Provided by: libgps-dev_3.25-2ubuntu2_amd64 
NAME
gpsd_json - gpsd request/response protocol
OVERVIEW
gpsd is a service daemon that can be used to monitor GPSes, DGPS receivers, Marine AIS
broadcasts, and various other location-related and kinematic sensors.
Clients may communicate with gpsd via textual requests and responses over a socket. It is
a bad idea for applications to speak the protocol directly: rather, they should use the
libgps client library (for C; bindings also exist for other languages) and take
appropriate care to check in their code for the expected major and minor protocol
versions.
The GPSD protocol is built on top of JSON, JavaScript Object Notation, as specified in
[RFC-7159]: The JavaScript Object Notation (JSON) Data Interchange Format. Similar to ECMA
404.
GPSD’s use of JSON is restricted in some ways that make parsing it in fixed-extent
languages (such as C) easier.
A request line is introduced by "?" and may include multiple commands. Commands begin with
a command identifier, followed either by a terminating ';' or by an equal sign "=" and a
JSON object treated as an argument. Any ';' or newline indication (either LF or CR-LF)
after the end of a command is ignored. All request lines must be composed of US-ASCII
characters and may be no more than 80 characters in length, exclusive of the trailing
newline.
Responses are single JSON objects that have a "class" attribute the value of which is the
object type . Object types include, but are not limited to: "TPV", "SKY", "DEVICE", and
"ERROR". Objects are sent both in response to commands, and periodically as gpsd sends
reports. Each object is terminated by a carriage return and a new line (CR-NL).
The order of JSON attributes within a response object is never significant, and you may
specify command attributes in any order. Responses never contain the special JSON value
null; instead, attributes with empty or undefined values are omitted. The length limit for
responses and reports is currently 10240 characters, including the trailing CR-NL. Longer
responses will be truncated, so client code must be prepared for the possibility of
invalid JSON fragments.
The default maximum message length is set by GPS_JSON_RESPONSE_MAX in include/gpsd_json.h.
at compile time.
In JSON reports, if an attribute is present only if the parent attribute is present or has
a particular range, then the parent attribute is emitted first.
There is one constraint on the order in which attributes will be omitted. If an optional
attribute is present only when a parent attribute has a specified value or range of
values, the parent attribute will be emitted first to make parsing easier.
The next subsection section documents the core GPSD protocol. Extensions are documented in
the following subsections. The extensions may not be supported in your gpsd instance if it
has been compiled with a restricted feature set.
The protocol was designed and documented by Eric S. Raymond.
CORE PROTOCOL RESPONSES
Here are the core-protocol responses.
TPV
A TPV object is a time-position-velocity report. The "class" and "mode" fields will
reliably be present. When "mode" is 0 (Unknown) there is likely no usable data in the
sentence. The remaining fields are optional, their presence depends on what data the GNSS
receiver has sent, and what gpsd may calculate from that data.
A TPV object will usually be sent at least once for every measurement epoch as determined
by the "time" field. Unless the receiver has a solid fix, and knows the current leap
second, the time may be random.
Multiple TPV objects are often sent per epoch. When the receiver dribbles data to gpsd,
then gpsd has no choice but to dribble it to the client in multiple TPV messages.
The optional "status" field (aka fix type), is a modifier (adjective) to mode. It is not a
replacement for, or superset of, the "mode" field. It is almost, but not quite, the same
as the NMEA 4.x xxGGA GPS Quality Indicator Values. Many GNSS receivers do not supply it.
Those that do interpret the specification in various incompatible ways. To save space in
the output, and avoid confusion, the JSON never includes status values of 0 or 1.
All error estimates (epc, epd, epe, eph, ept, epv, epx, epy) are guessed to be 95%
confidence, may also be 50%, one sigma, or two sigma confidence. Many GNSS receivers do
not specify a confidence level. None specify how the value is calculated. Use error
estimates with caution, and only as relative "goodness" indicators. If the GPS reports a
value to gpsd, then gpsd will report that value. Otherwise gpsd will try to compute the
value from the skyview.
See the file include/gps.h, especially struct gps_data_t, for expanded notes on the items
and values in the TPV message.
Table 1. TPV object
┌────────────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "TPV" │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│device │ No │ string │ Name of the │
│ │ │ │ originating device. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│mode │ Yes │ numeric │ NMEA mode: │
│ │ │ │ 0=unknown, │
│ │ │ │ 1=no fix, │
│ │ │ │ 2=2D, │
│ │ │ │ 3=3D. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│status │ No │ numeric │ GPS fix status: │
│ │ │ │ 0=Unknown, │
│ │ │ │ 1=Normal, │
│ │ │ │ 2=DGPS, │
│ │ │ │ 3=RTK Fixed, │
│ │ │ │ 4=RTK Floating, │
│ │ │ │ 5=DR, │
│ │ │ │ 6=GNSSDR, │
│ │ │ │ 7=Time (surveyed), │
│ │ │ │ 8=Simulated, │
│ │ │ │ 9=P(Y) │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│time │ No │ string │ Time/date stamp in │
│ │ │ │ ISO8601 format, │
│ │ │ │ UTC. May have a │
│ │ │ │ fractional part of │
│ │ │ │ up to .001sec │
│ │ │ │ precision. May be │
│ │ │ │ absent if the mode │
│ │ │ │ is not 2D or 3D. │
│ │ │ │ May be present, but │
│ │ │ │ invalid, if there │
│ │ │ │ is no fix. Verify 3 │
│ │ │ │ consecutive 3D │
│ │ │ │ fixes before │
│ │ │ │ believing it is │
│ │ │ │ UTC. Even then it │
│ │ │ │ may be off by │
│ │ │ │ several seconds │
│ │ │ │ until the current │
│ │ │ │ leap seconds is │
│ │ │ │ known. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│altHAE │ No │ numeric │ Altitude, height │
│ │ │ │ above ellipsoid, in │
│ │ │ │ meters. Probably │
│ │ │ │ WGS84. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│altMSL │ No │ numeric │ MSL Altitude in │
│ │ │ │ meters. The geoid │
│ │ │ │ used is rarely │
│ │ │ │ specified and is │
│ │ │ │ often inaccurate. │
│ │ │ │ See the comments │
│ │ │ │ below on geoidSep. │
│ │ │ │ altMSL is altHAE │
│ │ │ │ minus geoidSep. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│alt │ No │ numeric │ Deprecated. │
│ │ │ │ Undefined. Use │
│ │ │ │ altHAE or altMSL. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│climb │ No │ numeric │ Climb (positive) or │
│ │ │ │ sink (negative) │
│ │ │ │ rate, meters per │
│ │ │ │ second. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│datum │ No │ string │ Current datum. │
│ │ │ │ Hopefully WGS84. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│depth │ No │ numeric │ Depth in meters. │
│ │ │ │ Probably depth │
│ │ │ │ below the keel... │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│dgpsAge │ No │ numeric │ Age of DGPS data. │
│ │ │ │ In seconds │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│dgpsSta │ No │ numeric │ Station of DGPS │
│ │ │ │ data. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│epc │ No │ numeric │ Estimated climb │
│ │ │ │ error in meters per │
│ │ │ │ second. Certainty │
│ │ │ │ unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│epd │ No │ numeric │ Estimated track │
│ │ │ │ (direction) error │
│ │ │ │ in degrees. │
│ │ │ │ Certainty unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│eph │ No │ numeric │ Estimated │
│ │ │ │ horizontal Position │
│ │ │ │ (2D) Error in │
│ │ │ │ meters. Also known │
│ │ │ │ as Estimated │
│ │ │ │ Position Error │
│ │ │ │ (epe). Certainty │
│ │ │ │ unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│eps │ No │ numeric │ Estimated speed │
│ │ │ │ error in meters per │
│ │ │ │ second. Certainty │
│ │ │ │ unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ept │ No │ numeric │ Estimated time │
│ │ │ │ stamp error in │
│ │ │ │ seconds. Certainty │
│ │ │ │ unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│epx │ No │ numeric │ Longitude error │
│ │ │ │ estimate in meters. │
│ │ │ │ Certainty unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│epy │ No │ numeric │ Latitude error │
│ │ │ │ estimate in meters. │
│ │ │ │ Certainty unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│epv │ No │ numeric │ Estimated vertical │
│ │ │ │ error in meters. │
│ │ │ │ Certainty unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│geoidSep │ No │ numeric │ Geoid separation is │
│ │ │ │ the difference │
│ │ │ │ between the WGS84 │
│ │ │ │ reference ellipsoid │
│ │ │ │ and the geoid (Mean │
│ │ │ │ Sea Level) in │
│ │ │ │ meters. Almost no │
│ │ │ │ GNSS receiver │
│ │ │ │ specifies how they │
│ │ │ │ compute their │
│ │ │ │ geoid. gpsd │
│ │ │ │ interpolates the │
│ │ │ │ geoid from a 5x5 │
│ │ │ │ degree table of │
│ │ │ │ EGM2008 values when │
│ │ │ │ the receiver does │
│ │ │ │ not supply a geoid │
│ │ │ │ separation. The │
│ │ │ │ gpsd computed │
│ │ │ │ geoidSep is usually │
│ │ │ │ within one meter of │
│ │ │ │ the "true" value, │
│ │ │ │ but can be off as │
│ │ │ │ much as 12 meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│lat │ No │ numeric │ Latitude in │
│ │ │ │ degrees: +/- │
│ │ │ │ signifies │
│ │ │ │ North/South. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│leapseconds │ No │ integer │ Current leap │
│ │ │ │ seconds. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│lon │ No │ numeric │ Longitude in │
│ │ │ │ degrees: +/- │
│ │ │ │ signifies │
│ │ │ │ East/West. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│track │ No │ numeric │ Course over ground, │
│ │ │ │ degrees from true │
│ │ │ │ north. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│magtrack │ No │ numeric │ Course over ground, │
│ │ │ │ degrees magnetic. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│magvar │ No │ numeric │ Magnetic variation, │
│ │ │ │ degrees. Also known │
│ │ │ │ as the magnetic │
│ │ │ │ declination (the │
│ │ │ │ direction of the │
│ │ │ │ horizontal │
│ │ │ │ component of the │
│ │ │ │ magnetic field │
│ │ │ │ measured clockwise │
│ │ │ │ from north) in │
│ │ │ │ degrees, Positive │
│ │ │ │ is West variation. │
│ │ │ │ Negative is East │
│ │ │ │ variation. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│speed │ No │ numeric │ Speed over ground, │
│ │ │ │ meters per second. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ecefx │ No │ numeric │ ECEF X position in │
│ │ │ │ meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ecefy │ No │ numeric │ ECEF Y position in │
│ │ │ │ meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ecefz │ No │ numeric │ ECEF Z position in │
│ │ │ │ meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ecefpAcc │ No │ numeric │ ECEF position error │
│ │ │ │ in meters. │
│ │ │ │ Certainty unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ecefvx │ No │ numeric │ ECEF X velocity in │
│ │ │ │ meters per second. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ecefvy │ No │ numeric │ ECEF Y velocity in │
│ │ │ │ meters per second. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ecefvz │ No │ numeric │ ECEF Z velocity in │
│ │ │ │ meters per second. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ecefvAcc │ No │ numeric │ ECEF velocity error │
│ │ │ │ in meters per │
│ │ │ │ second. Certainty │
│ │ │ │ unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│sep │ No │ numeric │ Estimated Spherical │
│ │ │ │ (3D) Position Error │
│ │ │ │ in meters. Guessed │
│ │ │ │ to be 95% │
│ │ │ │ confidence, but │
│ │ │ │ many GNSS receivers │
│ │ │ │ do not specify, so │
│ │ │ │ certainty unknown. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│relD │ No │ numeric │ Down component of │
│ │ │ │ relative position │
│ │ │ │ vector in meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│relE │ No │ numeric │ East component of │
│ │ │ │ relative position │
│ │ │ │ vector in meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│relN │ No │ numeric │ North component of │
│ │ │ │ relative position │
│ │ │ │ vector in meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│velD │ No │ numeric │ Down velocity │
│ │ │ │ component in │
│ │ │ │ meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│velE │ No │ numeric │ East velocity │
│ │ │ │ component in │
│ │ │ │ meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│velN │ No │ numeric │ North velocity │
│ │ │ │ component in │
│ │ │ │ meters. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│wanglem │ No │ numeric │ Wind angle magnetic │
│ │ │ │ in degrees. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│wangler │ No │ numeric │ Wind angle relative │
│ │ │ │ in degrees. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│wanglet │ No │ numeric │ Wind angle true in │
│ │ │ │ degrees. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│wspeedr │ No │ numeric │ Wind speed relative │
│ │ │ │ in meters per │
│ │ │ │ second. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│wspeedt │ No │ numeric │ Wind speed true in │
│ │ │ │ meters per second. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│wtemp │ No │ numeric │ Water temperature │
│ │ │ │ in degrees Celsius. │
└────────────┴─────────┴─────────┴─────────────────────┘
When the C client library parses a response of this kind, it will assert validity bits in
the top-level set member for each field received; see gps.h for bitmask names and values.
Invalid or unknown floating-point values will be set to NAN. Always check floating point
values with isfinite() before use. isnan() is not sufficient.
Here’s an example TPV sentence:
{"class":"TPV","device":"/dev/pts/1",
"time":"2005-06-08T10:34:48.283Z","ept":0.005,
"lat":46.498293369,"lon":7.567411672,"alt":1343.127,
"eph":36.000,"epv":32.321,
"track":10.3788,"speed":0.091,"climb":-0.085,"mode":3}
SKY
A SKY object reports a sky view of the GPS satellite positions. If there is no GPS device
available, or no skyview has been reported yet, only the "class" field will reliably be
present.
Table 2. SKY object
┌───────────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "SKY" │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│device │ No │ string │ Name of originating │
│ │ │ │ device │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│nSat │ No │ numeric │ Number of satellite │
│ │ │ │ objects in │
│ │ │ │ "satellites" array. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│gdop │ No │ numeric │ Geometric │
│ │ │ │ (hyperspherical) │
│ │ │ │ dilution of │
│ │ │ │ precision, a │
│ │ │ │ combination of PDOP │
│ │ │ │ and TDOP. A │
│ │ │ │ dimensionless │
│ │ │ │ factor which should │
│ │ │ │ be multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│hdop │ No │ numeric │ Horizontal dilution │
│ │ │ │ of precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which should │
│ │ │ │ be multiplied by a │
│ │ │ │ base UERE to get a │
│ │ │ │ circular error │
│ │ │ │ estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│pdop │ No │ numeric │ Position │
│ │ │ │ (spherical/3D) │
│ │ │ │ dilution of │
│ │ │ │ precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which should │
│ │ │ │ be multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│prRes │ No │ numeric │ Pseudorange residue │
│ │ │ │ in meters. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│qual │ No │ numeric │ Quality Indicator │
│ │ │ │ 0=no signal │
│ │ │ │ 1=searching signal │
│ │ │ │ 2=signal acquired │
│ │ │ │ 3=signal detected │
│ │ │ │ but unusable │
│ │ │ │ 4=code locked and │
│ │ │ │ time synchronized │
│ │ │ │ 5, 6, 7=code and │
│ │ │ │ carrier locked and │
│ │ │ │ time synchronized │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│satellites │ No │ list │ List of satellite │
│ │ │ │ objects in skyview │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│tdop │ No │ numeric │ Time dilution of │
│ │ │ │ precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which should │
│ │ │ │ be multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│time │ No │ string │ Time/date stamp in │
│ │ │ │ ISO8601 format, │
│ │ │ │ UTC. May have a │
│ │ │ │ fractional part of │
│ │ │ │ up to .001sec │
│ │ │ │ precision. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│uSat │ No │ numeric │ Number of │
│ │ │ │ satellites used in │
│ │ │ │ navigation │
│ │ │ │ solution. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│vdop │ No │ numeric │ Vertical (altitude) │
│ │ │ │ dilution of │
│ │ │ │ precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which should │
│ │ │ │ be multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│xdop │ No │ numeric │ Longitudinal │
│ │ │ │ dilution of │
│ │ │ │ precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which should │
│ │ │ │ be multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ydop │ No │ numeric │ Latitudinal │
│ │ │ │ dilution of │
│ │ │ │ precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which should │
│ │ │ │ be multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error estimate. │
└───────────┴─────────┴─────────┴─────────────────────┘
Many devices compute dilution of precision factors but do not include them in their
reports. Many that do report DOPs report only HDOP, two-dimensional circular error. gpsd
always passes through whatever the device reports, then attempts to fill in other DOPs by
calculating the appropriate determinants in a covariance matrix based on the satellite
view. DOPs may be missing if some of these determinants are singular. It can even happen
that the device reports an error estimate in meters when the corresponding DOP is
unavailable; some devices use more sophisticated error modeling than the covariance
calculation.
The satellite list objects have the following elements:
Table 3. Satellite object
┌───────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│PRN │ Yes │ numeric │ PRN ID of the │
│ │ │ │ satellite. 1-63 are │
│ │ │ │ GNSS satellites, │
│ │ │ │ 64-96 are GLONASS │
│ │ │ │ satellites, 100-164 │
│ │ │ │ are SBAS satellites │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│az │ No │ numeric │ Azimuth, degrees │
│ │ │ │ from true north. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│el │ No │ numeric │ Elevation in │
│ │ │ │ degrees. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│ss │ No │ numeric │ Signal to Noise │
│ │ │ │ ratio in dBHz. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│used │ Yes │ boolean │ Used in current │
│ │ │ │ solution? │
│ │ │ │ (SBAS/WAAS/EGNOS │
│ │ │ │ satellites may be │
│ │ │ │ flagged used if the │
│ │ │ │ solution has │
│ │ │ │ corrections from │
│ │ │ │ them, but not all │
│ │ │ │ drivers make this │
│ │ │ │ information │
│ │ │ │ available.) │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│gnssid │ No │ numeric │ The GNSS ID, as │
│ │ │ │ defined by u-blox, │
│ │ │ │ not NMEA. 0=GPS, │
│ │ │ │ 2=Galileo, │
│ │ │ │ 3=Beidou, 5=QZSS, │
│ │ │ │ 6-GLONASS. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│svid │ No │ numeric │ The satellite ID │
│ │ │ │ within its │
│ │ │ │ constellation. As │
│ │ │ │ defined by u-blox, │
│ │ │ │ not NMEA). │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│sigid │ No │ numeric │ The signal ID of │
│ │ │ │ this signal. As │
│ │ │ │ defined by u-blox, │
│ │ │ │ not NMEA. See │
│ │ │ │ u-blox doc for │
│ │ │ │ details. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│freqid │ No │ numeric │ For GLONASS │
│ │ │ │ satellites only: │
│ │ │ │ the frequency ID of │
│ │ │ │ the signal. As │
│ │ │ │ defined by u-blox, │
│ │ │ │ range 0 to 13. The │
│ │ │ │ freqid is the │
│ │ │ │ frequency slot plus │
│ │ │ │ 7. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│health │ No │ numeric │ The health of this │
│ │ │ │ satellite. 0 is │
│ │ │ │ unknown, 1 is OK, │
│ │ │ │ and 2 is unhealthy. │
└───────┴─────────┴─────────┴─────────────────────┘
Note that satellite objects do not have a "class" field, as they are never shipped outside
of a SKY object.
When the C client library parses a SKY response, it will assert the SATELLITE_SET bit in
the top-level set member.
Here’s an example:
{"class":"SKY","device":"/dev/pts/1",
"time":"2005-07-08T11:28:07.114Z",
"xdop":1.55,"hdop":1.24,"pdop":1.99,
"satellites":[
{"PRN":23,"el":6,"az":84,"ss":0,"used":false},
{"PRN":28,"el":7,"az":160,"ss":0,"used":false},
{"PRN":8,"el":66,"az":189,"ss":44,"used":true},
{"PRN":29,"el":13,"az":273,"ss":0,"used":false},
{"PRN":10,"el":51,"az":304,"ss":29,"used":true},
{"PRN":4,"el":15,"az":199,"ss":36,"used":true},
{"PRN":2,"el":34,"az":241,"ss":43,"used":true},
{"PRN":27,"el":71,"az":76,"ss":43,"used":true}]}
GST
A GST object is a pseudorange noise report.
Table 4. GST object
┌───────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "GST" │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│device │ No │ string │ Name of originating │
│ │ │ │ device │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│time │ No │ string │ Time/date stamp in │
│ │ │ │ ISO8601 format, │
│ │ │ │ UTC. May have a │
│ │ │ │ fractional part of │
│ │ │ │ up to .001sec │
│ │ │ │ precision. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│rms │ No │ numeric │ Value of the │
│ │ │ │ standard deviation │
│ │ │ │ of the range inputs │
│ │ │ │ to the navigation │
│ │ │ │ process (range │
│ │ │ │ inputs include │
│ │ │ │ pseudoranges and │
│ │ │ │ DGPS corrections). │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│major │ No │ numeric │ Standard deviation │
│ │ │ │ of semi-major axis │
│ │ │ │ of error ellipse, │
│ │ │ │ in meters. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│minor │ No │ numeric │ Standard deviation │
│ │ │ │ of semi-minor axis │
│ │ │ │ of error ellipse, │
│ │ │ │ in meters. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│orient │ No │ numeric │ Orientation of │
│ │ │ │ semi-major axis of │
│ │ │ │ error ellipse, in │
│ │ │ │ degrees from true │
│ │ │ │ north. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│lat │ No │ numeric │ Standard deviation │
│ │ │ │ of latitude error, │
│ │ │ │ in meters. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│lon │ No │ numeric │ Standard deviation │
│ │ │ │ of longitude error, │
│ │ │ │ in meters. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│alt │ No │ numeric │ Standard deviation │
│ │ │ │ of altitude error, │
│ │ │ │ in meters. │
└───────┴─────────┴─────────┴─────────────────────┘
Here’s an example:
{"class":"GST","device":"/dev/ttyUSB0",
"time":"2010-12-07T10:23:07.096Z","rms":2.440,
"major":1.660,"minor":1.120,"orient":68.989,
"lat":1.600,"lon":1.200,"alt":2.520}
ATT
An ATT object is a vehicle-attitude report. It is returned by digital-compass and
gyroscope sensors; depending on device, it may include: heading, pitch, roll, yaw,
gyroscope, and magnetic-field readings. Because such sensors are often bundled as part of
marine-navigation systems, the ATT response may also include water depth.
The "class" and "mode" fields will reliably be present. Others may be reported or not
depending on the specific device type.
The ATT object is synchronous to the GNSS epoch. Some devices report attitude information
with arbitrary, even out of order, time scales. gpsd reports those in an IMU object. The
ATT and IMU objects have the same fields, but IMU objects are output as soon as possible.
Some devices output both types with arbitrary interleaving.
Table 5. ATT object
┌─────────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "ATT" │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│device │ Yes │ string │ Name of originating │
│ │ │ │ device │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│time │ No │ string │ Time/date stamp in │
│ │ │ │ ISO8601 format, │
│ │ │ │ UTC. May have a │
│ │ │ │ fractional part of │
│ │ │ │ up to .001sec │
│ │ │ │ precision. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│timeTag │ No │ string │ Arbitrary time tag │
│ │ │ │ of measurement. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│heading │ No │ numeric │ Heading, degrees │
│ │ │ │ from true north. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│mag_st │ No │ string │ Magnetometer │
│ │ │ │ status. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│mheading │ No │ numeric │ Heading, degrees │
│ │ │ │ from magnetic │
│ │ │ │ north. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│pitch │ No │ numeric │ Pitch in degrees. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│pitch_st │ No │ string │ Pitch sensor │
│ │ │ │ status. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│rot │ No │ numeric │ Rate of Turn in │
│ │ │ │ dgrees per minute. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│yaw │ No │ numeric │ Yaw in degrees │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│yaw_st │ No │ string │ Yaw sensor status. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│roll │ No │ numeric │ Roll in degrees. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│roll_st │ No │ string │ Roll sensor status. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│dip │ No │ numeric │ Local magnetic │
│ │ │ │ inclination, │
│ │ │ │ degrees, positive │
│ │ │ │ when the magnetic │
│ │ │ │ field points │
│ │ │ │ downward (into the │
│ │ │ │ Earth). │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│mag_len │ No │ numeric │ Scalar magnetic │
│ │ │ │ field strength. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│mag_x │ No │ numeric │ X component of │
│ │ │ │ magnetic field │
│ │ │ │ strength. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│mag_y │ No │ numeric │ Y component of │
│ │ │ │ magnetic field │
│ │ │ │ strength. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│mag_z │ No │ numeric │ Z component of │
│ │ │ │ magnetic field │
│ │ │ │ strength. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│acc_len │ No │ numeric │ Scalar │
│ │ │ │ acceleration. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│acc_x │ No │ numeric │ X component of │
│ │ │ │ acceleration │
│ │ │ │ (m/s^2). │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│acc_y │ No │ numeric │ Y component of │
│ │ │ │ acceleration │
│ │ │ │ (m/s^2). │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│acc_z │ No │ numeric │ Z component of │
│ │ │ │ acceleration │
│ │ │ │ (m/s^2). │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│gyro_x │ No │ numeric │ X component of │
│ │ │ │ angular rate │
│ │ │ │ (deg/s) │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│gyro_y │ No │ numeric │ Y component of │
│ │ │ │ angular rate │
│ │ │ │ (deg/s) │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│gyro_z │ No │ numeric │ Z component of │
│ │ │ │ angular rate │
│ │ │ │ (deg/s) │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│depth │ No │ numeric │ Water depth in │
│ │ │ │ meters. │
├─────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│temp │ No │ numeric │ Temperature at the │
│ │ │ │ sensor, degrees │
│ │ │ │ centigrade. │
└─────────┴─────────┴─────────┴─────────────────────┘
The heading, pitch, and roll status codes (if present) vary by device. For the TNT
Revolution digital compasses, they are coded as follows:
Table 6. Device flags
┌─────┬──────────────────────────────────┐
│ │ │
│Code │ Description │
├─────┼──────────────────────────────────┤
│ │ │
│C │ magnetometer calibration alarm │
├─────┼──────────────────────────────────┤
│ │ │
│L │ low alarm │
├─────┼──────────────────────────────────┤
│ │ │
│M │ low warning │
├─────┼──────────────────────────────────┤
│ │ │
│N │ normal │
├─────┼──────────────────────────────────┤
│ │ │
│O │ high warning │
├─────┼──────────────────────────────────┤
│ │ │
│P │ high alarm │
├─────┼──────────────────────────────────┤
│ │ │
│V │ magnetometer voltage level alarm │
└─────┴──────────────────────────────────┘
When the C client library parses a response of this kind, it will assert ATT_IS.
Here’s an example:
{"class":"ATT","time":1270938096.843,
"heading":14223.00,"mag_st":"N",
"pitch":169.00,"pitch_st":"N", "roll":-43.00,"roll_st":"N",
"dip":13641.000,"mag_x":2454.000}
IMU
The IMU object is asynchronous to the GNSS epoch. It is reported with arbitrary, even out
of order, time scales.
The ATT and IMU objects have the same fields, but IMU objects are output as soon as
possible.
Seee the ATT onject description for field details.
TOFF
This message is emitted on each cycle and reports the offset between the host’s clock time
and the GPS time at top of the second (actually, when the first data for the reporting
cycle is received).
This message exactly mirrors the PPS message.
The TOFF message reports the GPS time as derived from the GPS serial data stream. The PPS
message reports the GPS time as derived from the GPS PPS pulse.
A TOFF object has the following elements:
Table 7. TOFF object
┌───────────┬─────────┬─────────┬────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├───────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "TOFF" │
├───────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│device │ Yes │ string │ Name of the │
│ │ │ │ originating device │
├───────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│real_sec │ Yes │ numeric │ seconds from the │
│ │ │ │ GPS clock │
├───────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│real_nsec │ Yes │ numeric │ nanoseconds from │
│ │ │ │ the GPS clock │
├───────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│clock_sec │ Yes │ numeric │ seconds from the │
│ │ │ │ system clock │
├───────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│clock_nsec │ Yes │ numeric │ nanoseconds from │
│ │ │ │ the system clock │
└───────────┴─────────┴─────────┴────────────────────┘
This message is emitted once per second to watchers of a device and is intended to report
the timestamps of the in-band report of the GPS and seconds as reported by the system
clock (which may be NTP-corrected) when the first valid time stamp of the reporting cycle
was seen.
The message contains two second/nanosecond pairs: real_sec and real_nsec contain the time
the GPS thinks it was at the start of the current cycle; clock_sec and clock_nsec contain
the time the system clock thinks it was on receipt of the first timing message of the
cycle. real_nsec is always to nanosecond precision. clock_nsec is nanosecond precision on
most systems.
Here’s an example:
{"class":"TOFF","device":"/dev/ttyUSB0",
"real_sec":1330212592, "real_nsec":343182,
"clock_sec":1330212592,"clock_nsec":343184,
"precision":-2}
PPS
This message is emitted each time the daemon sees a valid PPS (Pulse Per Second) strobe
from a device.
This message exactly mirrors the TOFF message.
The TOFF message reports the GPS time as derived from the GPS serial data stream. The PPS
message reports the GPS time as derived from the GPS PPS pulse.
There are various sources of error in the reported clock times. The speed of the serial
connection between the GPS and the system adds a delay to the start of cycle detection. An
even bigger error is added by the variable computation time inside the GPS. Taken together
the time derived from the start of the GPS cycle can have offsets of 10 milliseconds to
700 milliseconds and combined jitter and wander of 100 to 300 milliseconds.
See the NTP documentation for their definition of precision.
A PPS object has the following elements:
Table 8. PPS object
┌───────────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "PPS" │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│device │ Yes │ string │ Name of the │
│ │ │ │ originating device │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│real_sec │ Yes │ numeric │ seconds from the │
│ │ │ │ PPS source │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│real_nsec │ Yes │ numeric │ nanoseconds from │
│ │ │ │ the PPS source │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│clock_sec │ Yes │ numeric │ seconds from the │
│ │ │ │ system clock │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│clock_nsec │ Yes │ numeric │ nanoseconds from │
│ │ │ │ the system clock │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│precision │ Yes │ numeric │ NTP style estimate │
│ │ │ │ of PPS precision │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│shm │ Yes │ string │ shm key of this PPS │
├───────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│qErr │ No │ numeric │ Quantization error │
│ │ │ │ of the PPS, in │
│ │ │ │ picoseconds. │
│ │ │ │ Sometimes called │
│ │ │ │ the "sawtooth" │
│ │ │ │ error. │
└───────────┴─────────┴─────────┴─────────────────────┘
This message is emitted once per second to watchers of a device emitting PPS, and reports
the time of the start of the GPS second (when the 1PPS arrives) and seconds as reported by
the system clock (which may be NTP-corrected) at that moment.
The message contains two second/nanosecond pairs: real_sec and real_nsec contain the time
the GPS thinks it was at the PPS edge; clock_sec and clock_nsec contain the time the
system clock thinks it was at the PPS edge. real_nsec is always to nanosecond precision.
clock_nsec is nanosecond precision on most systems.
There are various sources of error in the reported clock times. For PPS delivered via a
real serial-line strobe, serial-interrupt latency plus processing time to the timer call
should be bounded above by about 10 microseconds; that can be reduced to less than 1
microsecond if your kernel supports [RFC-2783]. USB1.1-to-serial control-line emulation is
limited to about 1 millisecond. seconds.
Here’s an example:
{"class":"PPS","device":"/dev/ttyUSB0",
"real_sec":1330212592, "real_nsec":343182,
"clock_sec":1330212592,"clock_nsec":343184,
"precision":-3}
OSC
This message reports the status of a GPS-disciplined oscillator (GPSDO). The GPS PPS
output (which has excellent long-term stability) is typically used to discipline a local
oscillator with much better short-term stability (such as a rubidium atomic clock).
An OSC object has the following elements:
Table 9. OSC object
┌────────────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "OSC" │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│device │ Yes │ string │ Name of the │
│ │ │ │ originating device. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│running │ Yes │ boolean │ If true, the │
│ │ │ │ oscillator is │
│ │ │ │ currently running. │
│ │ │ │ Oscillators may │
│ │ │ │ require warm-up │
│ │ │ │ time at the start │
│ │ │ │ of the day. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│reference │ Yes │ boolean │ If true, the │
│ │ │ │ oscillator is │
│ │ │ │ receiving a GPS PPS │
│ │ │ │ signal. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│disciplined │ Yes │ boolean │ If true, the GPS │
│ │ │ │ PPS signal is │
│ │ │ │ sufficiently stable │
│ │ │ │ and is being used │
│ │ │ │ to discipline the │
│ │ │ │ local oscillator. │
├────────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│delta │ Yes │ numeric │ The time difference │
│ │ │ │ (in nanoseconds) │
│ │ │ │ between the │
│ │ │ │ GPS-disciplined │
│ │ │ │ oscillator PPS │
│ │ │ │ output pulse and │
│ │ │ │ the most recent GPS │
│ │ │ │ PPS input pulse. │
└────────────┴─────────┴─────────┴─────────────────────┘
Here’s an example:
{"class":"OSC","running":true,"device":"/dev/ttyUSB0",
"reference":true,"disciplined":true,"delta":67}
CORE PROTOCOL COMMANDS
And here are the commands you can send to gpsd.
?VERSION;
Returns an object with the following attributes:
Table 10. VERSION object
┌────────────┬─────────┬─────────┬────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├────────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "VERSION" │
├────────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│release │ Yes │ string │ Public release │
│ │ │ │ level │
├────────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│rev │ Yes │ string │ Internal │
│ │ │ │ revision-control │
│ │ │ │ level. │
├────────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│proto_major │ Yes │ numeric │ API major revision │
│ │ │ │ level. │
├────────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│proto_minor │ Yes │ numeric │ API minor revision │
│ │ │ │ level. │
├────────────┼─────────┼─────────┼────────────────────┤
│ │ │ │ │
│remote │ No │ string │ URL of the remote │
│ │ │ │ daemon reporting │
│ │ │ │ this version. If │
│ │ │ │ empty, this is the │
│ │ │ │ version of the │
│ │ │ │ local daemon. │
└────────────┴─────────┴─────────┴────────────────────┘
The daemon ships a VERSION response to each client when the client first connects to it.
When the C client library parses a response of this kind, it will assert the VERSION_SET
bit in the top-level set member.
Here’s an example:
{"class":"VERSION","version":"2.40dev",
"rev":"06f62e14eae9886cde907dae61c124c53eb1101f",
"proto_major":3,"proto_minor":1
}
?DEVICES;
Returns a device list object with the following elements:
Table 11. DEVICES object
┌────────┬─────────┬────────┬────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├────────┼─────────┼────────┼────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "DEVICES" │
├────────┼─────────┼────────┼────────────────────┤
│ │ │ │ │
│devices │ Yes │ list │ List of device │
│ │ │ │ descriptions │
├────────┼─────────┼────────┼────────────────────┤
│ │ │ │ │
│remote │ No │ string │ URL of the remote │
│ │ │ │ daemon reporting │
│ │ │ │ the device set. If │
│ │ │ │ empty, this is a │
│ │ │ │ DEVICES response │
│ │ │ │ from the local │
│ │ │ │ daemon. │
└────────┴─────────┴────────┴────────────────────┘
When the C client library parses a response of this kind, it will assert the
DEVICELIST_SET bit in the top-level set member.
Here’s an example:
{"class"="DEVICES","devices":[
{"class":"DEVICE","path":"/dev/pts/1","flags":1,"driver":"SiRF binary"},
{"class":"DEVICE","path":"/dev/pts/3","flags":4,"driver":"AIVDM"}]}
The daemon occasionally ships a bare DEVICE object to the client (that is, one not inside
a DEVICES wrapper). The data content of these objects will be described later as a
response to the ?DEVICE command.
?WATCH;
This command sets watcher mode. It also sets or elicits a report of per-subscriber policy
and the raw bit. An argument WATCH object changes the subscriber’s policy. The response
describes the subscriber’s policy. The response will also include a DEVICES object.
A WATCH object has the following elements:
Table 12. WATCH object
┌────────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "WATCH" │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│enable │ No │ boolean │ Enable (true) or │
│ │ │ │ disable (false) │
│ │ │ │ watcher mode. │
│ │ │ │ Default is true. │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│json │ No │ boolean │ Enable (true) or │
│ │ │ │ disable (false) │
│ │ │ │ dumping of JSON │
│ │ │ │ reports. Default is │
│ │ │ │ false. │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│nmea │ No │ boolean │ Enable (true) or │
│ │ │ │ disable (false) │
│ │ │ │ dumping of binary │
│ │ │ │ packets as │
│ │ │ │ pseudo-NMEA. │
│ │ │ │ Default is false. │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│raw │ No │ integer │ Controls 'raw' │
│ │ │ │ mode. When this │
│ │ │ │ attribute is set to │
│ │ │ │ 1 for a channel, │
│ │ │ │ gpsd reports the │
│ │ │ │ unprocessed NMEA or │
│ │ │ │ AIVDM data stream │
│ │ │ │ from whatever │
│ │ │ │ device is attached. │
│ │ │ │ Binary GPS packets │
│ │ │ │ are hex-dumped. │
│ │ │ │ RTCM2 and RTCM3 │
│ │ │ │ packets are not │
│ │ │ │ dumped in raw mode. │
│ │ │ │ When this attribute │
│ │ │ │ is set to 2 for a │
│ │ │ │ channel that │
│ │ │ │ processes binary │
│ │ │ │ data, gpsd reports │
│ │ │ │ the received data │
│ │ │ │ verbatim without │
│ │ │ │ hex-dumping. │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│scaled │ No │ boolean │ If true, apply │
│ │ │ │ scaling divisors to │
│ │ │ │ output before │
│ │ │ │ dumping; default is │
│ │ │ │ false. │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│split24 │ No │ boolean │ If true, aggregate │
│ │ │ │ AIS type24 sentence │
│ │ │ │ parts. If false, │
│ │ │ │ report each part as │
│ │ │ │ a separate JSON │
│ │ │ │ object, leaving the │
│ │ │ │ client to match │
│ │ │ │ MMSIs and │
│ │ │ │ aggregate. Default │
│ │ │ │ is false. Applies │
│ │ │ │ only to AIS │
│ │ │ │ reports. │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│pps │ No │ boolean │ If true, emit the │
│ │ │ │ TOFF JSON message │
│ │ │ │ on each cycle and a │
│ │ │ │ PPS JSON message │
│ │ │ │ when the device │
│ │ │ │ issues 1PPS. │
│ │ │ │ Default is false. │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│device │ No │ string │ If present, enable │
│ │ │ │ watching only of │
│ │ │ │ the specified │
│ │ │ │ device rather than │
│ │ │ │ all devices. Useful │
│ │ │ │ with raw and NMEA │
│ │ │ │ modes in which │
│ │ │ │ device responses │
│ │ │ │ aren’t tagged. Has │
│ │ │ │ no effect when used │
│ │ │ │ with enable:false. │
├────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│remote │ No │ string │ URL of the remote │
│ │ │ │ daemon reporting │
│ │ │ │ the watch set. If │
│ │ │ │ empty, this is a │
│ │ │ │ WATCH response from │
│ │ │ │ the local daemon. │
└────────┴─────────┴─────────┴─────────────────────┘
There is an additional boolean "timing" attribute which is undocumented because that
portion of the interface is considered unstable and for developer use only.
In watcher mode, GPS reports are dumped as TPV and SKY responses. AIS, Subframe and RTCM
reporting is described in the next section.
When the C client library parses a response of this kind, it will assert the POLICY_SET
bit in the top-level set member.
Here’s an example:
{"class":"WATCH", "raw":1,"scaled":true}
?POLL;
The POLL command requests data from the last-seen fixes on all active GPS devices. Devices
must previously have been activated by ?WATCH to be pollable.
Polling can lead to possibly surprising results when it is used on a device such as an
NMEA GPS for which a complete fix has to be accumulated from several sentences. If you
poll while those sentences are being emitted, the response will contain only the fix data
collected so far in the current epoch. It may be as much as one cycle time (typically 1
second) stale.
The POLL response will contain a timestamped list of TPV objects describing cached data,
and a timestamped list of SKY objects describing satellite configuration. If a device has
not seen fixes, it will be reported with a mode field of zero.
Table 13. POLL object
┌───────┬─────────┬────────────┬────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├───────┼─────────┼────────────┼────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "POLL" │
├───────┼─────────┼────────────┼────────────────────┤
│ │ │ │ │
│time │ Yes │ Numeric │ Timestamp in ISO │
│ │ │ │ 8601 format. May │
│ │ │ │ have a fractional │
│ │ │ │ part of up to │
│ │ │ │ .001sec precision. │
├───────┼─────────┼────────────┼────────────────────┤
│ │ │ │ │
│active │ Yes │ Numeric │ Count of active │
│ │ │ │ devices. │
├───────┼─────────┼────────────┼────────────────────┤
│ │ │ │ │
│tpv │ Yes │ JSON array │ Comma-separated │
│ │ │ │ list of TPV │
│ │ │ │ objects. │
├───────┼─────────┼────────────┼────────────────────┤
│ │ │ │ │
│sky │ Yes │ JSON array │ Comma-separated │
│ │ │ │ list of SKY │
│ │ │ │ objects. │
└───────┴─────────┴────────────┴────────────────────┘
Here’s an example of a POLL response:
{"class":"POLL","time":"2010-06-04T10:31:00.289Z","active":1,
"tpv":[{"class":"TPV","device":"/dev/ttyUSB0",
"time":"2010-09-08T13:33:06.095Z",
"ept":0.005,"lat":40.035093060,
"lon":-75.519748733,"track":99.4319,"speed":0.123,"mode":2}],
"sky":[{"class":"SKY","device":"/dev/ttyUSB0",
"time":1270517264.240,"hdop":9.20,
"satellites":[{"PRN":16,"el":55,"az":42,"ss":36,"used":true},
{"PRN":19,"el":25,"az":177,"ss":0,"used":false},
{"PRN":7,"el":13,"az":295,"ss":0,"used":false},
{"PRN":6,"el":56,"az":135,"ss":32,"used":true},
{"PRN":13,"el":47,"az":304,"ss":0,"used":false},
{"PRN":23,"el":66,"az":259,"ss":0,"used":false},
{"PRN":20,"el":7,"az":226,"ss":0,"used":false},
{"PRN":3,"el":52,"az":163,"ss":32,"used":true},
{"PRN":31,"el":16,"az":102,"ss":0,"used":false}
]}]}
Note
Client software should not assume the field inventory of the POLL response is fixed
for all time. As gpsd collects and caches more data from more sensor types, those data
are likely to find their way into this response.
?DEVICE; ?DEVICE=
This command reports (when followed by ';') the state of a device, or sets (when followed
by '=' and a DEVICE object) device-specific control bits, notably the device’s speed and
serial mode and the native-mode bit. The parameter-setting form will be rejected if more
than one client is attached to the channel and a device path is not specified.
Pay attention to the response, because it is possible for this command to fail if the GPS
does not support a command or only supports some combinations of modes. In case of
failure, the daemon and GPS will continue to communicate at the old speed.
Use the parameter-setting form with caution. On USB and Bluetooth GPSes it is also
possible for serial mode setting to fail either because the serial adaptor chip does not
support non-8N1 modes or because the device firmware does not properly synchronize the
serial adaptor chip with the UART on the GPS chipset when the speed changes. These
failures can hang your device, possibly requiring a GPS power cycle or (in extreme cases)
physically disconnecting the NVRAM backup battery.
A DEVICE object has the following elements:
Table 14. DEVICE object
┌──────────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "DEVICE" │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│activated │ No │ string │ Time the device was │
│ │ │ │ activated as an │
│ │ │ │ ISO8601 time stamp. │
│ │ │ │ If the device is │
│ │ │ │ inactive this │
│ │ │ │ attribute is │
│ │ │ │ absent. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│bps │ No │ integer │ Device speed in │
│ │ │ │ bits per second. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│cycle │ No │ real │ Device cycle time │
│ │ │ │ in seconds. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│driver │ No │ string │ GPSD’s name for the │
│ │ │ │ device driver type. │
│ │ │ │ Won’t be reported │
│ │ │ │ before gpsd has │
│ │ │ │ seen identifiable │
│ │ │ │ packets from the │
│ │ │ │ device. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│flags │ No │ integer │ Bit vector of │
│ │ │ │ property flags. │
│ │ │ │ Currently defined │
│ │ │ │ flags are: describe │
│ │ │ │ packet types seen │
│ │ │ │ so far (GPS, RTCM2, │
│ │ │ │ RTCM3, AIS). Won’t │
│ │ │ │ be reported if │
│ │ │ │ empty, e.g. before │
│ │ │ │ gpsd has seen │
│ │ │ │ identifiable │
│ │ │ │ packets from the │
│ │ │ │ device. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│hexdata │ No │ string │ Data, in bare │
│ │ │ │ hexadecimal, to │
│ │ │ │ send to the GNSS │
│ │ │ │ receiver. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│mincycle │ No │ real │ Device minimum │
│ │ │ │ cycle time in │
│ │ │ │ seconds. Reported │
│ │ │ │ from ?DEVICE when │
│ │ │ │ (and only when) the │
│ │ │ │ rate is switchable. │
│ │ │ │ It is read-only and │
│ │ │ │ not settable. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│native │ No │ integer │ 0 means NMEA mode │
│ │ │ │ and 1 means │
│ │ │ │ alternate mode │
│ │ │ │ (binary if it has │
│ │ │ │ one, for SiRF and │
│ │ │ │ Evermore chipsets │
│ │ │ │ in particular). │
│ │ │ │ Attempting to set │
│ │ │ │ this mode on a │
│ │ │ │ non-GPS device will │
│ │ │ │ yield an error. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│parity │ No │ string │ N, O or E for no │
│ │ │ │ parity, odd, or │
│ │ │ │ even. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│path │ No │ string │ Name the device for │
│ │ │ │ which the control │
│ │ │ │ bits are being │
│ │ │ │ reported, or for │
│ │ │ │ which they are to │
│ │ │ │ be applied. This │
│ │ │ │ attribute may be │
│ │ │ │ omitted only when │
│ │ │ │ there is exactly │
│ │ │ │ one subscribed │
│ │ │ │ channel. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│readonly │ No │ boolean │ True if device is │
│ │ │ │ read-only. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│stopbits │ Yes │ string │ Stop bits (1 or 2). │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│subtype │ No │ string │ Whatever version │
│ │ │ │ information the │
│ │ │ │ device driver │
│ │ │ │ returned. │
├──────────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│subtype1 │ No │ string │ More version │
│ │ │ │ information the │
│ │ │ │ device driver │
│ │ │ │ returned. │
└──────────┴─────────┴─────────┴─────────────────────┘
The serial parameters will (bps, parity, stopbits) be omitted in a response describing a
TCP/IP source such as an Ntrip, DGPSIP, or AIS feed; on a serial device they will always
be present.
The contents of the flags field should be interpreted as follows:
Table 15. Device flags
┌───────────┬───────┬──────────────────────────┐
│ │ │ │
│C #define │ Value │ Description │
├───────────┼───────┼──────────────────────────┤
│ │ │ │
│SEEN_GPS │ 0x01 │ GPS data has been seen │
│ │ │ on this device │
├───────────┼───────┼──────────────────────────┤
│ │ │ │
│SEEN_RTCM2 │ 0x02 │ RTCM2 data has been seen │
│ │ │ on this device │
├───────────┼───────┼──────────────────────────┤
│ │ │ │
│SEEN_RTCM3 │ 0x04 │ RTCM3 data has been seen │
│ │ │ on this device │
├───────────┼───────┼──────────────────────────┤
│ │ │ │
│SEEN_AIS │ 0x08 │ AIS data has been seen │
│ │ │ on this device │
└───────────┴───────┴──────────────────────────┘
When the C client library parses a response of this kind, it will assert the DEVICE_SET
bit in the top-level set member.
Here’s an example:
{"class":"DEVICE","bps":4800,"parity":"N","stopbits":1,"native":0}
When a client is in watcher mode, the daemon will ship it DEVICE notifications when a
device is added to the pool or deactivated.
When the C client library parses a response of this kind, it will assert the DEVICE_SET
bit in the top-level set member.
Examples
A notice of a deactivated device:
{"class":"DEVICE","path":"/dev/pts1","activated":0}
A send a u-blox receiver at /dev/ttyUSB2 a request for a UBX-MON-VER message:
?DEVICE={"path":"/dev/ttyUSB2","hexdata":"b5620a0400000e34"}
The gpsd daemon will respond with an ACK on success:
{"class":"ACK"}
ERROR
The daemon may ship an error object in response to a syntactically invalid command line or
unknown command. It has the following elements:
Table 16. ERROR notification object
┌────────┬─────────┬────────┬────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├────────┼─────────┼────────┼────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "ERROR" │
├────────┼─────────┼────────┼────────────────┤
│ │ │ │ │
│message │ Yes │ string │ Textual error │
│ │ │ │ message │
└────────┴─────────┴────────┴────────────────┘
Here’s an example:
{"class":"ERROR","message":"Unrecognized request '?FOO'"}
When the C client library parses a response of this kind, it will assert the ERR_SET bit
in the top-level set member.
RTCM2
RTCM-104 is a family of serial protocols used for broadcasting pseudorange corrections
from differential-GPS reference stations. Many GPS receivers can accept these corrections
to improve their reporting accuracy.
RTCM-104 comes in two major and incompatible flavors, 2.x and 3.x. Each major flavor has
minor (compatible) revisions.
The applicable standard for RTCM Version 2.x is RTCM Recommended Standards for
Differential NAVSTAR GPS Service RTCM Paper 194-93/SC 104-STD. For RTCM 3.1 it is RTCM
Paper 177-2006-SC104-STD. Ordering instructions for both standards are accessible from the
website of the Radio Technical Commission for Maritime Services <https://www.rtcm.org/>
under "Publications".
RTCM WIRE TRANSMISSIONS
Differential-GPS correction stations consist of a GPS reference receiver coupled to a low
frequency (LF) transmitter. The GPS reference receiver is a survey-grade GPS that does GPS
carrier tracking and can work out its position to a few millimeters. It generates range
and range-rate corrections and encodes them into RTCM104. It ships the RTCM104 to the LF
transmitter over serial rs-232 signal at 100 baud or 200 baud depending on the
requirements of the transmitter.
The LF transmitter broadcasts the approximately 300khz radio signal that differential-GPS
radio receivers pick up. Transmitters that are meant to have a higher range will need to
transmit at a slower rate. The higher the data rate the harder it will be for the remote
radio receiver to receive with a good signal-to-noise ration. (Higher data rate signals
can’t be averaged over as long a time frame, hence they appear noisier.)
RTCM WIRE FORMATS
An RTCM 2.x message consists of a sequence of up to 33 30-bit words. The 24 most
significant bits of each word are data and the six least significant bits are parity. The
parity algorithm used is the same ISGPS-2000 as that used on GPS satellite downlinks. Each
RTCM 2.x message consists of two header words followed by zero or more data words,
depending upon the message type.
An RTCM 3.x message begins with a fixed leader byte 0xD3. That is followed by six bits of
version information and 10 bits of payload length information. Following that is the
payload; following the payload is a 3-byte checksum of the payload using the Qualcomm
CRC-24Q algorithm.
RTCM2 JSON FORMAT
Each RTCM2 message is dumped as a single JSON object per message, with the message fields
as attributes of that object. Arrays of satellite, station, and constellation statistics
become arrays of JSON sub-objects. Each sentence will normally also have a "device" field
containing the pathname of the originating device.
All attributes other than the device field are mandatory. Header attributes are emitted
before others.
Header portion
Table 17. SKY object
┌───────────────┬─────────┬─────────────────────────────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├───────────────┼─────────┼─────────────────────────────────────────────┤
│ │ │ │
│class │ string │ Fixed: "RTCM2". │
├───────────────┼─────────┼─────────────────────────────────────────────┤
│ │ │ │
│type │ integer │ Message type (1-9). │
├───────────────┼─────────┼─────────────────────────────────────────────┤
│ │ │ │
│station_id │ integer │ The id of the GPS │
│ │ │ reference receiver. The │
│ │ │ LF transmitters also │
│ │ │ have (different) id │
│ │ │ numbers. │
├───────────────┼─────────┼─────────────────────────────────────────────┤
│ │ │ │
│zcount │ real │ The reference time of │
│ │ │ the corrections in the │
│ │ │ message in seconds │
│ │ │ within the current hour. │
│ │ │ Note that it is in GPS │
│ │ │ time, which is some │
│ │ │ seconds ahead of UTC │
│ │ │ (see the U.S. Naval │
│ │ │ Observatory’s table of │
│ │ │ leap second │
│ │ │ <ftp://maia.usno.navy.mil/ser7/tai-utc.dat> │
│ │ │ corrections" ). │
├───────────────┼─────────┼─────────────────────────────────────────────┤
│ │ │ │
│seqnum │ integer │ Sequence number. Only 3 bits wide, wraps │
│ │ │ after 7. │
├───────────────┼─────────┼─────────────────────────────────────────────┤
│ │ │ │
│length │ integer │ The number of words after the header that │
│ │ │ comprise the message. │
├───────────────┼─────────┼─────────────────────────────────────────────┤
│ │ │ │
│station_health │ integer │ Station transmission status. Indicates the │
│ │ │ health of the beacon as a reference source. │
│ │ │ Any nonzero value means the satellite is │
│ │ │ probably transmitting bad data and should │
│ │ │ not be used in a fix. 6 means the │
│ │ │ transmission is unmonitored. 7 means the │
│ │ │ station is not working properly. Other │
│ │ │ values are defined by the beacon operator. │
└───────────────┴─────────┴─────────────────────────────────────────────┘
<message type> is one of
1
full corrections — one message containing corrections for all GPS satellites in view.
This is not common.
3
reference station parameters — the position of the reference station GPS antenna.
4
datum — the datum to which the DGPS data is referred.
5
constellation health — information about the satellites the beacon can see.
6
null message — just a filler.
7
radio beacon almanac — information about this or other beacons.
9
subset corrections — a message containing corrections for only a subset of the GPS
satellites in view.
16
special message — a text message from the beacon operator.
31
GLONASS subset corrections — a message containing corrections for a set of the GLONASS
satellites in view.
Type 1 and 9: Correction data
One or more satellite objects follow the header for type 1 or type 9 messages. Here is the
format:
Table 18. Satellite object
┌──────┬─────────┬──────────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├──────┼─────────┼──────────────────────────┤
│ │ │ │
│ident │ integer │ The PRN number of the │
│ │ │ satellite for which this │
│ │ │ is correction data. │
├──────┼─────────┼──────────────────────────┤
│ │ │ │
│udre │ integer │ User Differential Range │
│ │ │ Error (0-3). See the │
│ │ │ table following for │
│ │ │ values. │
├──────┼─────────┼──────────────────────────┤
│ │ │ │
│iod │ integer │ Issue Of Data, matching │
│ │ │ the IOD for the current │
│ │ │ ephemeris of this │
│ │ │ satellite, as │
│ │ │ transmitted by the │
│ │ │ satellite. The IOD is a │
│ │ │ unique tag that │
│ │ │ identifies the │
│ │ │ ephemeris; the GPS using │
│ │ │ the DGPS correction and │
│ │ │ the DGPS generating the │
│ │ │ data must use the same │
│ │ │ orbital positions for │
│ │ │ the satellite. │
├──────┼─────────┼──────────────────────────┤
│ │ │ │
│prc │ real │ The pseudorange error in │
│ │ │ meters for this │
│ │ │ satellite as measured by │
│ │ │ the beacon reference │
│ │ │ receiver at the epoch │
│ │ │ indicated by the z_count │
│ │ │ in the parent record. │
├──────┼─────────┼──────────────────────────┤
│ │ │ │
│rrc │ real │ The rate of change of │
│ │ │ pseudorange error in │
│ │ │ meters/sec for this │
│ │ │ satellite as measured by │
│ │ │ the beacon reference │
│ │ │ receiver at the epoch │
│ │ │ indicated by the z_count │
│ │ │ field in the parent │
│ │ │ record. This is used to │
│ │ │ calculate pseudorange │
│ │ │ errors at other epochs, │
│ │ │ if required by the GPS │
│ │ │ receiver. │
└──────┴─────────┴──────────────────────────┘
User Differential Range Error values are as follows:
Table 19. UDRE values
┌──┬─────────────────────┐
│ │ │
│0 │ 1-sigma error ⇐ 1 m │
├──┼─────────────────────┤
│ │ │
│1 │ 1-sigma error ⇐ 4 m │
├──┼─────────────────────┤
│ │ │
│2 │ 1-sigma error ⇐ 8 m │
├──┼─────────────────────┤
│ │ │
│3 │ 1-sigma error > 8 m │
└──┴─────────────────────┘
Here’s an example:
{"class":"RTCM2","type":1,
"station_id":688,"zcount":843.0,"seqnum":5,"length":19,"station_health":6,
"satellites":[
{"ident":10,"udre":0,"iod":46,"prc":-2.400,"rrc":0.000},
{"ident":13,"udre":0,"iod":94,"prc":-4.420,"rrc":0.000},
{"ident":7,"udre":0,"iod":22,"prc":-5.160,"rrc":0.002},
{"ident":2,"udre":0,"iod":34,"prc":-6.480,"rrc":0.000},
{"ident":4,"udre":0,"iod":47,"prc":-8.860,"rrc":0.000},
{"ident":8,"udre":0,"iod":76,"prc":-7.980,"rrc":0.002},
{"ident":5,"udre":0,"iod":99,"prc":-8.260,"rrc":0.002},
{"ident":23,"udre":0,"iod":81,"prc":-8.060,"rrc":0.000},
{"ident":16,"udre":0,"iod":70,"prc":-11.740,"rrc":0.000},
{"ident":30,"udre":0,"iod":4,"prc":-18.960,"rrc":-0.006},
{"ident":29,"udre":0,"iod":101,"prc":-24.960,"rrc":-0.002}
]}
Type 3: Reference Station Parameters
Here are the payload members of a type 3 (Reference Station Parameters) message:
Table 20. Reference Station Parameters
┌─────┬──────┬────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├─────┼──────┼────────────────────┤
│ │ │ │
│x │ real │ ECEF X coordinate. │
├─────┼──────┼────────────────────┤
│ │ │ │
│y │ real │ ECEF Y coordinate. │
├─────┼──────┼────────────────────┤
│ │ │ │
│z │ real │ ECEF Z coordinate. │
└─────┴──────┴────────────────────┘
The coordinates are the position of the station, in meters to two decimal places, in Earth
Centred Earth Fixed coordinates. These are usually referred to the WGS84 reference frame,
but may be referred to NAD83 in the US (essentially identical to WGS84 for all except
geodesists), or some other reference frame in other parts of the world.
An invalid reference message is represented by a type 3 header without payload fields.
Here’s an example:
{"class":"RTCM2","type":3,
"station_id":652,"zcount":1657.2,"seqnum":2,"length":4,"station_health":6,
"x":3878620.92,"y":670281.40,"z":5002093.59
}
Type 4: Datum
Here are the payload members of a type 4 (Datum) message:
Table 21. Datum
┌───────────┬─────────┬──────────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│dgnss_type │ string │ Either "GPS", "GLONASS", │
│ │ │ "GALILEO", or "UNKNOWN". │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│dat │ integer │ 0 or 1 and indicates the │
│ │ │ sense of the offset │
│ │ │ shift given by dx, dy, │
│ │ │ dz. dat = 0 means that │
│ │ │ the station coordinates │
│ │ │ (in the reference │
│ │ │ message) are referred to │
│ │ │ a local datum and that │
│ │ │ adding dx, dy, dz to │
│ │ │ that position will │
│ │ │ render it in GNSS │
│ │ │ coordinates (WGS84 for │
│ │ │ GPS). If dat = 1 then │
│ │ │ the ref station position │
│ │ │ is in GNSS coordinates │
│ │ │ and adding dx, dy, dz │
│ │ │ will give it referred to │
│ │ │ the local datum. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│datum_name │ string │ A standard name for the │
│ │ │ datum. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│dx │ real │ X offset. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│dy │ real │ Y offset. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│dz │ real │ Z offset. │
└───────────┴─────────┴──────────────────────────┘
<dx> <dy> <dz> are offsets to convert from local datum to GNSS datum or vice versa. These
fields are optional.
An invalid datum message is represented by a type 4 header without payload fields.
Type 5: Constellation Health
One or more of these follow the header for type 5 messages — one for each satellite.
Here is the format:
Table 22. Constellation health
┌────────────┬─────────┬──────────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├────────────┼─────────┼──────────────────────────┤
│ │ │ │
│ident │ integer │ The PRN number of the │
│ │ │ satellite. │
├────────────┼─────────┼──────────────────────────┤
│ │ │ │
│iodl │ bool │ True indicates that this │
│ │ │ information relates to │
│ │ │ the satellite │
│ │ │ information in an │
│ │ │ accompanying type 1 or │
│ │ │ type 9 message. │
├────────────┼─────────┼──────────────────────────┤
│ │ │ │
│health │ integer │ 0 indicates that the │
│ │ │ satellite is healthy. │
│ │ │ Any other value │
│ │ │ indicates a problem │
│ │ │ (coding is not known). │
├────────────┼─────────┼──────────────────────────┤
│ │ │ │
│snr │ integer │ The carrier/noise ratio │
│ │ │ of the received signal │
│ │ │ in the range 25 to 55 │
│ │ │ dB(Hz). │
├────────────┼─────────┼──────────────────────────┤
│ │ │ │
│health_en │ bool │ If set to True it │
│ │ │ indicates that the │
│ │ │ satellite is healthy │
│ │ │ even if the satellite │
│ │ │ navigation data says it │
│ │ │ is unhealthy. │
├────────────┼─────────┼──────────────────────────┤
│ │ │ │
│new_data │ bool │ True indicates that the │
│ │ │ IOD for this satellite │
│ │ │ will soon be updated in │
│ │ │ type 1 or 9 messages. │
├────────────┼─────────┼──────────────────────────┤
│ │ │ │
│los_warning │ bool │ Line-of-sight warning. │
│ │ │ True indicates that the │
│ │ │ satellite will shortly │
│ │ │ go unhealthy. │
├────────────┼─────────┼──────────────────────────┤
│ │ │ │
│tou │ integer │ Healthy time remaining │
│ │ │ in seconds. │
└────────────┴─────────┴──────────────────────────┘
Type 6: Null
This just indicates a null message. There are no payload fields.
Unknown message
This format is used to dump message words in hexadecimal when the message type field
doesn’t match any of the known ones.
Here is the format:
Table 23. Unknown Message
┌─────┬──────┬────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├─────┼──────┼────────────────────┤
│ │ │ │
│data │ list │ A list of strings. │
└─────┴──────┴────────────────────┘
Each string in the array is a hex literal representing 30 bits of information, after
parity checks and inversion. The high two bits should be ignored.
Type 7: Radio Beacon Almanac
Here is the format:
Table 24. Constellation health
┌───────────┬─────────┬──────────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│lat │ real │ Latitude in degrees, of │
│ │ │ the LF transmitter │
│ │ │ antenna for the station │
│ │ │ for which this is an │
│ │ │ almanac. North is │
│ │ │ positive. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│lon │ real │ Longitude in degrees, of │
│ │ │ the LF transmitter │
│ │ │ antenna for the station │
│ │ │ for which this is an │
│ │ │ almanac. East is │
│ │ │ positive. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│range │ integer │ Published range of the │
│ │ │ station in km. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│frequency │ real │ Station broadcast │
│ │ │ frequency in kHz. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│health │ integer │ <health> is the health │
│ │ │ of the station for which │
│ │ │ this is an almanac. If │
│ │ │ it is non-zero, the │
│ │ │ station is issuing │
│ │ │ suspect data and should │
│ │ │ not be used for fixes. │
│ │ │ The ITU and RTCM104 │
│ │ │ standards differ about │
│ │ │ the mode detailed │
│ │ │ interpretation of the │
│ │ │ <health> field and even │
│ │ │ about its bit width. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│station_id │ integer │ The id of the │
│ │ │ transmitter. This is not │
│ │ │ the same as the │
│ │ │ reference id in the │
│ │ │ header, the latter being │
│ │ │ the id of the reference │
│ │ │ receiver. │
├───────────┼─────────┼──────────────────────────┤
│ │ │ │
│bitrate │ integer │ The transmitted bitrate. │
└───────────┴─────────┴──────────────────────────┘
Here’s an example:
{"class":"RTCM2","type":9,"station_id":268,"zcount":252.6,
"seqnum":4,"length":5,"station_health":0,
"satellites":[
{"ident":13,"udre":0,"iod":3,"prc":-25.940,"rrc":0.066},
{"ident":2,"udre":0,"iod":73,"prc":0.920,"rrc":-0.080},
{"ident":8,"udre":0,"iod":22,"prc":23.820,"rrc":0.014}
]}
Type 13: GPS Time of Week
Here are the payload members of a type 13 (Groumf Tramitter Parameters) message:
Table 25. Ground Transmitter Parameters
┌──────────┬─────────┬──────────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├──────────┼─────────┼──────────────────────────┤
│ │ │ │
│status │ bool │ If True, signals user to │
│ │ │ expect a type 16 │
│ │ │ explanatory message │
│ │ │ associated with this │
│ │ │ station. Probably │
│ │ │ indicates some sort of │
│ │ │ unusual event. │
├──────────┼─────────┼──────────────────────────┤
│ │ │ │
│rangeflag │ bool │ If True, indicates that │
│ │ │ the estimated range is │
│ │ │ different from that │
│ │ │ found in the type 7 │
│ │ │ message (which contains │
│ │ │ the beacon’s listed │
│ │ │ range). Generally │
│ │ │ indicates a range │
│ │ │ reduction due to causes │
│ │ │ such as poor ionospheric │
│ │ │ conditions or reduced │
│ │ │ transmission power. │
├──────────┼─────────┼──────────────────────────┤
│ │ │ │
│lat │ real │ Degrees latitude, │
│ │ │ signed. Positive is N, │
│ │ │ negative is S. │
├──────────┼─────────┼──────────────────────────┤
│ │ │ │
│lon │ real │ Degrees longitude, │
│ │ │ signed. Positive is E, │
│ │ │ negative is W. │
├──────────┼─────────┼──────────────────────────┤
│ │ │ │
│range │ integer │ Transmission range in km │
│ │ │ (1-1024). │
└──────────┴─────────┴──────────────────────────┘
This message type replaces message type 3 (Reference Station Parameters) in RTCM 2.3.
Type 14: GPS Time of Week
Here are the payload members of a type 14 (GPS Time of Week) message:
Table 26. Reference Station Parameters
┌─────────┬─────────┬───────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├─────────┼─────────┼───────────────────────┤
│ │ │ │
│week │ integer │ GPS week (0-123). │
├─────────┼─────────┼───────────────────────┤
│ │ │ │
│hour │ integer │ Hour of week (0-167). │
├─────────┼─────────┼───────────────────────┤
│ │ │ │
│leapsecs │ integer │ Leap Seconds (0-63). │
└─────────┴─────────┴───────────────────────┘
Here’s an example:
{"class":"RTCM2","type":14,"station_id":652,"zcount":1657.2,
"seqnum":3,"length":1,"station_health":6,"week":601,"hour":109,
"leapsecs":15}
Type 16: Special Message
Table 27. Special Message
┌────────┬────────┬────────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├────────┼────────┼────────────────────────┤
│ │ │ │
│message │ string │ A text message sent by │
│ │ │ the beacon operator. │
└────────┴────────┴────────────────────────┘
Type 31: Correction data
One or more GLONASS satellite objects follow the header for type 1 or type 9 messages.
Here is the format:
Table 28. Satellite object
┌───────┬──────────┬──────────────────────────┐
│ │ │ │
│Name │ Type │ Description │
├───────┼──────────┼──────────────────────────┤
│ │ │ │
│ident │ integer │ The PRN number of the │
│ │ │ satellite for which this │
│ │ │ is correction data. │
├───────┼──────────┼──────────────────────────┤
│ │ │ │
│udre │ integer │ User Differential Range │
│ │ │ Error (0-3). See the │
│ │ │ table following for │
│ │ │ values. │
├───────┼──────────┼──────────────────────────┤
│ │ │ │
│change │ boolean │ Change-of-ephemeris bit. │
├───────┼──────────┼──────────────────────────┤
│ │ │ │
│tod │ uinteger │ Count of 30-second │
│ │ │ periods since the top of │
│ │ │ the hour. │
├───────┼──────────┼──────────────────────────┤
│ │ │ │
│prc │ real │ The pseudorange error in │
│ │ │ meters for this │
│ │ │ satellite as measured by │
│ │ │ the beacon reference │
│ │ │ receiver at the epoch │
│ │ │ indicated by the z_count │
│ │ │ in the parent record. │
├───────┼──────────┼──────────────────────────┤
│ │ │ │
│rrc │ real │ The rate of change of │
│ │ │ pseudorange error in │
│ │ │ meters/sec for this │
│ │ │ satellite as measured by │
│ │ │ the beacon reference │
│ │ │ receiver at the epoch │
│ │ │ indicated by the z_count │
│ │ │ field in the parent │
│ │ │ record. This is used to │
│ │ │ calculate pseudorange │
│ │ │ errors at other epochs, │
│ │ │ if required by the GPS │
│ │ │ receiver. │
└───────┴──────────┴──────────────────────────┘
Here’s an example:
{"class":"RTCM2","type":31,"station_id":652,"zcount":1642.2,
"seqnum":0,"length":14,"station_health":6,
"satellites":[
{"ident":5,"udre":0,"change":false,"tod":0,"prc":132.360,"rrc":0.000},
{"ident":15,"udre":0,"change":false,"tod":0,"prc":134.840,"rrc":0.002},
{"ident":14,"udre":0,"change":false,"tod":0,"prc":141.520,"rrc":0.000},
{"ident":6,"udre":0,"change":false,"tod":0,"prc":127.000,"rrc":0.000},
{"ident":21,"udre":0,"change":false,"tod":0,"prc":128.780,"rrc":0.000},
{"ident":22,"udre":0,"change":false,"tod":0,"prc":125.260,"rrc":0.002},
{"ident":20,"udre":0,"change":false,"tod":0,"prc":117.280,"rrc":-0.004},
{"ident":16,"udre":0,"change":false,"tod":17,"prc":113.460,"rrc":0.018}
]}
RTCM3 DUMP FORMAT
The support for RTCM104v3 dumping is incomplete and buggy. Do not attempt to use it for
production! Anyone interested in it should read the source code.
AIS DUMP FORMATS
AIS support is an extension. It may not be present if your instance of gpsd has been built
with a restricted feature set.
AIS packets are dumped as JSON objects with class "AIS". Each AIS report object contains a
"type" field giving the AIS message type and a "scaled" field telling whether the
remainder of the fields are dumped in scaled or unscaled form. (These will be emitted
before any type-specific fields.) It will also contain a "device" field naming the data
source. Other fields have names and types as specified in the AIVDM/AIVDO Protocol
Decoding document on the GPSD project website; each message field table may be directly
interpreted as a specification for the members of the corresponding JSON object type.
By default, certain scaling and conversion operations are performed for JSON output.
Latitudes and longitudes are scaled to decimal degrees rather than the native AIS unit of
1/10000th of a minute of arc. Ship (but not air) speeds are scaled to knots rather than
tenth-of-knot units. Rate of turn may appear as "nan" if is unavailable, or as one of the
strings "fastright" or "fastleft" if it is out of the AIS encoding range; otherwise it is
quadratically mapped back to the turn sensor number in degrees per minute. Vessel draughts
are converted to decimal meters rather than native AIS decimeters. Various other scaling
conversions are described in "AIVDM/AIVDO Protocol Decoding".
SUBFRAME DUMP FORMATS
Subframe support is always compiled into gpsd but many GPSes do not output subframe data
or the gpsd driver may not support subframes.
Subframe packets are dumped as JSON objects with class "SUBFRAME". Each subframe report
object contains a "frame" field giving the subframe number, a "tSV" field for the
transmitting satellite number, a "TOW17" field containing the 17 MSBs of the start of the
next 12-second message and a "scaled" field telling whether the remainder of the fields
are dumped in scaled or unscaled form. It will also contain a "device" field naming the
data source. Each SUBFRAME object will have a sub-object specific to that subframe page
type. Those sub-object fields have names and types similar to those specified in the
IS-GPS-200 document; each message field table may be directly interpreted as a
specification for the members of the corresponding JSON object type.
Table 29. SUBFRAME object
┌───────┬─────────┬─────────┬─────────────────────┐
│ │ │ │ │
│Name │ Always? │ Type │ Description │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│class │ Yes │ string │ Fixed: "SUBFRAME" │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│device │ Yes │ string │ Name of the │
│ │ │ │ originating device. │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│gnssId │ Yes │ integer │ Constellation of │
│ │ │ │ transmitting │
│ │ │ │ satellite │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│tSV │ Yes │ integer │ ID of transmitting │
│ │ │ │ satellite (Not PRN) │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│TOW17 │ No │ integer │ Type 17 bits of the │
│ │ │ │ next GPS Time Of │
│ │ │ │ Week │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│frame │ No │ integer │ Frame number │
├───────┼─────────┼─────────┼─────────────────────┤
│ │ │ │ │
│scaled │ Yes │ boolean │ True is values │
│ │ │ │ scaled │
└───────┴─────────┴─────────┴─────────────────────┘
READING
Reading the raw JSON can be tedious. You can pretty print, and colorize, your JSON with
[jq] to make reading easier. Using jq ito pretty pring a JSON file can be as simple as:
$ jq . GPSD.json
To grab 10 seconds of live gpsd JSON, and pretty print it:
$ gpspipe -w -x 10 | jq .
If you only want to see the TPV messages:
$ gpspipe -w -x 10 | fgrep TPV | jq .
SEE ALSO
gpsd(8), libgps(3), libgpsmm(3), jq(1)
RESOURCES
Project web site: https://gpsd.io/
[RFC 2783]: https://datatracker.ietf.org/doc/html/rfc2783
Pulse-Per-Second API for UNIX-like Operating Systems
[RFC 7159]: https://datatracker.ietf.org/doc/html/rfc7159
The JavaScript Object Notation (JSON) Data Interchange Format
[jq]: https://github.com/stedolan/jq
COPYING
This file is Copyright 2013 by the GPSD project
SPDX-License-Identifier: BSD-2-clause
AUTHOR
Eric S. Raymond