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 MKT-3301, iTrax, Motorola OnCore, Sony CXD2951, and Ashtech/Thales devices. It can also interpret the binary protocols used by EverMore, Garmin, Navcom, Rockwell/Zodiac, SiRF, Trimble, and uBlox ANTARIS devices. It can read heading and attitude information from the Oceanserver 5000 digital compass.
The GPS protocols supported by your instance 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 instance, 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.
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 will shrink position errors by roughly a factor of four. 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). See the section called lqACCURACYrq and the section called lqFILESrq for discussion.
The program accepts the following options:
-F
-S
-b
-G
-l
-n
-N
-h
-P
-D
-V
Arguments are interpreted as the names of data sources. Normally, a data source is the name of a local serial device from which the daemon may expect GPS data.
A data source name may also be a URL pointing to a specific differential-GPS service (DGPSIP server or Ntrip broadcaster).If the URL starts with "ntrip://" Ntrip will be used; if the URL starts with "dgpsip://", DGPSIP will be used. Gpsd also defaults to DGPSIP if no protocol is defined. For Ntrip services that require authentication, a prefix of the form "username:password@" can be added before the name of the Ntrip broadcaster. If a suffix of the service name begins with ":" it is interpreted as a port number, overriding the default IANA-assigned port of 2101. For Ntrip service you also need to specify which stream to use; the stream is given in the form "/streamname". So, an example DGPSIP URL could be "dgpsip://dgpsip.example.com" and a Ntrip URL could be "ntrip://foo:bar@ntrip.example.com:80/example-stream".
Internally, the daemon maintains a device list holding the pathnames of GPSes 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 (see the section called lqGPS DEVICE MANAGEMENTrq for details on this). Daemon startup will abort with an error if neither any devices nor a control socket are specified.
At any given time, each client will be listening to only one of the GPSes on the device list. By default, a client's device is the one that most recently shipped information to the daemon at the time the client first requests GPS information.
Warning
It is planned that the command protocol described below will change radically at API version 4. It is a bad idea for applications to speak this protocol directly: rather, they should use the libgps client library and take appropriate care to conditionalize their code on the major and minor API version symbols. See the GPSD project website for more information on the protocol and API change.
The request protocol for gpsd clients is very simple. Each request normally consists of a single ASCII character followed by a newline. Case of the request character is ignored. Each request returns a line of response text ended by a CR/LF. Requests and responses are as follows, with %f standing for a decimal float numeral and %d for decimal integer numeral:
Any command other than L, F, K, W=0 or R=0 is considered a request for GPS information and will cause a GPS device to be connected to the client's channel.
a
b
The command "B=%d" sets the baud rate, not changing parity or stop bits; The command "B=%d [78] [NOE] [12]" (B followed by whiteapace; followed by a numeric speed; followed by one of the digits 7 or 8; followed by whitespace; followed by one of the letters 'N', 'O', or 'E'; followed by whitespace; followed by one of the digits '1' or 2') sets not only speed but word length, parity, and stop bits.
For both forms, watch the response, because it is possible for this to fail if the GPS does not support a speed-switching command or only supports some combinations of serial modes. In case of failure, the daemon and GPS will continue to communicate at the old speed. Both forms are rejected if more than one client is attached to the channel.
Use this command 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 serrial adaptor chip with the UART on the GPS chipset whjen 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.
(Older versions of gpsd supported changing speed only.)
c
If the driver has the capability to change sampling rate the command "C=%f" does so, setting a new cycle time in seconds. The "C=" form is rejected if more than one client is attached to the channel.
If the driver has the capability to change sampling rate, this command always returns "C=%f %f" giving the current cycle time in seconds and the minimum possible cycle time. If the driver does not have the capability to change sampling rate, this returns, as "C=%f", the cycle time in seconds only.
Either number may be fractional, indicating a GPS cycle shorter than a second; however, if >1 the cycle time must be a whole number. Also note that relatively few GPSes have the ability to set sub-second cycle times; consult your hardware protocol description to make sure this works.
This command will return "C=?" at start of session, before the first full packet has been received from the GPS, because the GPS type is not yet known. To set up conditions for a real answer, issue it after some command that reads position/velocity/time information from the device.
d
e
f
The 'f=' command may fail if the specified device name is not on the daemon's device list. This device list is initialized with the paths given on the command line, if any were specified. For security reasons, ordinary clients cannot change this device list; instead, this must be done via the daemon's local control socket declared with the -F option.
Once an 'f=' command succeeds, the client is tied to the specified device until the client disconnects.
Whether the command is 'f' or 'f=' or not, and whether it succeeds or not, the response always lists the name of the client's device.
(At protocol level 1, the F command failed if more than one client was attached, and multiple devices were not supported.)
g
This command returns either '?' if no device of the specified type(s) could be assigned, otherwise a string ('GPS' or 'RTCM104v2') identifying the kind of information the attached device returns.
(Earlier versions accepted 'RTCM104' and returned 'RTCM104' rather than 'RTCM104v2')
i
j
(At protocol level below 3, there was no J command. Note, this command is experimental and its semantics are subject to change.)
k
(At protocol level 1, there was no K command.)
l
m
n
o
tag
timestamp
time error
latitude
longitude
altitude
horizontal error estimate
vertical error estimate
course over ground
speed over ground
climb/sink
estimated error in course over ground
estimated error in speed over ground
estimated error in climb/sink
mode
p
q
Note: Older versions of gpsd reported only the first three DOP numbers, omitting time DOP and total DOP.
r
The command 'r' immediately followed by the digit '1' or the plus sign '+' sets raw mode. The command 'r' immediately followed by the digit '2' sets super-raw mode; for non-NMEA (binary) GPSes or RTCM-104 sources this dumps the raw binary packet. The command 'r' followed by the digit '0' or the minus sign '-' clears raw mode. The command 'r' with neither suffix toggles raw mode.
Note: older versions of gpsd did not support super-raw mode.
s
t
u
v
w
x
(At protocol level 1, the nonzero response was always 1.)
y
(At protocol level 1, this response had no tag or timestamp.)
z
$
Note that a response consisting of just ? following the = means that there is no valid data available. This may mean either that the device being queried is offline, or (for position/velocity/time queries) that it is online but has no fix.
Requests can be concatenated and sent as a string; gpsd will then respond with a comma-separated list of replies.
Every gpsd reply will start with the string "GPSD" followed by the replies. Examples:
query: "p\n"
reply: "GPSD,P=36.000000 123.000000\r\n"
query: "d\n"
reply: "GPSD,D=2002-11-16T02:45:05.12Z\r\n"
query: "va\n"
reply: "GPSD,V=0.000000,A=37.900000\r\n"
When clients are active but the GPS is not responding, gpsd will spin trying to open the GPS device once per second. Thus, it can be left running in background and survive having a GPS repeatedly unplugged and plugged back in. When it is properly installed along with hotplug notifier scripts feeding it device-add commands, gpsd should require no configuration or user action to find devices.
The recommended mode for clients is watcher mode. In watcher mode gpsd ships a line of data to the client each time the GPS gets either a fix update or a satellite picture, but rather than being raw NMEA the line is a gpsd 'o' or 'y' response. Additionally, watching clients get notifications in the form X=0 or X=%f when the online/offline status of the GPS changes, and an I response giving the device type when the user is assigned a device.
Clients should be prepared for the possibility that additional fields (such as heading or roll/pitch/yaw) may be added to the O command, and not treat the occurrence of extra fields as an error. The protocol number will be incremented if and when such fields are added.
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.
gpsd maintains an internal list of GPS devices. 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.
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 reponse 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 list. 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 to atmospheric signal lag than latitude/longitude, 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 will reduce UERE by a factor of 4, provided you are within about 100mi (160km) of a DGPS ground station from which you are eceiving 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, 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 50km/h (31mi/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 0.000,000,340 = 3.4×10-7 seconds. A more important limit is the GPS tick rate. While the one-per-second PPS pulses emitted by serial GPS units are timed to the GPS system's intrinsic accuracy limit,the satellites only emit navigation messages at 0.01-second intervals, and the timestamps in them only carry 0.01-second precision. Thus, the timestamps that gpsd reports in time/position/velocity messages are normally accurate only to 1/100th of a second.
gpsd can provide reference clock information to ntpd, to keep the system clock synchronized to the time provided by the GPS receiver. This facility is only available when the daemon is started from root. If you're going to use gpsd you probably want to run it -n mode so the clock will be updated even when no clients are active.
Note that deriving time from messages received from the GPS is not as accurate as you might expect. Messages are often delayed in the receiver and on the link by several hundred milliseconds, and this delay is not constant. 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.
When gpsd receives a sentence with a timestamp, it packages the received timestamp with current local time and sends it to a shared-memory segment with an ID known to ntpd, the network time synchronization daemon. If ntpd has been properly configured to receive this message, it will be used to correct the system clock.
Here is a sample ntp.conf configuration stanza telling ntpd how to read the GPS notfications:
server 127.127.28.0 minpoll 4 maxpoll 4 fudge 127.127.28.0 time1 0.420 refid GPS server 127.127.28.1 minpoll 4 maxpoll 4 prefer fudge 127.127.28.1 refid GPS1
The magic pseudo-IP address 127.127.28.0 identifies unit 0 of the ntpd shared-memory driver; 127.127.28.1 identifies unit 1. Unit 0 is used for message-decoded time and unit 1 for the (more accurate, when available) time derived from the PPS synchronization pulse. Splitting these notifications allows ntpd to use its normal heuristics to weight them.
With this configuration, ntpd will read the timestamp posted by gpsd every 16 seconds and send it to unit 0. The number after the parameter time1 is an offset in seconds. You can use it to adjust out some of the fixed delays in the system. 0.035 is a good starting value for the Garmin GPS-18/USB, 0.420 for the Garmin GPS-18/LVC.
After restarting ntpd, a line similar to the one below should appear in the output of the command "ntpq -p" (after allowing a couple of minutes):
remote refid st t when poll reach delay offset jitter ========================================================================= +SHM(0) .GPS. 0 l 13 16 377 0.000 0.885 0.882
If you are running PPS then it will look like this:
remote refid st t when poll reach delay offset jitter ========================================================================= -SHM(0) .GPS. 0 l 13 16 377 0.000 0.885 0.882 *SHM(1) .GPS1. 0 l 11 16 377 0.000 -0.059 0.006
When the value under "reach" remains zero, check that gpsd is running; and some application is connected to it or the '-n' option was used. Make sure the receiver is locked on to at least one satellite, and the receiver is in SiRF binary, Garmin binary or NMEA/PPS mode. Plain NMEA will also drive ntpd, but the accuracy as bad as one second. When the SHM(0) line does not appear at all, check the system logs for error messages from ntpd.
When no other reference clocks appear in the NTP configuration, the system clock will lock onto the GPS clock. When you have previously used ntpd, and other reference clocks appear in your configuration, there may be a fixed offset between the GPS clock and other clocks. The gpsd developers would like to receive information about the offsets observed by users for each type of receiver. Please send us the output of the "ntpq -p" command and the make and type of receiver.
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. Before doing any processing of GPS data, it tries to drop root privileges by setting its UID to "nobody" (or another userid as set by configure) 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), an 'O' 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.
If your GPS uses a SiRF chipset at firmware level 231, and it is after 31 May 2007, reported UTC time may be off by the difference between 13 seconds and whatever leap-second correction is currently applicable, from startup until complete subframe information is received (normally about six seconds). 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.
When using SiRF chips, the VDOP/TDOP/GDOP figures and associated error estimates are computed by gpsd rather than reported by the chip. The computation does not exactly match what SiRF chips do internally, which includes some satellite weighting using parameters gpsd cannot see.
Autobauding on the Trimble GPSes can take as long as 5 seconds if the device speed is not matched to the GPS speed.
If you are using an NMEA-only GPS (that is, not using SiRF or Garmin or Zodiac binary mode) and the GPS does not emit GPZDA at the start of its update cycle (which most consumer-grade NMEA GPSes do not) and it is after 2099, then the century part of the dates gpsd delivers will be wrong.
/dev/ttyS0
The official NMEA protocol standard is available on paper from the National Marine Electronics Association[1], but is proprietary and expensive; the maintainers of gpsd have made a point of not looking at it. The GPSD website[2] 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. It recognizes these with either the normal GP talker-ID prefix, or with the II prefix emitted by Seahawk Autohelm marine navigation systems, or with the IN prefix emitted by some Garmin units. It recognizes one vendor extension, the PGRME emitted by some Garmin GPS models.
Note that gpsd returns pure decimal degrees, not the hybrid degree/minute format described in the NMEA standard.
Differential-GPS corrections are conveyed by the RTCM-104 proocol. The applicable standard for RTCM-104 V2 is RTCM Recommended Standards for Differential NAVSTAR GPS Service RTCM Paper 194-93/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.
gps(1), libgps(3), libgpsd(3), gpsprof(1), gpsfake(1), gpsctl(1), gpscat(1), rtcm-104(5).
Remco Treffcorn, Derrick Brashear, Russ Nelson, Eric S. Raymond, Chris Kuethe. This manual page by Eric S. Raymond <esr@thyrsus.com>. There is a project site at here[2].