1GPSD(8) GPSD Documentation GPSD(8)
2
6 gpsd - interface daemon for GPS receivers
7
9 gpsd [OPTIONS]
10 gpsd -h
12 gpsd -V
14
16 If you have a GPS attached on the lowest-numbered USB port of a Linux
17 system, and want to read reports from it on TCP/IP port 2947, it will
18 normally suffice to do this, as root:
19 # gpsd /dev/ttyUSB0
21 For the lowest-numbered serial port:
23 # gpsd /dev/ttyS0
25 gpsd may be started as a normal user, or by using sudo, but some
27 functionality will be lost.
28 Change the device number as appropriate if you need to use a different
30 port. Command-line flags enable verbose logging, a control port, and
31 other optional extras but should not be needed for basic operation; the
32 one exception, on very badly designed hardware, might be -b (which
33 see).
34 On Linux systems supporting udev, gpsd is normally started
36 automatically when a USB plugin event fires (if it is not already
37 running) and is handed the name of the newly active device. In that
38 case no invocation is required at all.
39 For your initial tests set your GPS hardware to speak NMEA, as gpsd is
41 guaranteed to be able to process that. If your GPS has a native or
42 binary mode with better performance that gpsd knows how to speak, gpsd
43 may autoconfigure that mode.
44 You can verify correct operation by first starting gpsd. Then xgps, the
46 X windows test client or cgps the curses based terminal client.
47 If you have problems, the GPSD project maintains helpful information
49 online. Some links are at the end of this man page.
50
52 gpsd is a monitor daemon that collects information from GPSes,
53 differential-GPS radios, or AIS receivers attached to the host machine.
54 Each GPS, DGPS radio, or AIS receiver is expected to be directly
55 connected to the host via a USB or RS232C serial device. The serial
56 device may be specified to gpsd at startup, or it may be set via a
57 command shipped down a local control socket (e.g. by a USB hotplug
58 script). Given a GPS device by either means, gpsd discovers the correct
59 port speed and protocol for it.
60 gpsd should be able to query any GPS that speaks either the standard
62 textual NMEA 0183 protocol, or the (differing) extended NMEA dialects
63 used by MKT-3301, iTrax, Motorola OnCore, Sony CXD2951, Ashtech/Thales
64 and some other devices. It can also interpret the binary protocols used
65 by EverMore, Garmin, Javad, Navcom, Rockwell/Zodiac, SiRF, Trimble, and
66 u-blox devices. Under Linux it can read NMEA2000 packets through the
67 kernel CAN socket. It can read heading and attitude information from
68 the Oceanserver 5000 or TNT Revolution digital compasses.
69 The GPS reporting formats supported by your binary of gpsd may differ
71 depending on how it was compiled; general-purpose versions support
72 many, but it can be built with protocol subsets down to a singleton for
73 use in constrained environments. For a list of the GPS protocols
74 supported by your binary, see the output of gpsd -l
75 gpsd effectively hides the differences among the GPS types it supports.
77 It also knows about and uses commands that tune these GPSes for lower
78 latency. By using gpsd as an intermediary, applications avoid
79 contention for serial devices.
80 gpsd can use differential-GPS corrections from a DGPS radio or over the
82 net, from a ground station running a DGPSIP server or a Ntrip
83 broadcaster that reports RTCM-104 data; this may shrink position
84 errors. When gpsd opens a serial device emitting RTCM-104, it
85 automatically recognizes this and uses the device as a correction
86 source for all connected GPSes that accept RTCM corrections (this is
87 dependent on the type of the GPS; not all GPSes have the firmware
88 capability to accept RTCM correction packets).
89 Client applications typically communicate with gpsd via a TCP/IP port,
91 port 2947 by default. Both IPv4 and IPv6 connections are supported and
92 a client may connect via either.
93
95 The program accepts the following options:
96 -?, -h, ---help
98 Display help message and terminate.
99 -b, --readonly
101 Broken-device-safety mode, otherwise known as read-only mode. A few
102 bluetooth and USB receivers lock up or become totally inaccessible
103 when probed or reconfigured; see the hardware compatibility list on
104 the GPSD project website for details. This switch prevents gpsd
105 from writing to a receiver. This means that gpsd cannot configure
106 the receiver for optimal performance, but it also means that gpsd
107 cannot break the receiver. A better solution would be for Bluetooth
108 to not be so fragile. A platform independent method to identify
109 serial-over-Bluetooth devices would also be nice.
110 -D LVL, --debug LVL
112 Set debug level. Default is 0. At debug levels 2 and above, gpsd
113 reports incoming sentence and actions to standard error if gpsd is
114 in the foreground (-N) or to syslog if in the background. See
115 LOGGING below.
116 -F FILE, --sockfile FILE
118 Create a control socket for device addition and removal commands.
119 Default is None. You must specify a valid pathname on your local
120 filesystem; this will be created as a Unix-domain socket to which
121 you can write commands that edit the daemon’s internal device list.
122 -f FRAME, --framing FRAME
124 Fix the framing to the GNSS device. The framing parameter is of the
125 form: [78][ENO][012]. Most GNSS are 8N1. Some Trimble default to
126 8O1. The default is to search for the correct framing.
127 -G, --listenany
129 This flag causes gpsd to listen on all addresses (INADDR_ANY)
130 rather than just the loop back (INADDR_LOOPBACK) address. For the
131 sake of privacy and security, gpsd information is private by
132 default to the local machine until the user makes an effort to
133 expose this to the world.
134 -l, --drivers
136 List all drivers compiled into this gpsd instance. The letters to
137 the left of each driver name are the gpsd control commands
138 supported by that driver. Then exit.
139 -n, --nowait
141 Don’t wait for a client to connect before polling whatever GPS is
142 associated with it. Some RS232 GPSes wait in a standby mode
143 (drawing less power) when the host machine is not asserting DTR,
144 and some cellphone and handheld embedded GPSes have similar
145 behaviors. Accordingly, waiting for a watch request to open the
146 device may save battery power. (This capability is rare in
147 consumer-grade devices). You should use this option if you plan to
148 use gpsd to provide reference clock information to ntpd or chronyd.
149 This option will also enable clients to see data from the receiver
150 sooner on connection.
151 -N, --foreground
153 Don’t daemonize; run in foreground. This switch is mainly useful
154 for debugging.
155 -p, --passive
157 Passive mode. Do not autoconfigure the receiver, but allow manual
158 configuration changes.
159 -P FILE, --pidfile FILE
161 Specify the name and path to record the daemon’s process ID.
162 -r, --badtime
164 Use GPS time even with no current fix. Some GPSs have battery
165 powered Real Time Clocks (RTC’s) built in, making them a valid time
166 source even before a fix is acquired. This can be useful on a
167 Raspberry Pi, or other device that has no battery powered RTC, and
168 thus has no valid time at startup. Use with caution.
169 --port PORT, -S PORT
171 Set TCP/IP port on which to listen for GPSD clients (default is
172 2947).
173 -s SPEED, --speed SPEED
175 Fix the serial port speed to the GNSS device. Allowed speeds are:
176 4800, 9600, 19200, 38400, 57600, 115200, 230400, 460800 and 921600.
177 The default is to autobaud. Note that some devices with integrated
178 USB ignore port speed.
179 -V, --version
181 Dump version and exit.
182 Arguments are interpreted as the names of data sources. Normally, a
184 data source is the device pathname of a local device from which the
185 daemon may expect GPS data. But there are three other special source
186 types recognized, for a total of four:
187 Local serial or USB device
189 A normal Unix device name of a serial or USB device to which a
190 sensor is attached. Examples: /dev/ttyS0, or /dev/ttyUSB0.
191 Local PPS device
193 A normal Unix device name of a PPS device to which a PPS source is
194 attached. The device name must start with "/dev/pps" and a local
195 serial or USB GPS device must also be available. Example:
196 /dev/pps0.
197 TCP feed
199 A URI with the prefix "tcp://", followed by a hostname, a colon,
200 and a port number. The daemon will open a socket to the indicated
201 address and port and read data packets from it, which will be
202 interpreted as though they had been issued by a serial device.
203 Example: tcp://data.aishub.net:4006.
204 UDP feed
206 A URI with the prefix "udp://", followed by a hostname, a colon,
207 and a port number. The daemon will open a socket listening for UDP
208 datagrams arriving in the indicated address and port, which will be
209 interpreted as though they had been issued by a serial device.
210 Example: udp://127.0.0.1:5000.
211 Ntrip caster
213 A URI with the prefix "ntrip://" followed by the name of an Ntrip
214 caster (Ntrip is a protocol for broadcasting differential-GPS fixes
215 over the net). For Ntrip services that require authentication, a
216 prefix of the form "username:password@" can be added before the
217 name of the Ntrip broadcaster. For Ntrip service, you must specify
218 which stream to use; the stream is given in the form "/streamname".
219 An example DGPSIP URI could be "dgpsip://dgpsip.example.com" and a
220 Ntrip URI could be
221 "ntrip://foo:bar@ntrip.example.com:80/example-stream". Corrections
222 from the caster will be sent to each attached GPS with the
223 capability to accept them.
224 DGPSIP server
226 A URI with the prefix "dgpsip://" followed by a hostname, a colon,
227 and an optional colon-separated port number (defaulting to 2101).
228 The daemon will handshake with the DGPSIP server and read RTCM2
229 correction data from it. Corrections from the server will be set to
230 each attached GPS with the capability to accept them. Example:
231 dgpsip://dgps.wsrcc.com:2101.
232 Remote gpsd feed
234 A URI with the prefix "gpsd://", followed by a hostname and
235 optionally a colon and a port number (if the port is absent the
236 default gpsd port will be used). Then followed optionally by a
237 second colon and the remote device name The daemon will open a
238 socket to the indicated address and port and emulate a gpsd client,
239 collecting JSON reports from the remote gpsd instance that will be
240 passed to local clients. Example: gpsd://gpsd.io:2947:/dev/ttyAMA0.
241 NMEA2000 CAN data
243 A URI with the prefix "nmea2000://", followed by a CAN devicename.
244 Only Linux socket CAN interfaces are supported. The interface must
245 be configured to receive CAN messages before gpsd can be started.
246 If there is more than one unit on the CAN bus that provides GPS
247 data, gpsd chooses the unit from which a GPS message is first seen.
248 Example: nmea2000://can0.
249 (The "ais:://" source type supported in some older versions of the
251 daemon has been retired in favor of the more general "tcp://".)
252 Additionally, two serial device name have a side effect, if your binary
254 was compiled with the MAGIC_HAT option:
255 /dev/ttyAMA0
257 The UART device on a Raspberry Pi. Has the side effect of opening
258 /dev/pps0 for RFC2783 1PPS data.
259 /dev/gpsd0
261 Generic GPS device 0. Has the side effect of opening /dev/pps0 for
262 RFC2783 1PPS data.
263 Note, however, that if /dev/pps0 is the fake "ktimer" PPS, then
265 /dev/pps1 will be used instead.
266 Internally, the daemon maintains a device pool holding the pathnames of
268 devices and remote servers known to the daemon. Initially, this list is
269 the list of device-name arguments specified on the command line. That
270 list may be empty, in which case the daemon will have no devices on its
271 search list until they are added by a control-socket command. Daemon
272 startup will abort with an error if no devices and no control socket is
273 specified.
274 When a device is activated (i.e. a client requests data from it), gpsd
276 attempts to execute a hook from /etc/gpsd/device-hook with first
277 command line argument set to the pathname of the device and the second
278 to ACTIVATE. On deactivation, it does the same passing DEACTIVATE for
279 the second argument.
280 gpsd can export data to client applications in three ways: via a
282 sockets interface, via a shared-memory segment, and via D-Bus. The next
283 three major sections describe these interfaces.
284
286 When gpsd is running as a daemon (not in the foreground) is sends all
287 of its logging to syslog(3). Logging is sent with ident "gpsd" using
288 facility "LOG_USER".
289 If you are using syslog-ng, then you can send all gpsd logs to a file
291 /var/log/gpsd. Put this at the end of your
292 /etc/syslog-ng/syslog-ng.conf file:
293 destination gpsdf { file("/var/log/gpsd"); };
295 filter f_gpsd { program("gpsd"); };
296 log { source(src); filter(f_gpsd); destination(gpsdf); };
297
299 Clients may communicate with the daemon via textual request and
300 responses over a socket. It is a bad idea for applications to speak the
301 protocol directly: rather, they should use the libgps client library
302 and take appropriate care to conditionalize their code on the major and
303 minor protocol version symbols.
304 The request-response protocol for the socket interface is fully
306 documented in gpsd_json(5).
307
309 gpsd has two other (read-only) interfaces.
310 Whenever the daemon recognizes a packet from any attached device, it
312 writes the accumulated state from that device to a shared memory
313 segment. The C and C++ client libraries shipped with GPSD can read this
314 segment. Client methods, and various restrictions associated with the
315 read-only nature of this interface, are documented at libgps(3). The
316 shared-memory interface is intended primarily for embedded deployments
317 in which gpsd monitors a single device, and its principal advantage is
318 that a daemon instance configured with shared memory but without the
319 sockets interface loses a significant amount of runtime weight.
320 The daemon may be configured to emit a D-Bus signal each time an
322 attached device delivers a fix. The signal path is "path /org/gpsd",
323 the signal interface is "org.gpsd", and the signal name is "fix". The
324 signal payload layout is as follows:
325 Table 1. Satellite object
327 ┌─────────────────┬───────────────────────────┐
328 │ │ │
329 │Type │ Description │
330 ├─────────────────┼───────────────────────────┤
331 │ │ │
332 │DBUS_TYPE_DOUBLE │ Time (seconds since Unix │
333 │ │ epoch) │
334 ├─────────────────┼───────────────────────────┤
335 │ │ │
336 │DBUS_TYPE_INT32 │ mode │
337 ├─────────────────┼───────────────────────────┤
338 │ │ │
339 │DBUS_TYPE_DOUBLE │ Time uncertainty │
340 │ │ (seconds). │
341 ├─────────────────┼───────────────────────────┤
342 │ │ │
343 │DBUS_TYPE_DOUBLE │ Latitude in degrees. │
344 ├─────────────────┼───────────────────────────┤
345 │ │ │
346 │DBUS_TYPE_DOUBLE │ Longitude in degrees. │
347 ├─────────────────┼───────────────────────────┤
348 │ │ │
349 │DBUS_TYPE_DOUBLE │ Horizontal uncertainty in │
350 │ │ meter. │
351 ├─────────────────┼───────────────────────────┤
352 │ │ │
353 │DBUS_TYPE_DOUBLE │ Altitude MSL in meters. │
354 ├─────────────────┼───────────────────────────┤
355 │ │ │
356 │DBUS_TYPE_DOUBLE │ Altitude uncertainty in │
357 │ │ meters. │
358 ├─────────────────┼───────────────────────────┤
359 │ │ │
360 │DBUS_TYPE_DOUBLE │ Course in degrees from │
361 │ │ true north. │
362 ├─────────────────┼───────────────────────────┤
363 │ │ │
364 │DBUS_TYPE_DOUBLE │ Course uncertainty in │
365 │ │ meters │
366 ├─────────────────┼───────────────────────────┤
367 │ │ │
368 │DBUS_TYPE_DOUBLE │ Speed, meters per second. │
369 ├─────────────────┼───────────────────────────┤
370 │ │ │
371 │DBUS_TYPE_DOUBLE │ Speed uncertainty in │
372 │ │ meters per second. │
373 ├─────────────────┼───────────────────────────┤
374 │ │ │
375 │DBUS_TYPE_DOUBLE │ Climb, meters per second. │
376 ├─────────────────┼───────────────────────────┤
377 │ │ │
378 │DBUS_TYPE_DOUBLE │ Climb uncertainty in │
379 │ │ meters per second. │
380 ├─────────────────┼───────────────────────────┤
381 │ │ │
382 │DBUS_TYPE_STRING │ Device name │
383 └─────────────────┴───────────────────────────┘
384 Uncertainty values are provided by the GNSS receiver. Check your
386 receiver documentation to see if is specifies what its "uncertainty"
387 means.
388
390 gpsd maintains an internal list of GPS devices (the "device pool"). If
391 you specify devices on the command line, the list is initialized with
392 those pathnames, otherwise the list starts empty. Commands to add and
393 remove GPS device paths from the daemon’s device list must be written
394 to a local Unix-domain socket which will be accessible only to programs
395 running as root. This control socket will be located wherever the -F
396 option specifies it.
397 A device may will also be dropped from the pool if GPSD gets a zero
399 length read from it. This end-of-file condition indicates that the
400 device has been disconnected.
401 When gpsd is installed along with working hotplug notifier scripts
403 feeding it device-add commands over the control socket, gpsd should
404 require no configuration or user action to find hotplug devices.
405 Sending SIGHUP to a running gpsd forces it to close all GPSes and all
407 client connections. It will then attempt to reconnect to any GPSes on
408 its device list and resume listening for client connections. This may
409 be useful if your GPS enters a wedged or confused state but can be
410 soft-reset by pulling down DTR.
411 When gpsd is called with no initial devices (thus, expecting devices to
413 be passed to it by notifications to the control socket), and reaches a
414 state where there are no devices connected and no subscribers after
415 some devices have been seen, it shuts down gracefully. It is expected
416 that future device hotplug events will reactivate it.
417 To point gpsd at a device that may be a GPS, write to the control
419 socket a plus sign ('+') followed by the device name followed by LF or
420 CR-LF. Thus, to point the daemon at /dev/foo. send "+/dev/foo\n". To
421 tell the daemon that a device has been disconnected and is no longer
422 available, send a minus sign ('-') followed by the device name followed
423 by LF or CR-LF. Thus, to remove /dev/foo from the search list, send
424 "-/dev/foo\n".
425 To send a control string to a specified device, write to the control
427 socket a '!', followed by the device name, followed by '=', followed by
428 the control string.
429 To send a binary control string to a specified device, write to the
431 control socket a '&', followed by the device name, followed by '=',
432 followed by the control string in paired hex digits.
433 Your client may await a response, which will be a line beginning with
435 either "OK" or "ERROR". An ERROR response to an 'add' command means the
436 device did not emit data recognizable as GPS packets, an ERROR response
437 to a remove command means the specified device was not in gpsd's device
438 pool. An ERROR response to a '!' command means the daemon did not
439 recognize the devicename specified.
440 The control socket is intended for use by hotplug scripts and other
442 device-discovery services. This control channel is separate from the
443 public gpsd service port, and only locally accessible, in order to
444 prevent remote denial-of-service and spoofing attacks.
445
447 The base User Estimated Range Error (UERE) of GPSes is 8 meters or less
448 at 66% confidence, 15 meters or less at 95% confidence. Actual
449 horizontal error will be UERE times a dilution factor dependent on
450 current satellite position. Altitude determination is more sensitive to
451 variability in ionospheric signal lag than latitude/longitude is, and
452 is also subject to errors in the estimation of local mean sea level,
453 base error is 12 meters at 66% confidence, 23 meters at 95% confidence.
454 Again, this will be multiplied by a vertical dilution of precision
455 (VDOP) dependent on satellite geometry, and VDOP is typically larger
456 than HDOP. Users should not rely on GPS altitude for life-critical
457 tasks such as landing an airplane.
458 These errors are intrinsic to the design and physics of the GPS system.
460 gpsd does its internal computations at sufficient accuracy that it will
461 add no measurable position error of its own.
462 DGPS correction may reduce UERE, provided you are within about 100
464 miles (160 km) of a DGPS ground station from which you are receiving
465 corrections.
466 On a 4800bps connection, the time latency of fixes provided by gpsd
468 will be one second or less 95% of the time. Most of this lag is due to
469 the fact that GPSes normally emit fixes once per second, thus expected
470 latency is 0.5sec. On the personal-computer hardware available in 2005
471 and later, computation lag induced by gpsd will be negligible, on the
472 order of a millisecond. Nevertheless, latency can introduce significant
473 errors for vehicles in motion, at 50 km/h (31 mi/h) of speed over
474 ground, 1 second of lag corresponds to 13.8 meters change in position
475 between updates.
476 The time reporting of the GPS system itself has an intrinsic accuracy
478 limit of 14 nanoseconds, but this can only be approximated by
479 specialized receivers using that send the high-accuracy PPS
480 (Pulse-Per-Second) over RS232 to cue a clock crystal. Most GPS
481 receivers only report time to a precision of 0.01s or 0.001s, and with
482 no accuracy guarantees below 1sec.
483 If your GPS uses a SiRF chipset at firmware level 231, reported UTC
485 time may be off by the difference between whatever default leap-second
486 offset has been compiled in and whatever leap-second correction is
487 currently applicable, from startup until complete subframe information
488 is received. Firmware levels 232 and up don’t have this problem. You
489 may run gpsd at debug level 4 to see the chipset type and firmware
490 revision level.
491 There are exactly two circumstances under which gpsd relies on the
493 host-system clock:
494 In the GPS broadcast signal, GPS time is represented using a week
496 number that rolls over after 2^10 or 2^13 weeks (about 19.6 years, or
497 157 years), depending on the spacecraft. Receivers are required to
498 disambiguate this to the correct date, but may have difficulty due to
499 not knowing time to within half this interval, or may have bugs. Users
500 have reported incorrect dates which appear to be due to this issue.
501 gpsd uses the startup time of the daemon detect and compensate for
502 rollovers while it is running, but otherwise reports the date as it is
503 reported by the receiver without attempting to correct it.
504 If you are using an NMEA-only GPS (that is, not using SiRF or Garmin or
506 Zodiac binary mode), gpsd relies on the system clock to tell it the
507 current century. If the system clock returns an invalid value near
508 zero, and the GPS does not emit GPZDA at the start of its update cycle
509 (which most consumer-grade NMEA GPSes do not) then the century part of
510 the dates gpsd delivers may be wrong. Additionally, near the century
511 turnover, a range of dates as wide in seconds as the accuracy of your
512 system clock may be referred to the wrong century.
513
515 gpsd can provide reference clock information to ntpd or chronyd, to
516 keep the system clock synchronized to the time provided by the GPS
517 receiver.
518 On Linux, gpsd includes support for interpreting the PPS pulses emitted
520 at the start of every clock second on the carrier-detect lines of some
521 serial GPSes, this pulse can be used to update NTP at much higher
522 accuracy than message time provides. You can determine whether your GPS
523 emits this pulse by running at -D 5 and watching for carrier-detect
524 state change messages in the logfile. In addition, if your kernel
525 provides the RFC 2783 kernel PPS API then gpsd will use that for extra
526 accuracy.
527 gpsd communicates with ntpd using shared memory segments (SHMs). The
529 SHMs are numbered SHM(0) to SHM(7). and have a name (key) of NTP0 to
530 NTP7. The first local time source on the command line may use NTP0 and
531 NTP1. The second may use NTP2 and NTP3, etc.
532 You can see the output of gpsd to ntpd in real time with the ntpshmmon
534 command.
535 Other daemons, such as ptp4l may also be using the same SHMs to talk to
537 ntpd.
538 Detailed instructions for using GPSD to set up a high-quality time
540 service can be found among the documentation on the GPSD website.
541
543 On operating systems that support D-BUS, gpsd can be built to broadcast
544 GPS fixes to D-BUS-aware applications. As D-BUS is still at a pre-1.0
545 stage, we will not attempt to document this interface here. Read the
546 gpsd source code to learn more.
547
549 gpsd, if given the -G flag, will listen for connections from any
550 reachable host, and then disclose the current position. Before using
551 the -G flag, consider whether you consider your computer’s location to
552 be sensitive data to be kept private or something that you wish to
553 publish.
554 gpsd must start up as root in order to open the NTPD shared-memory
556 segment, open its logfile, and create its local control socket. Note
557 that starting gpsd with sudo is not the same as starting as root.
558 Before doing any processing of GPS data, it tries to drop root
559 privileges by setting its UID to "nobody" (or another configured
560 userid) and its group ID to the group of the initial GPS passed on the
561 command line — or, if that device doesn’t exist, to the group of
562 /dev/ttyS0.
563 Privilege-dropping is a hedge against the possibility that carefully
565 crafted data, either presented from a client socket or from a subverted
566 serial device posing as a GPS, could be used to induce misbehavior in
567 the internals of gpsd. It ensures that any such compromises cannot be
568 used for privilege elevation to root.
569 The assumption behind gpsd's particular behavior is that all the tty
571 devices to which a GPS might be connected are owned by the same
572 non-root group and allow group read/write, though the group may vary
573 because of distribution-specific or local administrative practice. If
574 this assumption is false, gpsd may not be able to open GPS devices in
575 order to read them (such failures will be logged).
576 In order to fend off inadvertent denial-of-service attacks by port
578 scanners (not to mention deliberate ones), gpsd will time out inactive
579 client connections. Before the client has issued a command that
580 requests a channel assignment, a short timeout (60 seconds) applies.
581 There is no timeout for clients in watcher or raw modes; rather, gpsd
582 drops these clients if they fail to read data long enough for the
583 outbound socket write buffer to fill. Clients with an assigned device
584 in polling mode are subject to a longer timeout (15 minutes).
585
587 If multiple NMEA talkers are feeding RMC, GLL, and GGA sentences to the
588 same serial device (possible with an RS422 adapter hooked up to some
589 marine-navigation systems), a 'TPV' response may mix an altitude from
590 one device’s GGA with latitude/longitude from another’s RMC/GLL after
591 the second sentence has arrived.
592 gpsd may change control settings on your GPS (such as the emission
594 frequency of various sentences or packets) and not restore the original
595 settings on exit. This is a result of inadequacies in NMEA and the
596 vendor binary GPS protocols, which often do not give clients any way to
597 query the values of control settings in order to be able to restore
598 them later.
599 Some receivers do not report VDOP/TDOP/GDOP figures and associated
601 error estimates. In that case they may be computed by gpsd instead.
602 This computation does not exactly match what chips do internally, which
603 includes some satellite weighting using parameters gpsd cannot see.
604 Autobauding on the Trimble GPSes can take as long as 20 seconds, or
606 more, if the device speed is not matched to the GPS speed. Use the -s
607 option to avoid autobaud delays.
608 Generation of position error estimates (eph, epv, epd, eps, epc) from
610 the incomplete data handed back by GPS reporting protocols involves
611 both a lot of mathematical black art and fragile device-dependent
612 assumptions. This code has been bug-prone in the past and problems may
613 still lurk there.
614 AIDVM decoding of types 16-17, 22-23, and 25-26 is unverified.
616 GPSD presently fully recognizes only the 2.1 level of RTCM2 (message
618 types 1, 3, 4, 5, 6, 7, 9, 16). The 2.3 message types 13, 14, and 31
619 are recognized and reported. Message types 8, 10-12, 15-27, 28-30
620 (undefined), 31-37, 38-58 (undefined), and 60-63 are not yet supported.
621 The ISGPS used for RTCM2 and subframes decoder logic is sufficiently
623 convoluted to confuse some compiler optimizers, notably in GCC 3.x at
624 -O2, into generating bad code.
625 Devices meant to use PPS for high-precision timekeeping may fail if
627 they are specified after startup by a control-socket command, as
628 opposed to on the daemon’s original command line. Root privileges are
629 dropped early, and some Unix variants require them in order to set the
630 PPS line discipline. Under Linux the POSIX capability to set the line
631 discipline is retained, but other platforms cannot use this code.
632 USB GPS devices often do not identify themselves through the USB
634 subsystem; they typically present as the class 00h (undefined) or class
635 FFh (vendor-specific) of USB-to-serial adapters. Because of this, the
636 Linux hotplug scripts must tell gpsd to sniff data from every
637 USB-to-serial adapter that goes active and is known to be of a type
638 used in GPSes. No such device is sent configuration strings until after
639 it has been identified as a GPS, and gpsd never opens a device that is
640 opened by another process. But there is a tiny window for non-GPS
641 devices not opened; if the application that wants them loses a race
642 with GPSD its device open will fail and have to be retried after GPSD
643 sniffs the device (normally less than a second later).
644
646 /dev/ttyS0
647 Prototype TTY device. After startup, gpsd sets its group ID to the
648 owning group of this device if no GPS device was specified on the
649 command line does not exist.
650 /etc/gpsd/device-hook
652 Optional file containing the device activation/deactivation script.
653 Note that while /etc/gpsd is the default system configuration
654 directory, it is possible to build the GPSD source code with
655 different assumptions. See above for further details on the
656 device-hook mechanism.
657 /@RUNDIR@/chrony.XXX.sock
659 Optional Unix domain socket provided by chronyd configured with a
660 SOCK reference clock, where XXX is the name of the serial device
661 (e.g. ttyS0). When gpsd is started, it will try to connect to this
662 socket. If connected, it will be sending PPS timing information to
663 chronyd in order to synchronize the system clock.
664 /@RUNDIR@/chrony.clk.XXX.sock
666 Similar to chrony.XXX.sock, but will be used to pass timing
667 information to chronyd that is derived from the serial (likely
668 NMEA) timing instead of PPS.
669
671 By setting the environment variable GPSD_SHM_KEY, you can control the
672 key value used to create the shared-memory segment used for
673 communication with the client library. This will be useful mainly when
674 isolating test instances of gpsd from production ones.
675
677 0
678 on success.
679 1
681 on failure
682
684 The official NMEA protocol standards for NMEA0183 and NMEA2000 are
685 available from the National Marine Electronics Association, but are
686 proprietary and expensive; the maintainers of gpsd have made a point of
687 not looking at them. The GPSD project website links to several
688 documents that collect publicly disclosed information about the
689 protocol.
690 gpsd parses the following NMEA sentences: RMC, GGA, GLL, GSA, GSV, VTG,
692 ZDA, GBS, HDT, DBT, GST. It recognizes these with either the normal GP
693 talker-ID prefix, or with the GN prefix used by GLONASS, or with the II
694 prefix emitted by Seahawk Autohelm marine navigation systems, or with
695 the IN prefix emitted by some Garmin units, or with the EC prefix
696 emitted by ECDIS units, or with the SD prefix emitted by depth
697 sounders, or with the HC and TI prefix emitted by some Airmar
698 equipment. It recognizes some vendor extensions: the PGRME emitted by
699 some Garmin GPS models, the OHPR emitted by Oceanserver digital
700 compasses, the PTNTHTM emitted by True North digital compasses, the
701 PMTK omitted by some San Jose Navigation GPSes, and the PASHR sentences
702 emitted by some Ashtech GPSes.
703 Note that gpsd JSON returns pure decimal degrees, not the hybrid
705 degree/minute format described in the NMEA standard.
706 Differential-GPS corrections are conveyed by the RTCM protocols. The
708 applicable standard for RTCM-104 V2 is RTCM Recommended Standards for
709 Differential GNSS (Global Navigation Satellite) Service RTCM Paper
710 136-2001/SC 104-STD. The applicable standard for RTCM-104 V3 is RTCM
711 Standard 10403.1 for Differential GNSS Services - Version 3 RTCM Paper
712 177-2006-SC104-STD. Ordering instructions for the RTCM standards are
713 accessible from the website of the Radio Technical Commission for
714 Maritime Services under "Publications".
715 AIS is defined by ITU Recommendation M.1371, Technical Characteristics
717 for a Universal Shipborne Automatic Identification System Using Time
718 Division Multiple Access. The AIVDM/AIVDO format understood by this
719 program is defined by IEC-PAS 61162-100, Maritime navigation and
720 radiocommunication equipment and systems. A more accessible description
721 of both can be found at AIVDM/AIVDO Protocol Decoding, on the
722 references page of the GPSD project website.
723 Subframe data is defined by IS-GPS-200, GLOBAL POSITIONING SYSTEM WING
725 (GPSW) SYSTEMS ENGINEERING & INTEGRATION, INTERFACE SPECIFICATION
726 IS-GPS-200. The format understood by this program is defined in Section
727 20 (Appendix II),
728 JSON is specified by RFC 7159, The JavaScript Object Notation (JSON)
730 Data Interchange Format.
731 The API for PPS time service is specified by RFC 2783, Pulse-Per-Second
733 API for UNIX-like Operating Systems, Version 1.0
734
736 Authors: Eric S. Raymond, Chris Kuethe, Gary Miller. Former authors
737 whose bits have been plowed under by code turnover: Remco Treffcorn,
738 Derrick Brashear, Russ Nelson.
739 This manual page by Eric S. Raymond esr@thyrsus.com.
741
743 gpsctl(1), gps(1), gpsprof(1), gpsfake(1). gpscat(1), *ntpshmmon(1),
744 libgps(3), libgpsmm(3)
746 gpsd_json(5)
748 chronyd(8), gpsd(8), gpsdctl(8), ntpd(8)
750
752 Project web site: https://gpsd.io/ GPSD FAQ: https://gpsd.io/faq.html
753 GPSD Time Service HOWTO: https://gpsd.io/gpsd-time-service-howto.html
754 Introduction to Time Service: https://gpsd.io/time-service-intro.html
755
757 This file is Copyright 2013 by the GPSD project
758 SPDX-License-Identifier: BSD-2-clause
759GPSD, Version 3.25 2023-01-10 GPSD(8)