maritime Navigation and Radiocommunication
  Equipment and Systems – Class B Shipborne Equipment of the
  Automatic Identification System (AIS)" Communicated by Mike Greene;
  I have not looked at it.

- [[[ITU-MID]]] https://www.itu.int/en/ITU-R/terrestrial/fmd/Pages/mid.aspx[Table
  of Maritime Identification Digits]

- [[[RAIM]]] https://en.wikipedia.org/wiki/RAIM[Receiver Autonomous
  Integrity Monitoring]

- [[[C2]]] http://www.uais.org/AISspecificationCorrigendum2.pdf[AIS
  Specification Corrigendum 2]

- [[[MMSI]]] http://www.navcen.uscg.gov/marcomms/gmdss/mmsi.htm#format[MMSI
  Format]

- [[[AISHUB]]] http://www.aishub.net/[AIS Hub, the AIS data sharing center]

- [[[PILOTPLUG]]] http://www.pilotplug.com/

- [[[IALA-A126]]] http://www.iala-aism.org/iala/publications/documentspdf/doc_299_eng.pdf[]

- [[[MSGS123]]] https://www.navcen.uscg.gov/ais-class-a-reports[Class A AIS Position Report (Messages 1, 2, and 3)]

- [[[SART]]] https://en.wikipedia.org/wiki/AIS-SART

- [[[PATREON]]] https://www.patreon.com/esr

- [[[NMEA-ADVANCE]]] https://vdocuments.net/nmea-0183-advancements-2pdfl-luft-1010-nmea-0183-advancements-slide-11-new.html

- [[[Schwehr]]] https://ccom.unh.edu/user/schwehr


== Change history

Version 1.0 was the initial release covering messages 1-3, 4, and 5.

Version 1.1 adds message breakdowns for 9 and 18, explanation of the
Repeat Indicator field, and the explanation of USCG extended AIVDM.

Version 1.2 adds information on the ITU1371 edition 3 maneuver field,
and the RAIM flag. It also adds an important clarification about
six-bit decoding.

Version 1.3 adds information on message types 6, 7, 12, and 13, and
attempts to demystify bit-stuffing.

Version 1.4 adds explicit decoding tables for ASCII armoring and
six-bit ASCII.

Version 1.5 corrects the interpretation of field 7 in AIVDM
ASCII-armored sentences.

Version 1.6 corrects some minor errors in the interpretation of Type
5 messages.

Version 1.7 adds descriptions for Type 10, 11, 19, 21, and 24
messages, information about ITU-1371-3 flags in message type 18,
and the new section on Improving This Document.

Version 1.8 fixes some broken markup and adds information about JSON-AIS.

Version 1.9 adds more information on JSON and the member names.

Version 1.10 fixes a typo in the formula for undoing 6-bit armoring.

Version 1.11 describes message types 15, 16, and 17.

Version 1.12 describes messages 20 and 22, and adds navigation aid
type codes.

Version 1.13 documents more out-of-band values and treats radio status
blocks more uniformly.

Version 1.14 documents message 23.

Version 1.15 corrects an incorrect member name in message 5. It didn't
match my C code, but had no effect on conformance with the standard. I
corrected it because it confused someone working on a Python decoder.

Version 1.16 incorporated various minor fixes and corrections from
Neal Arundale.  One 'standard' fieldname changed, in message type 21:
type -> aid_type.

Version 1.17 clarifies the role of @ as a terminator in 6-bit text.

Version 1.18 notes a possible off-by-two error in the standards'
description of type 14, and noted that type 25 and 26 have not
been observed in the wild.  It also adds a more complete description
of AIS data types and some pragmatics about spare and reserved fields.

Version 1.19 adds a description of AIS Hub.

Version 1.20 adds a list of AIS feed sites - just two, so far.

Version 1.21 describes JSON-AIS more completely.  It adds descriptions
for AIS messages type 25 and 26, not yet observed in the wild.

Version 1.22 describes the problem with message length checks.
Notes on EPFD value 15 and shiptype values > 99 are added.  Added
another AIS feed. Corrections and more details on message 22.

Version 1.23 corrects some typos and numbering errors in the
description of message 19 (field widths were correct, though).
Also, AISLive no longer offers free delayed access.

Version 1.24 breaks the Type 6 and 8 application_id field into
DAC and FID and adds tables for known DAC/FID pairs and their sources.
Unspecified fields are now omitted in JSON dumps.  A new section
"AIS byte alignment, bit stuffing, and padding", reveals some
particularly black magic.

Version 1.25 adds clarifications and more message subtypes for
U.S. Coast Guard PAWSS messages.

Version 1.26 corrects an error in describing rate-of-turn decoding in
AIS Type 1, 2, and 3 messages,

Version 1.27 describes the sometime U.S. practice of omitting the leading
'3' region code from MMSIs.

Version 1.28 merges updates from IMO289, communicated by Kurt Schwehr.

Version 1.29 added much information on WMO special message formats in
types 6 and 8.

Version 1.30 was revised because M.1371-4 is now a free download.
Also, we describe "pilot plugs" and AIS message type 27.  We get
much more explicit about defaults in IMO236 and IMO289 messages.

Version 1.31 shortened some C names in the Meteorological/Hydrological
messages and fixed typos.

Version 1.32 adds descriptions of IMO Area Notice and Environmental
messages.  It adds explicit type information to the tables.

Version 1.33 notes that the home location of "NMEA Revealed" became
unstable in late 2011.

Version 1.34 fixes a typo in the MMSI 2 field offset of message 7.

Version 1.35 notes that 24A and 24B messages don't necessarily come
in neat adjacent pairs and that decoders need to handle this.

Version 1.36 corrects erroneous scale factors in the IMO289 Area
Notice and Time to Enter Port descriptions.

Version 1.36 adds breakdowns of Aid to Navigation monitoring messages
used in the UK and the Republic of Ireland.

Version 1.37 corrects a field signedness error in the IMO236 and
IMO289 Met/Hydro messages.

Version 1.38 corrects a minor bug in the description of the Navigation
Message ROT field. Adds information on AIS-SART.

Version 1.39 corrects signedness errors in the description of the Air
Temperature and Water Temperature fields in the IMO289 Weather Report
From Ship message.

Version 1.40 notes that AIS channel codes '1' and '2' may be
encountered in the wild.

Version 1.41 has been slightly amended because type 27s have started
showing up  on satellite feeds.

Version 1.42 fixes a typo in the Type 4 message description.

Version 1.43 describes the ITU-R 1371-4 breakdown of the Type 24
Vendor ID field and adds information on AIS standards for inland
waterways. Also Inland AIS standard messages are now covered.

Version 1.44 adds substantial new information on MMSIs and updates
the <<MMSI>> link, which was stale.

Version 1.45 adds a description of variant AIS talker IDs and NMEA
4.10 tag blocks.

Version 1.46 corrects an error in the specification on Inland AIS Type
10s. It also adds a bit more information on AIS tag blocks.

Version 1.47 corrects some minor field type errors in Type 18 and Type
19 speed fields.  Also corrected Type 9 radio field length.

Version 1.48 corrects two typos in the WMO289 Area Notice and Weather
Report From Ship definitions.

Version 1.49 adds a technical detail about non-AIS encapsulated
sentences.

Version 1.50 fixes a couple of typos in the ISO289 Berthing Data and
Tidal Window message descriptions.

Version 1.51 fixes an incorrect bit length in the IMO289 Hydrological
Data message.

Version 1.52 fixes many small typos. Change Gratipay to Patreon.

Version 1.53 fixes several small typos and formatting errors, adds detail
to the Types 6 and 8 Area Notice message, and adds reference to the IALA
Application-Specific Message (ASM) registry.

Version 1.54 adds information on subarea fields in IMO289 Area Notice
messages.

Version 1.55 add Table of Contents.  Markup cleanup.

Version 1.56 fix typos per Jim J. Jewett

Version 1.57 Update Type 1/2/3 Nav Statusa, and EPFD, Descriptions. Fix dead
link to <<MSGS123>>.

Version 1.58 Fix many dead links



// end
