gpsd(8)

gpsd - interface daemon for GPS receivers

gpsd [OPTIONS]

gpsd -h

gpsd -V

If you have a GPS attached on the lowest-numbered USB port of a Linux system, and want to read reports from it on TCP/IP port 2947, it will normally suffice to do this, as root:

For the lowest-numbered serial port:

gpsd may be started as a normal user, or by using sudo, but some functionality will be lost.

Change the device number as appropriate if you need to use a different port. Command-line flags enable verbose logging, a control port, and other optional extras but should not be needed for basic operation; the one exception, on very badly designed hardware, might be -b (which see).

On Linux systems supporting udev, gpsd is normally started automatically when a USB plugin event fires (if it is not already running) and is handed the name of the newly active device. In that case no invocation is required at all.

For your initial tests set your GPS hardware to speak NMEA, as gpsd is guaranteed to be able to process that. If your GPS has a native or binary mode with better performance that gpsd knows how to speak, gpsd may autoconfigure that mode.

You can verify correct operation by first starting gpsd. Then xgps, the X windows test client or cgps the curses based terminal client.

If you have problems, the GPSD project maintains helpful information online. Some links are at the end of this man page.

gpsd is a monitor daemon that collects information from GPSes, differential-GPS radios, or AIS receivers attached to the host machine. Each GPS, DGPS radio, or AIS receiver is expected to be directly connected to the host via a USB or RS232C serial device. The serial device may be specified to gpsd at startup, or it may be set via a command shipped down a local control socket (e.g. by a USB hotplug script). Given a GPS device by either means, gpsd discovers the correct port speed and protocol for it.

gpsd should be able to query any GPS that speaks either the standard textual NMEA 0183 protocol, or the (differing) extended NMEA dialects used by MTK-3301, iTrax, Motorola OnCore, Sony CXD2951, Ashtech/Thales and some other devices. It can also interpret the binary protocols used by EverMore, Garmin, Javad, Navcom, Rockwell/Zodiac, SiRF, Trimble, and u-blox devices. Under Linux it can read NMEA2000 packets through the kernel CAN socket. It can read heading and attitude information from the TNT Revolution digital compasses.

The GPS reporting formats supported by your binary of gpsd may differ depending on how it was compiled; general-purpose versions support many, but it can be built with protocol subsets down to a singleton for use in constrained environments. For a list of the GPS protocols supported by your binary, see the output of gpsd -l

gpsd effectively hides the differences among the GPS types it supports. It also knows about and uses commands that tune these GPSes for lower latency. By using gpsd as an intermediary, applications avoid contention for serial devices.

gpsd can use differential-GPS corrections from a DGPS radio or over the net, from a ground station running a DGPSIP server or a Ntrip broadcaster that reports RTCM-104 data; this may shrink position errors. When gpsd opens a serial device emitting RTCM-104, it automatically recognizes this and uses the device as a correction source for all connected GPSes that accept RTCM corrections (this is dependent on the type of the GPS; not all GPSes have the firmware capability to accept RTCM correction packets).

Client applications typically communicate with gpsd via a TCP/IP port, port 2947 by default. Both IPv4 and IPv6 connections are supported and a client may connect via either.

The program accepts the following options:

Arguments are interpreted as the names of data sources. Normally, a data source is the device pathname of a local device from which the daemon may expect GPS data. But there are three other special source types recognized, for a total of four:

(The "ais:://" source type supported in some older versions of the daemon has been retired in favor of the more general "tcp://".)

Additionally, two serial device name have a side effect, if your binary was compiled with the MAGIC_HAT option:

Note, however, that if /dev/pps0 is the fake "ktimer" PPS, then /dev/pps1 will be used instead.

Internally, the daemon maintains a device pool holding the pathnames of devices and remote servers known to the daemon. Initially, this list is the list of device-name arguments specified on the command line. That list may be empty, in which case the daemon will have no devices on its search list until they are added by a control-socket command. Daemon startup will abort with an error if no devices and no control socket is specified.

When a device is activated (i.e. a client requests data from it), gpsd attempts to execute a hook from /etc/gpsd/device-hook with first command line argument set to the pathname of the device and the second to ACTIVATE. On deactivation, it does the same passing DEACTIVATE for the second argument.

gpsd can export data to client applications in three ways: via a sockets interface, via a shared-memory segment, and via D-Bus. The next three major sections describe these interfaces.

When gpsd is running as a daemon (not in the foreground) is sends all of its logging to syslog(3). Logging is sent with ident "gpsd" using facility "LOG_USER".

If you are using syslog-ng, then you can send all gpsd logs to a file /var/log/gpsd. Put this at the end of your /etc/syslog-ng/syslog-ng.conf file:

Clients may communicate with the daemon via textual request and responses over a socket. It is a bad idea for applications to speak the protocol directly: rather, they should use the libgps client library and take appropriate care to conditionalize their code on the major and minor protocol version symbols.

The request-response protocol for the socket interface is fully documented in gpsd_json(5).

gpsd has two other (read-only) interfaces.

Whenever the daemon recognizes a packet from any attached device, it writes the accumulated state from that device to a shared memory segment. The C and C++ client libraries shipped with GPSD can read this segment. Client methods, and various restrictions associated with the read-only nature of this interface, are documented at libgps(3). The shared-memory interface is intended primarily for embedded deployments in which gpsd monitors a single device, and its principal advantage is that a daemon instance configured with shared memory but without the sockets interface loses a significant amount of runtime weight.

The daemon may be configured to emit a D-Bus signal each time an attached device delivers a fix. The signal path is "path /org/gpsd", the signal interface is "org.gpsd", and the signal name is "fix". The signal payload layout is as follows:

Uncertainty values are provided by the GNSS receiver. Check your receiver documentation to see if is specifies what its "uncertainty" means.

gpsd maintains an internal list of GPS devices (the "device pool"). If you specify devices on the command line, the list is initialized with those pathnames, otherwise the list starts empty. Commands to add and remove GPS device paths from the daemon’s device list must be written to a local Unix-domain socket which will be accessible only to programs running as root. This control socket will be located wherever the -F option specifies it.

A device may will also be dropped from the pool if GPSD gets a zero length read from it. This end-of-file condition indicates that the device has been disconnected.

When gpsd is installed along with working hotplug notifier scripts feeding it device-add commands over the control socket, gpsd should require no configuration or user action to find hotplug devices.

Sending SIGHUP to a running gpsd forces it to close all GPSes and all client connections. It will then attempt to reconnect to any GPSes on its device list and resume listening for client connections. This may be useful if your GPS enters a wedged or confused state but can be soft-reset by pulling down DTR.

When gpsd is called with no initial devices (thus, expecting devices to be passed to it by notifications to the control socket), and reaches a state where there are no devices connected and no subscribers after some devices have been seen, it shuts down gracefully. It is expected that future device hotplug events will reactivate it.

To point gpsd at a device that may be a GPS, write to the control socket a plus sign ('+') followed by the device name followed by LF or CR-LF. Thus, to point the daemon at /dev/foo. send "+/dev/foo\n". To tell the daemon that a device has been disconnected and is no longer available, send a minus sign ('-') followed by the device name followed by LF or CR-LF. Thus, to remove /dev/foo from the search list, send "-/dev/foo\n".

To send a control string to a specified device, write to the control socket a '!', followed by the device name, followed by '=', followed by the control string.

To send a binary control string to a specified device, write to the control socket a '&', followed by the device name, followed by '=', followed by the control string in paired hex digits.

Your client may await a response, which will be a line beginning with either "OK" or "ERROR". An ERROR response to an 'add' command means the device did not emit data recognizable as GPS packets, an ERROR response to a remove command means the specified device was not in gpsd's device pool. An ERROR response to a '!' command means the daemon did not recognize the devicename specified.

The control socket is intended for use by hotplug scripts and other device-discovery services. This control channel is separate from the public gpsd service port, and only locally accessible, in order to prevent remote denial-of-service and spoofing attacks.

The base User Estimated Range Error (UERE) of GPSes is 8 meters or less at 66% confidence, 15 meters or less at 95% confidence. Actual horizontal error will be UERE times a dilution factor dependent on current satellite position. Altitude determination is more sensitive to variability in ionospheric signal lag than latitude/longitude is, and is also subject to errors in the estimation of local mean sea level, base error is 12 meters at 66% confidence, 23 meters at 95% confidence. Again, this will be multiplied by a vertical dilution of precision (VDOP) dependent on satellite geometry, and VDOP is typically larger than HDOP. Users should not rely on GPS altitude for life-critical tasks such as landing an airplane.

These errors are intrinsic to the design and physics of the GPS system. gpsd does its internal computations at sufficient accuracy that it will add no measurable position error of its own.

DGPS correction may reduce UERE, provided you are within about 100 miles (160 km) of a DGPS ground station from which you are receiving corrections.

On a 4800bps connection, the time latency of fixes provided by gpsd will be one second or less 95% of the time. Most of this lag is due to the fact that GPSes normally emit fixes once per second, thus expected latency is 0.5sec. On the personal-computer hardware available in 2005 and later, computation lag induced by gpsd will be negligible, on the order of a millisecond. Nevertheless, latency can introduce significant errors for vehicles in motion, at 50 km/h (31 mi/h) of speed over ground, 1 second of lag corresponds to 13.8 meters change in position between updates.

The time reporting of the GPS system itself has an intrinsic accuracy limit of 14 nanoseconds, but this can only be approximated by specialized receivers using that send the high-accuracy PPS (Pulse-Per-Second) over RS232 to cue a clock crystal. Most GPS receivers only report time to a precision of 0.01s or 0.001s, and with no accuracy guarantees below 1sec.

If your GPS uses a SiRF chipset at firmware level 231, reported UTC time may be off by the difference between whatever default leap-second offset has been compiled in and whatever leap-second correction is currently applicable, from startup until complete subframe information is received. Firmware levels 232 and up don’t have this problem. You may run gpsd at debug level 4 to see the chipset type and firmware revision level.

There are exactly two circumstances under which gpsd relies on the host-system clock:

In the GPS broadcast signal, GPS time is represented using a week number that rolls over after 2^10 or 2^13 weeks (about 19.6 years, or 157 years), depending on the spacecraft. Receivers are required to disambiguate this to the correct date, but may have difficulty due to not knowing time to within half this interval, or may have bugs. Users have reported incorrect dates which appear to be due to this issue. gpsd uses the startup time of the daemon detect and compensate for rollovers while it is running, but otherwise reports the date as it is reported by the receiver without attempting to correct it.

If you are using an NMEA-only GPS (that is, not using SiRF or Garmin or Zodiac binary mode), gpsd relies on the system clock to tell it the current century. If the system clock returns an invalid value near zero, and the GPS does not emit GPZDA at the start of its update cycle (which most consumer-grade NMEA GPSes do not) then the century part of the dates gpsd delivers may be wrong. Additionally, near the century turnover, a range of dates as wide in seconds as the accuracy of your system clock may be referred to the wrong century.

gpsd can provide reference clock information to ntpd or chronyd, to keep the system clock synchronized to the time provided by the GPS receiver.

On Linux, gpsd includes support for interpreting the PPS pulses emitted at the start of every clock second on the carrier-detect lines of some serial GPSes, this pulse can be used to update NTP at much higher accuracy than message time provides. You can determine whether your GPS emits this pulse by running at -D 5 and watching for carrier-detect state change messages in the logfile. In addition, if your kernel provides the RFC 2783 kernel PPS API then gpsd will use that for extra accuracy.

gpsd communicates with ntpd using shared memory segments (SHMs). The SHMs are numbered SHM(0) to SHM(7). and have a name (key) of NTP0 to NTP7. The first local time source on the command line may use NTP0 and NTP1. The second may use NTP2 and NTP3, etc.

You can see the output of gpsd to ntpd in real time with the ntpshmmon command.

Other daemons, such as ptp4l may also be using the same SHMs to talk to ntpd.

Detailed instructions for using GPSD to set up a high-quality time service can be found among the documentation on the GPSD website.

On operating systems that support D-BUS, gpsd can be built to broadcast GPS fixes to D-BUS-aware applications. As D-BUS is still at a pre-1.0 stage, we will not attempt to document this interface here. Read the gpsd source code to learn more.

gpsd, if given the -G flag, will listen for connections from any reachable host, and then disclose the current position. Before using the -G flag, consider whether you consider your computer’s location to be sensitive data to be kept private or something that you wish to publish.

gpsd must start up as root in order to open the NTPD shared-memory segment, open its logfile, and create its local control socket. Note that starting gpsd with sudo is not the same as starting as root. Before doing any processing of GPS data, it tries to drop root privileges by setting its UID to "nobody" (or another configured userid) and its group ID to the group of the initial GPS passed on the command line — or, if that device doesn’t exist, to the group of /dev/ttyS0.

Privilege-dropping is a hedge against the possibility that carefully crafted data, either presented from a client socket or from a subverted serial device posing as a GPS, could be used to induce misbehavior in the internals of gpsd. It ensures that any such compromises cannot be used for privilege elevation to root.

The assumption behind gpsd's particular behavior is that all the tty devices to which a GPS might be connected are owned by the same non-root group and allow group read/write, though the group may vary because of distribution-specific or local administrative practice. If this assumption is false, gpsd may not be able to open GPS devices in order to read them (such failures will be logged).

In order to fend off inadvertent denial-of-service attacks by port scanners (not to mention deliberate ones), gpsd will time out inactive client connections. Before the client has issued a command that requests a channel assignment, a short timeout (60 seconds) applies. There is no timeout for clients in watcher or raw modes; rather, gpsd drops these clients if they fail to read data long enough for the outbound socket write buffer to fill. Clients with an assigned device in polling mode are subject to a longer timeout (15 minutes).

If multiple NMEA talkers are feeding RMC, GLL, and GGA sentences to the same serial device (possible with an RS422 adapter hooked up to some marine-navigation systems), a 'TPV' response may mix an altitude from one device’s GGA with latitude/longitude from another’s RMC/GLL after the second sentence has arrived.

gpsd may change control settings on your GPS (such as the emission frequency of various sentences or packets) and not restore the original settings on exit. This is a result of inadequacies in NMEA and the vendor binary GPS protocols, which often do not give clients any way to query the values of control settings in order to be able to restore them later.

Some receivers do not report VDOP/TDOP/GDOP figures and associated error estimates. In that case they may be computed by gpsd instead. This computation does not exactly match what chips do internally, which includes some satellite weighting using parameters gpsd cannot see.

Autobauding on the Trimble GPSes can take as long as 20 seconds, or more, if the device speed is not matched to the GPS speed. Use the -s option to avoid autobaud delays.

Generation of position error estimates (eph, epv, epd, eps, epc) from the incomplete data handed back by GPS reporting protocols involves both a lot of mathematical black art and fragile device-dependent assumptions. This code has been bug-prone in the past and problems may still lurk there.

AIDVM decoding of types 16-17, 22-23, and 25-26 is unverified.

GPSD presently fully recognizes only the 2.1 level of RTCM2 (message types 1, 3, 4, 5, 6, 7, 9, 16). The 2.3 message types 13, 14, and 31 are recognized and reported. Message types 8, 10-12, 15-27, 28-30 (undefined), 31-37, 38-58 (undefined), and 60-63 are not yet supported.

The ISGPS used for RTCM2 and subframes decoder logic is sufficiently convoluted to confuse some compiler optimizers, notably in GCC 3.x at -O2, into generating bad code.

Devices meant to use PPS for high-precision timekeeping may fail if they are specified after startup by a control-socket command, as opposed to on the daemon’s original command line. Root privileges are dropped early, and some Unix variants require them in order to set the PPS line discipline. Under Linux the POSIX capability to set the line discipline is retained, but other platforms cannot use this code.

USB GPS devices often do not identify themselves through the USB subsystem; they typically present as the class 00h (undefined) or class FFh (vendor-specific) of USB-to-serial adapters. Because of this, the Linux hotplug scripts must tell gpsd to sniff data from every USB-to-serial adapter that goes active and is known to be of a type used in GPSes. No such device is sent configuration strings until after it has been identified as a GPS, and gpsd never opens a device that is opened by another process. But there is a tiny window for non-GPS devices not opened; if the application that wants them loses a race with GPSD its device open will fail and have to be retried after GPSD sniffs the device (normally less than a second later).

By setting the environment variable GPSD_SHM_KEY, you can control the key value used to create the shared-memory segment used for communication with the client library. This will be useful mainly when isolating test instances of gpsd from production ones.

The official NMEA protocol standards for NMEA0183 and NMEA2000 are available from the National Marine Electronics Association, but are proprietary and expensive; the maintainers of gpsd have made a point of not looking at them. The GPSD project website links to several documents that collect publicly disclosed information about the protocol.

gpsd parses the following NMEA sentences: RMC, GGA, GLL, GSA, GSV, VTG, ZDA, GBS, HDT, DBT, GST. It recognizes these with either the normal GP talker-ID prefix, or with the GN prefix used by GLONASS, or with the II prefix emitted by Seahawk Autohelm marine navigation systems, or with the IN prefix emitted by some Garmin units, or with the EC prefix emitted by ECDIS units, or with the SD prefix emitted by depth sounders, or with the HC and TI prefix emitted by some Airmar equipment. It recognizes some vendor extensions: the PGRME emitted by some Garmin GPS models, the PTNTHTM emitted by True North digital compasses, the PMTK omitted by some San Jose Navigation GPSes, and the PASHR sentences emitted by some Ashtech GPSes.

Note that gpsd JSON returns pure decimal degrees, not the hybrid degree/minute format described in the NMEA standard.

Differential-GPS corrections are conveyed by the RTCM protocols. The applicable standard for RTCM-104 V2 is RTCM Recommended Standards for Differential GNSS (Global Navigation Satellite) Service RTCM Paper 136-2001/SC 104-STD. The applicable standard for RTCM-104 V3 is RTCM Standard 10403.1 for Differential GNSS Services - Version 3 RTCM Paper 177-2006-SC104-STD. Ordering instructions for the RTCM standards are accessible from the website of the Radio Technical Commission for Maritime Services under "Publications".

AIS is defined by ITU Recommendation M.1371, Technical Characteristics for a Universal Shipborne Automatic Identification System Using Time Division Multiple Access. The AIVDM/AIVDO format understood by this program is defined by IEC-PAS 61162-100, Maritime navigation and radiocommunication equipment and systems. A more accessible description of both can be found at AIVDM/AIVDO Protocol Decoding, on the references page of the GPSD project website.

Subframe data is defined by IS-GPS-200, GLOBAL POSITIONING SYSTEM WING (GPSW) SYSTEMS ENGINEERING & INTEGRATION, INTERFACE SPECIFICATION IS-GPS-200. The format understood by this program is defined in Section 20 (Appendix II),

JSON is specified by RFC 7159, The JavaScript Object Notation (JSON) Data Interchange Format.

The API for PPS time service is specified by RFC 2783, Pulse-Per-Second API for UNIX-like Operating Systems, Version 1.0

Authors: Eric S. Raymond, Chris Kuethe, Gary Miller. Former authors whose bits have been plowed under by code turnover: Remco Treffcorn, Derrick Brashear, Russ Nelson.

This manual page by Eric S. Raymond <esr@thyrsus.com>.

gpsctl(1), gps(1), gpsprof(1), gpsfake(1). gpscat(1), *ntpshmmon(1),

libgps(3), libgpsmm(3)

gpsd_json(5)

chronyd(8), gpsd(8), gpsdctl(8), ntpd(8)

Project web site: https://gpsd.io/ GPSD FAQ: https://gpsd.io/faq.html GPSD Time Service HOWTO: https://gpsd.io/gpsd-time-service-howto.html Introduction to Time Service: https://gpsd.io/time-service-intro.html

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