1GPSD(8)                       GPSD Documentation                       GPSD(8)
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4

NAME

6       gpsd - interface daemon for GPS receivers
7

SYNOPSIS

9       gpsd [-b] [-D debuglevel] [-F control-socket] [-G] [-h] [-l] [-n] [-N]
10            [-P pidfile] [-r] [-S listener-port] [-V] [[source-name]...]
11

QUICK START

13       If you have a GPS attached on the lowest-numbered USB port of a Linux
14       system, and want to read reports from it on TCP/IP port 2947, it will
15       normally suffice to do this:
16
17           gpsd /dev/ttyUSB0
18
19       For the lowest-numbered serial port:
20
21           gpsd /dev/ttyS0
22
23       Change the device number as appropriate if you need to use a different
24       port. Command-line flags enable verbose logging, a control port, and
25       other optional extras but should not be needed for basic operation; the
26       one exception, on very badly designed hardware, might be -b (which
27       see).
28
29       On Linux systems supporting udev, gpsd is normally started
30       automatically when a USB plugin event fires (if it is not already
31       running) and is handed the name of the newly active device. In that
32       case no invocation is required at all.
33
34       For your initial tests set your GPS hardware to speak NMEA, as gpsd is
35       guaranteed to be able to process that. If your GPS has a native or
36       binary mode with better performance that gpsd knows how to speak, gpsd
37       will autoconfigure that mode.
38
39       You can verify correct operation by first starting gpsd and then xgps,
40       the X windows test client.
41
42       If you have problems, the GPSD project maintains a FAQ to assist
43       troubleshooting.
44

DESCRIPTION

46       gpsd is a monitor daemon that collects information from GPSes,
47       differential-GPS radios, or AIS receivers attached to the host machine.
48       Each GPS, DGPS radio, or AIS receiver is expected to be
49       direct-connected to the host via a USB or RS232C serial device. The
50       serial device may be specified to gpsd at startup, or it may be set via
51       a command shipped down a local control socket (e.g. by a USB hotplug
52       script). Given a GPS device by either means, gpsd discovers the correct
53       port speed and protocol for it.
54
55       gpsd should be able to query any GPS that speaks either the standard
56       textual NMEA 0183 protocol, or the (differing) extended NMEA dialects
57       used by MKT-3301, iTrax, Motorola OnCore, Sony CXD2951, and
58       Ashtech/Thales devices. It can also interpret the binary protocols used
59       by EverMore, Garmin, Navcom, Rockwell/Zodiac, SiRF, Trimble, and u-blox
60       ANTARIS devices. Under Linux it can read NMEA2000 packets through the
61       kernel CAN socket. It can read heading and attitude information from
62       the Oceanserver 5000 or TNT Revolution digital compasses.
63
64       The GPS reporting formats supported by your instance of gpsd may differ
65       depending on how it was compiled; general-purpose versions support
66       many, but it can be built with protocol subsets down to a singleton for
67       use in constrained environments. For a list of the GPS protocols
68       supported by your instance, see the output of gpsd -l
69
70       gpsd effectively hides the differences among the GPS types it supports.
71       It also knows about and uses commands that tune these GPSes for lower
72       latency. By using gpsd as an intermediary, applications avoid
73       contention for serial devices.
74
75       gpsd can use differential-GPS corrections from a DGPS radio or over the
76       net, from a ground station running a DGPSIP server or a Ntrip
77       broadcaster that reports RTCM-104 data; this will shrink position
78       errors by roughly a factor of four. When gpsd opens a serial device
79       emitting RTCM-104, it automatically recognizes this and uses the device
80       as a correction source for all connected GPSes that accept RTCM
81       corrections (this is dependent on the type of the GPS; not all GPSes
82       have the firmware capability to accept RTCM correction packets). See
83       the section called “ACCURACY” and the section called “FILES” for
84       discussion.
85
86       Client applications will communicate with gpsd via a TCP/IP port, 2947
87       by default). Both IPv4 and IPv6 connections are supported and a client
88       may connect via either.
89
90       The program accepts the following options:
91
92       -b
93           Broken-device-safety mode, otherwise known as read-only mode. A few
94           bluetooth and USB receivers lock up or become totally inaccessible
95           when probed or reconfigured; see the hardware compatibility list on
96           the GPSD project website for details. This switch prevents gpsd
97           from writing to a receiver. This means that gpsd cannot configure
98           the receiver for optimal performance, but it also means that gpsd
99           cannot break the receiver. A better solution would be for Bluetooth
100           to not be so fragile. A platform independent method to identify
101           serial-over-Bluetooth devices would also be nice.
102
103       -D
104           Set debug level. At debug levels 2 and above, gpsd reports incoming
105           sentence and actions to standard error if gpsd is in the foreground
106           (-N) or to syslog if in the background.
107
108       -F
109           Create a control socket for device addition and removal commands.
110           You must specify a valid pathname on your local filesystem; this
111           will be created as a Unix-domain socket to which you can write
112           commands that edit the daemon's internal device list.
113
114       -G
115           This flag causes gpsd to listen on all addresses (INADDR_ANY)
116           rather than just the loop back (INADDR_LOOPBACK) address. For the
117           sake of privacy and security, TPV information is now private to the
118           local machine until the user makes an effort to expose this to the
119           world.
120
121       -h
122           Display help message and terminate.
123
124       -l
125           List all drivers compiled into this gpsd instance. The letters to
126           the left of each driver name are the gpsd control commands
127           supported by that driver.
128
129       -n
130           Don't wait for a client to connect before polling whatever GPS is
131           associated with it. Some RS232 GPSes wait in a standby mode
132           (drawing less power) when the host machine is not asserting DTR,
133           and some cellphone and handheld embedded GPSes have similar
134           behaviors. Accordingly, waiting for a watch request to open the
135           device may save battery power. (This capability is rare in
136           consumer-grade devices).
137
138       -N
139           Don't daemonize; run in foreground. This switch is mainly useful
140           for debugging.
141
142       -r
143           Use GPS time even with no current fix. Some GPS's have battery
144           powered Real Time Clocks (RTC's) built in, makeing them a valid
145           time source even before a fix is acquired. This can be useful on a
146           Raspberry Pi, or other device that has no battery powered RTC, and
147           thus has no valid time at startup.
148
149       -P
150           Specify the name and path to record the daemon's process ID.
151
152       -S
153           Set TCP/IP port on which to listen for GPSD clients (default is
154           2947).
155
156       -V
157           Dump version and exit.
158
159       Arguments are interpreted as the names of data sources. Normally, a
160       data source is the device pathname of a local device from which the
161       daemon may expect GPS data. But there are three other special source
162       types recognized, for a total of four:
163
164       Local serial or USB device
165           A normal Unix device name of a serial or USB device to which a
166           sensor is attached. Example: /dev/ttyUSB0.
167
168       Local PPS device
169           A normal Unix device name of a PPS device to which a PPS source is
170           attached. The device name must start with "/dev/pps" and a local
171           serial or USB GPS device must also be available. Example:
172           /dev/pps0.
173
174       TCP feed
175           A URI with the prefix "tcp://", followed by a hostname, a colon,
176           and a port number. The daemon will open a socket to the indicated
177           address and port and read data packets from it, which will be
178           interpreted as though they had been issued by a serial device.
179           Example: tcp://data.aishub.net:4006.
180
181       UDP feed
182           A URI with the prefix "udp://", followed by a hostname, a colon,
183           and a port number. The daemon will open a socket listening for UDP
184           datagrams arriving on the indicated address and port, which will be
185           interpreted as though they had been issued by a serial device.
186           Example: udp://127.0.0.1:5000.
187
188       Ntrip caster
189           A URI with the prefix "ntrip://" followed by the name of an Ntrip
190           caster (Ntrip is a protocol for broadcasting differential-GPS fixes
191           over the net). For Ntrip services that require authentication, a
192           prefix of the form "username:password@" can be added before the
193           name of the Ntrip broadcaster. For Ntrip service, you must specify
194           which stream to use; the stream is given in the form "/streamname".
195           An example DGPSIP URI could be "dgpsip://dgpsip.example.com" and a
196           Ntrip URI could be
197           "ntrip://foo:bar@ntrip.example.com:80/example-stream". Corrections
198           from the caster will be send to each attached GPS with the
199           capability to accept them.
200
201       DGPSIP server
202           A URI with the prefix "dgpsip://" followed by a hostname, a colon,
203           and an optional colon-separated port number (defaulting to 2101).
204           The daemon will handshake with the DGPSIP server and read RTCM2
205           correction data from it. Corrections from the server will be set to
206           each attached GPS with the capability to accept them. Example:
207           dgpsip://dgps.wsrcc.com:2101.
208
209       Remote gpsd feed
210           A URI with the prefix "gpsd://", followed by a hostname and
211           optionally a colony and a port number (if the port is absent the
212           default gpsd port will be used). The daemon will open a socket to
213           the indicated address and port and emulate a gpsd client,
214           collecting JSON reports from the remote gpsd instance that will be
215           passed to local clients.
216
217       NMEA2000 CAN data
218           A URI with the prefix "nmea2000://", followed by a CAN devicename.
219           Only Linux socket CAN interfaces are supported. The interface must
220           be configured to receive CAN messages before gpsd can be started.
221           If there is more then one unit on the CAN bus that provides GPS
222           data, gpsd chooses the unit from which a GPS message is first seen.
223           Example: nmea2000://can0.
224
225       (The "ais:://" source type supported in some older versions of the
226       daemon has been retired in favor of the more general "tcp://".)
227
228       Additionally, two serial device names have a side effect:
229
230       /dev/ttyAMA0
231           The UART device on a Raspberry Pi. Has the side effect of opening
232           /dev/pps0 for RFC2783 1PPS data.
233
234       Generic GPS device 0. Has the side effect of opening /dev/pps0 for
235       RFC2783 1PPS data/
236
237       Internally, the daemon maintains a device pool holding the pathnames of
238       devices and remote servers known to the daemon. Initially, this list is
239       the list of device-name arguments specified on the command line. That
240       list may be empty, in which case the daemon will have no devices on its
241       search list until they are added by a control-socket command (see the
242       section called “GPS DEVICE MANAGEMENT” for details on this). Daemon
243       startup will abort with an error if neither any devices nor a control
244       socket are specified.
245
246       When a device is activated (i.e. a client requests data from it), gpsd
247       attempts to execute a hook from /etc/gpsd/device-hook with first
248       command line argument set to the pathname of the device and the second
249       to ACTIVATE. On deactivation it does the same passing DEACTIVATE for
250       the second argument.
251
252       gpsd can export data to client applications in three ways: via a
253       sockets interface, via a shared-memory segment, and via D-Bus. The next
254       three major sections describe these interfaces.
255

THE SOCKET INTERFACE

257       Clients may communicate with the daemon via textual request and
258       responses over a socket. It is a bad idea for applications to speak the
259       protocol directly: rather, they should use the libgps client library
260       and take appropriate care to conditionalize their code on the major and
261       minor protocol version symbols.
262
263       The request-response protocol for the socket interface is fully
264       documented in gpsd_json(5).
265

SHARED-MEMORY AND DBUS INTERFACES

267       gpsd has two other (read-only) interfaces.
268
269       Whenever the daemon recognizes a packet from any attached device, it
270       writes the accumulated state from that device to a shared memory
271       segment. The C and C++ client libraries shipped with GPSD can read this
272       segment. Client methods, and various restrictions associated with the
273       read-only nature of this interface, are documented at libgps(3). The
274       shared-memory interface is intended primarily for embedded deployments
275       in which gpsd monitors a single device, and its principal advantage is
276       that a daemon instance configured with shared memory but without the
277       sockets interface loses a significant amount of runtime weight.
278
279       The daemon may be configured to emit a D-Bus signal each time an
280       attached device delivers a fix. The signal path is path /org/gpsd, the
281       signal interface is "org.gpsd", and the signal name is "fix". The
282       signal payload layout is as follows:
283
284       Table 1. Satellite object
285       ┌─────────────────┬────────────────────────────┐
286Type             │                            │
287       │                 │        Description         │
288       ├─────────────────┼────────────────────────────┤
289       │DBUS_TYPE_DOUBLE │                            │
290       │                 │        Time (seconds since │
291       │                 │        Unix epoch)         │
292       ├─────────────────┼────────────────────────────┤
293       │DBUS_TYPE_INT32  │                            │
294       │                 │        mode                │
295       ├─────────────────┼────────────────────────────┤
296       │DBUS_TYPE_DOUBLE │                            │
297       │                 │        Time uncertainty    │
298       │                 │        (seconds).          │
299       ├─────────────────┼────────────────────────────┤
300       │DBUS_TYPE_DOUBLE │                            │
301       │                 │        Latitude in         │
302       │                 │        degrees.            │
303       ├─────────────────┼────────────────────────────┤
304       │DBUS_TYPE_DOUBLE │                            │
305       │                 │        Longitude in        │
306       │                 │        degrees.            │
307       ├─────────────────┼────────────────────────────┤
308       │DBUS_TYPE_DOUBLE │                            │
309       │                 │        Horizontal          │
310       │                 │        uncertainty in      │
311       │                 │        meters, 95%         │
312       │                 │        confidence.         │
313       ├─────────────────┼────────────────────────────┤
314       │DBUS_TYPE_DOUBLE │                            │
315       │                 │        Altitude in meters. │
316       ├─────────────────┼────────────────────────────┤
317       │DBUS_TYPE_DOUBLE │                            │
318       │                 │        Altitude            │
319       │                 │        uncertainty in      │
320       │                 │        meters, 95%         │
321       │                 │        confidence.         │
322       ├─────────────────┼────────────────────────────┤
323       │DBUS_TYPE_DOUBLE │                            │
324       │                 │        Course in degrees   │
325       │                 │        from true north.    │
326       ├─────────────────┼────────────────────────────┤
327       │DBUS_TYPE_DOUBLE │                            │
328       │                 │        Course uncertainty  │
329       │                 │        in meters, 95%      │
330       │                 │        confidence.         │
331       ├─────────────────┼────────────────────────────┤
332       │DBUS_TYPE_DOUBLE │                            │
333       │                 │        Speed, meters per   │
334       │                 │        second.             │
335       ├─────────────────┼────────────────────────────┤
336       │DBUS_TYPE_DOUBLE │                            │
337       │                 │        Speed uncertainty   │
338       │                 │        in meters per       │
339       │                 │        second, 95%         │
340       │                 │        confidence.         │
341       ├─────────────────┼────────────────────────────┤
342       │DBUS_TYPE_DOUBLE │                            │
343       │                 │        Climb, meters per   │
344       │                 │        second.             │
345       ├─────────────────┼────────────────────────────┤
346       │DBUS_TYPE_DOUBLE │                            │
347       │                 │        Climb uncertainty   │
348       │                 │        in meters per       │
349       │                 │        second, 95%         │
350       │                 │        confidence.         │
351       ├─────────────────┼────────────────────────────┤
352       │DBUS_TYPE_STRING │                            │
353       │                 │        Device name         │
354       └─────────────────┴────────────────────────────┘
355

GPS DEVICE MANAGEMENT

357       gpsd maintains an internal list of GPS devices (the "device pool"). If
358       you specify devices on the command line, the list is initialized with
359       those pathnames; otherwise the list starts empty. Commands to add and
360       remove GPS device paths from the daemon's device list must be written
361       to a local Unix-domain socket which will be accessible only to programs
362       running as root. This control socket will be located wherever the -F
363       option specifies it.
364
365       A device may will also be dropped from the pool if GPSD gets a zero
366       length read from it. This end-of-file condition indicates that the'
367       device has been disconnected.
368
369       When gpsd is properly installed along with hotplug notifier scripts
370       feeding it device-add commands over the control socket, gpsd should
371       require no configuration or user action to find devices.
372
373       Sending SIGHUP to a running gpsd forces it to close all GPSes and all
374       client connections. It will then attempt to reconnect to any GPSes on
375       its device list and resume listening for client connections. This may
376       be useful if your GPS enters a wedged or confused state but can be
377       soft-reset by pulling down DTR.
378
379       When gpsd is called with no initial devices (thus, expecting devices to
380       be passed to it by notifications to the control socket), and reaches a
381       state where there are no devices connected and no subscribers after
382       after some devices have been seen, it shuts down gracefully. It is
383       expected that future device hotplug events will reactivate it.
384
385       To point gpsd at a device that may be a GPS, write to the control
386       socket a plus sign ('+') followed by the device name followed by LF or
387       CR-LF. Thus, to point the daemon at /dev/foo. send "+/dev/foo\n". To
388       tell the daemon that a device has been disconnected and is no longer
389       available, send a minus sign ('-') followed by the device name followed
390       by LF or CR-LF. Thus, to remove /dev/foo from the search list, send
391       "-/dev/foo\n".
392
393       To send a control string to a specified device, write to the control
394       socket a '!', followed by the device name, followed by '=', followed by
395       the control string.
396
397       To send a binary control string to a specified device, write to the
398       control socket a '&', followed by the device name, followed by '=',
399       followed by the control string in paired hex digits.
400
401       Your client may await a response, which will be a line beginning with
402       either "OK" or "ERROR". An ERROR response to an add command means the
403       device did not emit data recognizable as GPS packets; an ERROR response
404       to a remove command means the specified device was not in gpsd's device
405       pool. An ERROR response to a ! command means the daemon did not
406       recognize the devicename specified.
407
408       The control socket is intended for use by hotplug scripts and other
409       device-discovery services. This control channel is separate from the
410       public gpsd service port, and only locally accessible, in order to
411       prevent remote denial-of-service and spoofing attacks.
412

ACCURACY

414       The base User Estimated Range Error (UERE) of GPSes is 8 meters or less
415       at 66% confidence, 15 meters or less at 95% confidence. Actual
416       horizontal error will be UERE times a dilution factor dependent on
417       current satellite position. Altitude determination is more sensitive to
418       variability in ionospheric signal lag than latitude/longitude is, and
419       is also subject to errors in the estimation of local mean sea level;
420       base error is 12 meters at 66% confidence, 23 meters at 95% confidence.
421       Again, this will be multiplied by a vertical dilution of precision
422       (VDOP) dependent on satellite geometry, and VDOP is typically larger
423       than HDOP. Users should not rely on GPS altitude for life-critical
424       tasks such as landing an airplane.
425
426       These errors are intrinsic to the design and physics of the GPS system.
427       gpsd does its internal computations at sufficient accuracy that it will
428       add no measurable position error of its own.
429
430       DGPS correction will reduce UERE by a factor of 4, provided you are
431       within about 100mi (160km) of a DGPS ground station from which you are
432       receiving corrections.
433
434       On a 4800bps connection, the time latency of fixes provided by gpsd
435       will be one second or less 95% of the time. Most of this lag is due to
436       the fact that GPSes normally emit fixes once per second, thus expected
437       latency is 0.5sec. On the personal-computer hardware available in 2005
438       and later, computation lag induced by gpsd will be negligible, on the
439       order of a millisecond. Nevertheless, latency can introduce significant
440       errors for vehicles in motion; at 50km/h (31mi/h) of speed over ground,
441       1 second of lag corresponds to 13.8 meters change in position between
442       updates.
443
444       The time reporting of the GPS system itself has an intrinsic accuracy
445       limit of 14 nanoseconds, but this can only be approximated by
446       specialized receivers using that send the high-accuracy PPS
447       (Pulse-Per-Second) over RS232 to cue a clock crystal. Most GPS
448       receivers only report time to a precision of 0.01s or 0.001s, and with
449       no accuracy guarantees below 1sec.
450
451       If your GPS uses a SiRF chipset at firmware level 231, reported UTC
452       time may be off by the difference between whatever default leap-second
453       offset has been compiled in and whatever leap-second correction is
454       currently applicable, from startup until complete subframe information
455       is received. Firmware levels 232 and up don't have this problem. You
456       may run gpsd at debug level 4 to see the chipset type and firmware
457       revision level.
458
459       There are exactly two circumstances under which gpsd relies on the
460       host-system clock:
461
462       In the GPS broadcast signal, GPS time is represented using a week
463       number that rolls over after 2^10 or 2^13 weeks (about 19.6 years, or
464       157 years), depending on the spacecraft. Receivers are required to
465       disambiguate this to the correct date, but may have difficulty due to
466       not knowing time to within half this interval, or may have bugs. Users
467       have reported incorrect dates which appear to be due to this issue.
468       gpsd uses the startup time of the daemon detect and compensate for
469       rollovers while it is running, but otherwise reports the date as it is
470       reported by the receiver without attempting to correct it.
471
472       If you are using an NMEA-only GPS (that is, not using SiRF or Garmin or
473       Zodiac binary mode), gpsd relies on the system clock to tell it the
474       current century. If the system clock returns an invalid value near
475       zero, and the GPS does not emit GPZDA at the start of its update cycle
476       (which most consumer-grade NMEA GPSes do not) then the century part of
477       the dates gpsd delivers may be wrong. Additionally, near the century
478       turnover, a range of dates as wide in seconds as the accuracy of your
479       system clock may be referred to the wrong century.
480

USE WITH NTP

482       gpsd can provide reference clock information to ntpd, to keep the
483       system clock synchronized to the time provided by the GPS receiver.
484
485       On Linux, gpsd includes support for interpreting the PPS pulses emitted
486       at the start of every clock second on the carrier-detect lines of some
487       serial GPSes; this pulse can be used to update NTP at much higher
488       accuracy than message time provides. You can determine whether your GPS
489       emits this pulse by running at -D 5 and watching for carrier-detect
490       state change messages in the logfile. In addition, if your kernel
491       provides the RFC 2783 kernel PPS API then gpsd will use that for extra
492       accuracy.
493
494       Detailed instructions for using GPSD to set up a high-quality time
495       service can be found among the documentation on the GPSD website.
496

USE WITH D-BUS

498       On operating systems that support D-BUS, gpsd can be built to broadcast
499       GPS fixes to D-BUS-aware applications. As D-BUS is still at a pre-1.0
500       stage, we will not attempt to document this interface here. Read the
501       gpsd source code to learn more.
502

SECURITY AND PERMISSIONS ISSUES

504       gpsd, if given the -G flag, will listen for connections from any
505       reachable host, and then disclose the current position. Before using
506       the -G flag, consider whether you consider your computer's location to
507       be sensitive data to be kept private or something that you wish to
508       publish.
509
510       gpsd must start up as root in order to open the NTPD shared-memory
511       segment, open its logfile, and create its local control socket. Before
512       doing any processing of GPS data, it tries to drop root privileges by
513       setting its UID to "nobody" (or another configured userid) and its
514       group ID to the group of the initial GPS passed on the command line —
515       or, if that device doesn't exist, to the group of /dev/ttyS0.
516
517       Privilege-dropping is a hedge against the possibility that carefully
518       crafted data, either presented from a client socket or from a subverted
519       serial device posing as a GPS, could be used to induce misbehavior in
520       the internals of gpsd. It ensures that any such compromises cannot be
521       used for privilege elevation to root.
522
523       The assumption behind gpsd's particular behavior is that all the tty
524       devices to which a GPS might be connected are owned by the same
525       non-root group and allow group read/write, though the group may vary
526       because of distribution-specific or local administrative practice. If
527       this assumption is false, gpsd may not be able to open GPS devices in
528       order to read them (such failures will be logged).
529
530       In order to fend off inadvertent denial-of-service attacks by port
531       scanners (not to mention deliberate ones), gpsd will time out inactive
532       client connections. Before the client has issued a command that
533       requests a channel assignment, a short timeout (60 seconds) applies.
534       There is no timeout for clients in watcher or raw modes; rather, gpsd
535       drops these clients if they fail to read data long enough for the
536       outbound socket write buffer to fill. Clients with an assigned device
537       in polling mode are subject to a longer timeout (15 minutes).
538

LIMITATIONS

540       If multiple NMEA talkers are feeding RMC, GLL, and GGA sentences to the
541       same serial device (possible with an RS422 adapter hooked up to some
542       marine-navigation systems), a 'TPV' response may mix an altitude from
543       one device's GGA with latitude/longitude from another's RMC/GLL after
544       the second sentence has arrived.
545
546       gpsd may change control settings on your GPS (such as the emission
547       frequency of various sentences or packets) and not restore the original
548       settings on exit. This is a result of inadequacies in NMEA and the
549       vendor binary GPS protocols, which often do not give clients any way to
550       query the values of control settings in order to be able to restore
551       them later.
552
553       When using SiRF chips, the VDOP/TDOP/GDOP figures and associated error
554       estimates are computed by gpsd rather than reported by the chip. The
555       computation does not exactly match what SiRF chips do internally, which
556       includes some satellite weighting using parameters gpsd cannot see.
557
558       Autobauding on the Trimble GPSes can take as long as 5 seconds if the
559       device speed is not matched to the GPS speed.
560
561       Generation of position error estimates (eph, epv, epd, eps, epc) from
562       the incomplete data handed back by GPS reporting protocols involves
563       both a lot of mathematical black art and fragile device-dependent
564       assumptions. This code has been bug-prone in tbe past and problems may
565       still lurk there.
566
567       AIDVM decoding of types 16-17, 22-23, and 25-26 is unverified.
568
569       GPSD presently fully recognizes only the 2.1 level of RTCM2 (message
570       types 1, 3, 4, 5, 6, 7, 9, 16). The 2.3 message types 13, 14, and 31
571       are recognized and reported. Message types 8, 10-12, 15-27, 28-30
572       (undefined), 31-37, 38-58 (undefined), and 60-63 are not yet supported.
573
574       The ISGPS used for RTCM2 and subframes decoder logic is sufficiently
575       convoluted to confuse some compiler optimizers, notably in GCC 3.x at
576       -O2, into generating bad code.
577
578       Devices meant to use PPS for high-precision timekeeping may fail if
579       they are specified after startup by a control-socket command, as
580       opposed to on the daemon's original command line. Root privileges are
581       dropped early, and some Unix variants require them in order to set the
582       PPS line discipline. Under Linux the POSIX capability to set the line
583       discipline is retained, but other platforms cannot use this code.
584
585       USB GPS devices often do not identify themselves through the USB
586       subsystem; they typically present as the class 00h (undefined) or class
587       FFh (vendor-specific) of USB-to-serial adapters. Because of this, the
588       Linux hotplug scripts must tell gpsd to sniff data from every
589       USB-to-serial adapter that goes active and is known to be of a type
590       used in GPSes. No such device is sent configuration strings until after
591       it has been identified as a GPS, and gpsd never opens a device that is
592       opened by another process. But there is a tiny window for non-GPS
593       devices not opened; if the application that wants them loses a race
594       with GPSD its device open will fail and have to be retried after GPSD
595       sniffs the device (normally less than a second later).
596

FILES

598       /dev/ttyS0
599           Prototype TTY device. After startup, gpsd sets its group ID to the
600           owning group of this device if no GPS device was specified on the
601           command line does not exist.
602
603       /etc/gpsd/device-hook
604           Optional file containing the device activation/deactivation script.
605           Note that while /etc/gpsd is the default system configuration
606           directory, it is possible to build the GPSD source code with
607           different assumptions.
608

ENVIRONMENT VARIABLES

610       By setting the environment variable GPSD_SHM_KEY, you can control the
611       key value used to create the shared-memory segment used for
612       communication with the client library. This will be useful mainly when
613       isolating test instances of gpsd from production ones.
614

APPLICABLE STANDARDS

616       The official NMEA protocol standards for NMEA0183 and NMEA2000 are
617       available from the National Marine Electronics Association, but are
618       proprietary and expensive; the maintainers of gpsd have made a point of
619       not looking at them. The GPSD project website links to several
620       documents that collect publicly disclosed information about the
621       protocol.
622
623       gpsd parses the following NMEA sentences: RMC, GGA, GLL, GSA, GSV, VTG,
624       ZDA, GBS, HDT, DBT, GST. It recognizes these with either the normal GP
625       talker-ID prefix, or with the GN prefix used by GLONASS, or with the II
626       prefix emitted by Seahawk Autohelm marine navigation systems, or with
627       the IN prefix emitted by some Garmin units, or with the EC prefix
628       emitted by ECDIS units, or with the SD prefix emitted by depth
629       sounders, or with the HC and TI prefix emitted by some Airmar
630       equipment. It recognizes some vendor extensions: the PGRME emitted by
631       some Garmin GPS models, the OHPR emitted by Oceanserver digital
632       compasses, the PTNTHTM emitted by True North digital compasses, the
633       PMTK omitted by some San Jose Navigation GPSes, and the PASHR sentences
634       emitted by some Ashtech GPSes.
635
636       Note that gpsd JSON returns pure decimal degrees, not the hybrid
637       degree/minute format described in the NMEA standard.
638
639       Differential-GPS corrections are conveyed by the RTCM protocols. The
640       applicable standard for RTCM-104 V2 is RTCM Recommended Standards for
641       Differential GNSS (Global Navigation Satellite) Service RTCM Paper
642       136-2001/SC 104-STD. The applicable standard for RTCM-104 V3 is RTCM
643       Standard 10403.1 for Differential GNSS Services - Version 3 RTCM Paper
644       177-2006-SC104-STD. Ordering instructions for the RTCM standards are
645       accessible from the website of the Radio Technical Commission for
646       Maritime Services under "Publications".
647
648       AIS is defined by ITU Recommendation M.1371, Technical Characteristics
649       for a Universal Shipborne Automatic Identification System Using Time
650       Division Multiple Access. The AIVDM/AIVDO format understood by this
651       program is defined by IEC-PAS 61162-100, Maritime navigation and
652       radiocommunication equipment and systems. A more accessible description
653       of both can be found at AIVDM/AIVDO Protocol Decoding, on the
654       references page of the GPSD project website.
655
656       Subframe data is defined by IS-GPS-200E, GLOBAL POSITIONING SYSTEM WING
657       (GPSW) SYSTEMS ENGINEERING & INTEGRATION, INTERFACE SPECIFICATION
658       IS-GPS-200 Revision E. The format understood by this program is defined
659       in Section 20 (Appendix II) of the IS-GPS-200E, GPS NAVIGATION DATA
660       STRUCTURE FOR DATA, D(t)
661
662       JSON is specified by RFC 7159, The JavaScript Object Notation (JSON)
663       Data Interchange Format.
664
665       The API for PPS time service is specified by RFC 2783, Pulse-Per-Second
666       API for UNIX-like Operating Systems, Version 1.0
667

SEE ALSO

669       gpsdctl(8), gps(1), libgps(3), gpsd_json(5), libgpsmm(3), gpsprof(1),
670       gpsfake(1), gpsctl(1), gpscat(1),
671

AUTHORS

673       Authors: Eric S. Raymond, Chris Kuethe, Gary Miller. Former authors
674       whose bits have been plowed under by code turnover: Remco Treffcorn,
675       Derrick Brashear, Russ Nelson. This manual page by Eric S. Raymond
676       <esr@thyrsus.com>.
677
678
679
680The GPSD Project                  9 Aug 2004                           GPSD(8)
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