= gpsctl(1)
:author: Eric S. Raymond
:date: 11 August 2024
:email: <esr@thyrsus.com.>
:keywords: gps, gpsd, gpsctl
:manmanual: GPSD Documentation
:mansource: GPSD, Version {gpsdver}
:robots: index,follow
:sectlinks:
:toc: macro
:type: manpage

include::../www/inc-menu.adoc[]

== NAME

gpsctl - control the modes of a GNSS receiver

== SYNOPSIS

*gpsctl* [OPTIONS] [serial-port]

*gpsctl* -h

*gpsctl* -V

== DESCRIPTION

*gpsctl* can switch a dual-mode GNSS receiver between NMEA and
vendor-binary modes.  It can also be used to set the device baud
rate. Note: Not all devices have these capabilities.

If you have only one GNSS receiver attached to your machine, and *gpsd*
is running, it is not necessary to specify the device; *gpsctl* does its
work through *gpsd*, which will locate it for you.

When *gpsd* is running, *gpsctl* may be run as any user, or as root.

When *gpsd* is not running, the device specification is required, and you
will need to be running as root or be a member of the device's owning
group in order to have write access to the device. On many Unix variants
the owning group will be named 'dialout'.

Running under *sudo* will cause some loss of functionality.

== OPTIONS

The program accepts the following options:

*-?*, *-h*, *--help*::
  Display program usage and exit.
*-b*, *--binary*::
  Put the GNSS receiver into native (binary) mode.
*-c RATE*, *--rate RATE*::
  Change the receivers's cycle time. Units are seconds. Note, most
  receivers have a fixed cycle time of 1 second.

*-D LVL*, *--debug LVL*::
  Set level of debug messages.
*-e*, *--echo*::
  Generate the packet from any other arguments specified and ship it to
  standard output instead of the device. This switch can be used with
  the *-t* option without specifying a device. Note: the packet data
  for a binary prototype will be raw, not ASCII-ized in any way.
*-f*, *--force*::
  Force low-level, direct, access (not through the daemon).
*-l*, *--list*::
  List a table showing which option switches can be applied to which
  device types, and exit.
*-n*, *--nmea*::
  Put the GNSS receiver into NMEA mode.
*-r*, *--reset*::
  Reset the GNSS receiver. Device port and type must be specified.
*-R*, *--rmshm*::
  Remove the GPSD shared-memory segment used for SHM export. This option
  will normally only be of interest to GPSD developers.
*-s SPEED*, *--speed SPEED*::
  Set the baud rate at which the receiver emits packets.

Use the *-s* option with caution. On USB and Bluetooth GPSes it is also
possible for serial mode setting to fail either because the serial
adaptor chip does not support non-8N1 modes or because the device
firmware does not properly synchronize the serial adaptor chip with
the UART on the GPS chipset when the speed changes. These failures can
hang your device, possibly requiring a GPS power cycle or (in extreme
cases) physically disconnecting the NVRAM backup battery.

*-t TYPE*, *--type TYPE*::
  Force the device type.
*-T TIMEOUT*, *--timeout TIMEOUT*::
  Change the sampling timeout. Defaults to 8 seconds, which should
  always be sufficient to get an identifying packet from a device
  emitting at the normal rate of 1 per second.
*-V*, *--version*::
  Display program version and exit.
*-x STR*, *--ship STR*::
  Send the specified control string to the GNSS receiver. C-style
  backslash escapes in the string are decoded.  Use \xNN for hex,
  \e will be replaced with ESC.

  In normal mode, through _gpsd_, the decoded string is passed through,
  unchanged top _gpsd_ which in turns sends it to the receiver.  Headers,
  checksums, and suffffices must be provided.

  In low-level, (direct) mode *gpsctl* will provide packet headers and
  trailers and checksum as appropriate for binary packet types, and
  whatever checksum and trailer is required for text packet types.

  You must include the leading $ for NMEA packets.

  When sending to a UBX, Allystar or CASIC device, the first two bytes
  of the string supplied will become the message class and type, and the
  remainder the payload.

  When sending to a Navcom NCT or Trimble TSIP device, the first byte is
  interpreted as the command ID and the rest as payload.

  When sending to a Zodiac device, the first two bytes are used as a
  message ID of type little-endian short, and the remainder as payload
  in byte pairs interpreted as little-endian short.

  For all other supported binary GPSes (notably including SiRF) the
  string is taken as the entire message payload and wrapped with
  appropriate header, trailer and checksum bytes.

The argument of the forcing option, *-t*, should be a string which is
contained in exactly one of the known driver names; for a list, do
*gpsctl -l*.

Forcing the device type behaves somewhat differently depending on
whether this tool is going through the daemon or not. In high-level
mode, if the device that daemon selects for you doesn't match the driver
you specified, *gpsctl* exits with a warning. (This may be useful in
scripts.)

In low-level mode, if the device identifies as a Generic NMEA, use the
selected driver instead. This will be useful if you have a GPS device of
known type that is in NMEA mode and not responding to probes. (This
option was originally implemented for talking to SiRFStar I chips, which
don't respond to the normal SiRF ID probe.)

If no options are given, the program will display a message identifying
the GPS type of the selected device and exit.

Reset (*-r*) operations must stand alone; others can be combined.
gpsctl will execute multiple options in this order: mode change (-b
or -n) first, speed changes (-s) second, cycle rate (-c) third and
control strings (-x) last.

[[environment]]
== ENVIRONMENT VARIABLES

By setting the environment variable *GPSD_SHM_KEY*, you can control
the key value used to designate the shared-memory segment removed with
the -R option. This will be useful mainly when isolating test instances
of *gpsd* from production ones.

== EXAMPLES

*gpsctl /dev/ttyUSB0*::
  Attempt to identify the device on USB serial device 0. Time out after
  the default number of seconds. Adding the *-f* will force low-level
  access and suppress the normal complaint when this tool can't find a
  GPSD to work through.

*gpsctl -f -n -s 9600 /dev/ttyUSB0*::
  Use low-level operations (not going through a *gpsd* instance) to switch
  a GPS to NMEA mode at 9600bps. The tool will identify the GPS type
  itself.

*gpsctl -x '\xb5\x62\x0a\x04\x00\x00\x0e\x34'*::
  Send a request for UBX-MON-VER to a _gpsd_ connected GNSS receiver.

== BUGS

SiRF GPSes can only be identified by the success of an attempt to flip
them into SiRF binary mode. Thus, the process of probing one of these
running in NMEA will change its behavior.

Baud rate and mode changes work in direct mode but are not reliable in
client mode. This will be fixed in a future release.

== RETURN VALUES

*0*:: on success.
*1*:: on failure

== SEE ALSO

*gpsd*(8), *gpsdctl*(1), *gps*(1), *ubxtool*(1), *zerk*(1)

== RESOURCES

*Project web site:* {gpsdweb}

== COPYING

This file is Copyright by the GPSD project +
SPDX-License-Identifier: BSD-2-clause
