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GPSCTL(1) GPSD Documentation GPSCTL(1)


gpsctl - control the modes of a GPS


gpsctl [-? | --binary | --nmea | --reset] [--debug LVL] [--direct] [--echo] [--help] [--list] [--rate] [--rmshm] [--rate rate] [--ship control] [--speed speed] [--timeout devicetype] [--version] [-b | -n | -r] [-D LVL] [-c rate] [-e] [-f] [-h] [-l] [-R] [-s speed] [-t devicetype] [-V] [-x control] [serial-port]


gpsctl can switch a dual-mode GPS 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 GPS 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 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'.

The program accepts the following options:

-?, -h, --help

Display program usage and exit.

-b, --binary

Put the GPS into native (binary) mode.

-c RATE, --rate RATE

Change the GPS's cycle time. Units are seconds. Note, most GPSes 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 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 GPS into NMEA mode.

-r, --reset

Reset the GPS. 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 GPS emits packets.

Use this 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.


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 a specified control string to the GPS; 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 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. C-style backslash escapes in the string, notably \xNN for hex, will be interpreted; additionally, \e will be replaced with ESC. This switch implies -f.

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. Multiple options will be executed in this order: mode changes (-b and -n) first, speed changes (-s) second, and control-string sends (-c) last.


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.


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.


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.


gpsd(8), gpsdctl(8), gps(1), libgps(3), libgpsmm(3), gpsprof(1), gpsfake(1).


Eric S. Raymond <>.

6 December 2020 The GPSD Project