Provided by: libgps-dev_3.17-5_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 conditionalize their code on the major and minor protocol version
symbols.
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. 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 JSON objects all of which have a "class" attribute the value of which is
either the name of the invoking command. There are reports (including but not limited to
as "TPV", "SKY", "DEVICE", and "ERROR") which are not direct responses to commands.
The order of JSON attributes within a response object is never significant, and you may
specify attributes in commands 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 1536 characters, including trailing newline; longer
responses will be truncated, so client code must be prepared for the possibility of
invalid JSON fragments.
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.
CORE SOCKET PROTOCOL
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. The "mode" field will be emitted before optional fields that may
be absent when there is no fix. Error estimates will be emitted after the fix
components they're associated with. Others may be reported or not depending on the fix
quality.
Table 1. TPV object
┌───────┬─────────┬─────────┬─────────────────────┐
│Name │ Always? │ Type │ Description │
├───────┼─────────┼─────────┼─────────────────────┤
│class │ Yes │ string │ Fixed: "TPV" │
├───────┼─────────┼─────────┼─────────────────────┤
│device │ No │ string │ Name of originating │
│ │ │ │ device. │
├───────┼─────────┼─────────┼─────────────────────┤
│status │ No │ numeric │ GPS status: %d, │
│ │ │ │ 2=DGPS fix, │
│ │ │ │ otherwise not │
│ │ │ │ present. │
├───────┼─────────┼─────────┼─────────────────────┤
│mode │ Yes │ numeric │ NMEA mode: %d, 0=no │
│ │ │ │ mode value yet │
│ │ │ │ seen, 1=no fix, │
│ │ │ │ 2=2D, 3=3D. │
├───────┼─────────┼─────────┼─────────────────────┤
│time │ No │ string │ Time/date stamp in │
│ │ │ │ ISO8601 format, │
│ │ │ │ UTC. May have a │
│ │ │ │ fractional │
│ │ │ │ part of up to │
│ │ │ │ .001sec precision. │
│ │ │ │ May be absent if │
│ │ │ │ mode │
│ │ │ │ is not 2 or │
│ │ │ │ 3. │
├───────┼─────────┼─────────┼─────────────────────┤
│ept │ No │ numeric │ Estimated timestamp │
│ │ │ │ error (%f, seconds, │
│ │ │ │ 95% confidence). │
│ │ │ │ Present if │
│ │ │ │ time is present. │
├───────┼─────────┼─────────┼─────────────────────┤
│lat │ No │ numeric │ Latitude in │
│ │ │ │ degrees: +/- │
│ │ │ │ signifies │
│ │ │ │ North/South. │
│ │ │ │ Present │
│ │ │ │ when mode │
│ │ │ │ is 2 or 3. │
├───────┼─────────┼─────────┼─────────────────────┤
│lon │ No │ numeric │ Longitude in │
│ │ │ │ degrees: +/- │
│ │ │ │ signifies │
│ │ │ │ East/West. Present │
│ │ │ │ when mode │
│ │ │ │ is 2 or 3. │
├───────┼─────────┼─────────┼─────────────────────┤
│alt │ No │ numeric │ Altitude in meters. │
│ │ │ │ Present if mode is │
│ │ │ │ 3. │
├───────┼─────────┼─────────┼─────────────────────┤
│epx │ No │ numeric │ Longitude error │
│ │ │ │ estimate in meters, │
│ │ │ │ 95% confidence. │
│ │ │ │ Present │
│ │ │ │ if mode is │
│ │ │ │ 2 or 3 and DOPs can │
│ │ │ │ be calculated from │
│ │ │ │ the satellite │
│ │ │ │ view. │
├───────┼─────────┼─────────┼─────────────────────┤
│epy │ No │ numeric │ Latitude error │
│ │ │ │ estimate in meters, │
│ │ │ │ 95% confidence. │
│ │ │ │ Present │
│ │ │ │ if mode is │
│ │ │ │ 2 or 3 and DOPs can │
│ │ │ │ be calculated from │
│ │ │ │ the satellite │
│ │ │ │ view. │
├───────┼─────────┼─────────┼─────────────────────┤
│epv │ No │ numeric │ Estimated vertical │
│ │ │ │ error in meters, │
│ │ │ │ 95% confidence. │
│ │ │ │ Present │
│ │ │ │ if mode is │
│ │ │ │ 3 and DOPs can be │
│ │ │ │ calculated from the │
│ │ │ │ satellite │
│ │ │ │ view. │
├───────┼─────────┼─────────┼─────────────────────┤
│track │ No │ numeric │ Course over ground, │
│ │ │ │ degrees from true │
│ │ │ │ north. │
├───────┼─────────┼─────────┼─────────────────────┤
│speed │ No │ numeric │ Speed over ground, │
│ │ │ │ meters per second. │
├───────┼─────────┼─────────┼─────────────────────┤
│climb │ No │ numeric │ Climb (positive) or │
│ │ │ │ sink (negative) │
│ │ │ │ rate, meters per │
│ │ │ │ second. │
├───────┼─────────┼─────────┼─────────────────────┤
│epd │ No │ numeric │ Direction error │
│ │ │ │ estimate in │
│ │ │ │ degrees, 95% │
│ │ │ │ confidence. │
├───────┼─────────┼─────────┼─────────────────────┤
│eps │ No │ numeric │ Speed error │
│ │ │ │ estinmate in │
│ │ │ │ meters/sec, 95% │
│ │ │ │ confidence. │
├───────┼─────────┼─────────┼─────────────────────┤
│epc │ No │ numeric │ Climb/sink error │
│ │ │ │ estimate in │
│ │ │ │ meters/sec, 95% │
│ │ │ │ confidence. │
└───────┴─────────┴─────────┴─────────────────────┘
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 actually received; see gps.h for bitmask
names and values.
Here's an example:
{"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 │
├───────────┼─────────┼─────────┼─────────────────────┤
│time │ No │ numeric │ Time/date stamp in │
│ │ │ │ ISO8601 format, │
│ │ │ │ UTC. May have a │
│ │ │ │ fractional │
│ │ │ │ part of up to │
│ │ │ │ .001sec precision. │
├───────────┼─────────┼─────────┼─────────────────────┤
│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. │
├───────────┼─────────┼─────────┼─────────────────────┤
│vdop │ No │ numeric │ Altitude dilution │
│ │ │ │ of precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which │
│ │ │ │ should be │
│ │ │ │ multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error │
│ │ │ │ estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│tdop │ No │ numeric │ Time dilution of │
│ │ │ │ precision, 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 │ Spherical dilution │
│ │ │ │ of precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which │
│ │ │ │ should be │
│ │ │ │ multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error │
│ │ │ │ estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│gdop │ No │ numeric │ Hyperspherical │
│ │ │ │ dilution of │
│ │ │ │ precision, a │
│ │ │ │ dimensionless │
│ │ │ │ factor which │
│ │ │ │ should be │
│ │ │ │ multiplied by a │
│ │ │ │ base UERE to get an │
│ │ │ │ error │
│ │ │ │ estimate. │
├───────────┼─────────┼─────────┼─────────────────────┤
│satellites │ Yes │ list │ List of satellite │
│ │ │ │ objects in skyview │
└───────────┴─────────┴─────────┴─────────────────────┘
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 actually 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 │ Yes │ numeric │ Azimuth, degrees │
│ │ │ │ from true north. │
├─────┼─────────┼─────────┼─────────────────────┤
│el │ Yes │ numeric │ Elevation in │
│ │ │ │ degrees. │
├─────┼─────────┼─────────┼─────────────────────┤
│ss │ Yes │ numeric │ Signal strength in │
│ │ │ │ dB. │
├─────┼─────────┼─────────┼─────────────────────┤
│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.) │
└─────┴─────────┴─────────┴─────────────────────┘
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 │ numeric │ Seconds since the │
│ │ │ │ Unix epoch, 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.
Table 5. ATT object
┌─────────┬─────────┬─────────┬─────────────────────┐
│Name │ Always? │ Type │ Description │
├─────────┼─────────┼─────────┼─────────────────────┤
│class │ Yes │ string │ Fixed: "ATT" │
├─────────┼─────────┼─────────┼─────────────────────┤
│device │ Yes │ string │ Name of originating │
│ │ │ │ device │
├─────────┼─────────┼─────────┼─────────────────────┤
│time │ No │ numeric │ Seconds since the │
│ │ │ │ Unix epoch, UTC. │
│ │ │ │ May have a │
│ │ │ │ fractional │
│ │ │ │ part of up to │
│ │ │ │ .001sec precision. │
├─────────┼─────────┼─────────┼─────────────────────┤
│heading │ No │ numeric │ Heading, degrees │
│ │ │ │ from true north. │
├─────────┼─────────┼─────────┼─────────────────────┤
│mag_st │ No │ string │ Magnetometer │
│ │ │ │ status. │
├─────────┼─────────┼─────────┼─────────────────────┤
│pitch │ No │ numeric │ Pitch in degrees. │
├─────────┼─────────┼─────────┼─────────────────────┤
│pitch_st │ No │ string │ Pitch sensor │
│ │ │ │ status. │
├─────────┼─────────┼─────────┼─────────────────────┤
│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. │
├─────────┼─────────┼─────────┼─────────────────────┤
│acc_y │ No │ numeric │ Y component of │
│ │ │ │ acceleration. │
├─────────┼─────────┼─────────┼─────────────────────┤
│acc_z │ No │ numeric │ Z component of │
│ │ │ │ acceleration. │
├─────────┼─────────┼─────────┼─────────────────────┤
│gyro_x │ No │ numeric │ X component of │
│ │ │ │ acceleration. │
├─────────┼─────────┼─────────┼─────────────────────┤
│gyro_y │ No │ numeric │ Y component of │
│ │ │ │ acceleration. │
├─────────┼─────────┼─────────┼─────────────────────┤
│depth │ No │ numeric │ Water depth in │
│ │ │ │ meters. │
├─────────┼─────────┼─────────┼─────────────────────┤
│temp │ No │ numeric │ Temperature at │
│ │ │ │ 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}
And here are the commands:
?VERSION;
Returns an object with the following attributes:
Table 7. 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 8. 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 9. 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 the last
complete fix data and 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 10. 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.
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 second (actually, when the first data for the
reporting cycle is received).
This message exactly mirrors the PPS message except for two details.
TOFF emits no NTP precision, this is assumed to be -2. See the NTP documentation for
their definition of precision.
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 11. TOFF object
┌───────────┬─────────┬─────────┬─────────────────────┐
│Name │ Always? │ Type │ Description │
├───────────┼─────────┼─────────┼─────────────────────┤
│class │ Yes │ string │ Fixed: "TOFF" │
├───────────┼─────────┼─────────┼─────────────────────┤
│device │ Yes │ string │ Name of 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 time stamps 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 timestamp 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 except for two details.
PPS emits the NTP precision. See the NTP documentation for their definition of
precision.
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 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 millisecond to 700 milliseconds and combined jjitter and wander of 100 to 300
millisecond.
A PPS object has the following elements:
Table 12. PPS object
┌───────────┬─────────┬─────────┬─────────────────────┐
│Name │ Always? │ Type │ Description │
├───────────┼─────────┼─────────┼─────────────────────┤
│class │ Yes │ string │ Fixed: "PPS" │
├───────────┼─────────┼─────────┼─────────────────────┤
│device │ Yes │ string │ Name of 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 │
└───────────┴─────────┴─────────┴─────────────────────┘
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 13. OSC object
┌────────────┬─────────┬─────────┬─────────────────────┐
│Name │ Always? │ Type │ Description │
├────────────┼─────────┼─────────┼─────────────────────┤
│class │ Yes │ string │ Fixed: "OSC" │
├────────────┼─────────┼─────────┼─────────────────────┤
│device │ Yes │ string │ Name of originating │
│ │ │ │ device. │
├────────────┼─────────┼─────────┼─────────────────────┤
│running │ Yes │ boolean │ If true, the │
│ │ │ │ oscillator is │
│ │ │ │ currently running. │
│ │ │ │ Oscillators may │
│ │ │ │ require warm-up │
│ │ │ │ time at start of │
│ │ │ │ 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}
?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.
Pay attention to the response, because it is possible for this command to fail if the
GPS does not support a speed-switching command or only supports some combinations of
serial modes. In case of failure, the daemon and GPS will continue to communicate at
the old speed.
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" │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│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. │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│activated │ No │ string │ Time the device was │
│ │ │ │ activated as an │
│ │ │ │ ISO8601 │
│ │ │ │ timestamp. │
│ │ │ │ If the device is │
│ │ │ │ inactive this │
│ │ │ │ attribute is │
│ │ │ │ absent. │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│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. │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│driver │ No │ string │ GPSD's name for the │
│ │ │ │ device driver type. │
│ │ │ │ Won't be reported │
│ │ │ │ before gpsd │
│ │ │ │ has seen │
│ │ │ │ identifiable │
│ │ │ │ packets from │
│ │ │ │ the device. │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│subtype │ When the daemon │ string │ Whatever version │
│ │ sees a delayed │ │ information the │
│ │ response to a probe │ │ device returned. │
│ │ for subtype or │ │ │
│ │ firmware-version │ │ │
│ │ information. │ │ │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│bps │ No │ integer │ Device speed in │
│ │ │ │ bits per second. │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│parity │ No │ string │ N, O or E for no │
│ │ │ │ parity, odd, or │
│ │ │ │ even. │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│stopbits │ Yes │ string │ Stop bits (1 or 2). │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│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. │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│cycle │ No │ real │ Device cycle time │
│ │ │ │ in seconds. │
├──────────┼─────────────────────┼─────────┼─────────────────────┤
│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. │
└──────────┴─────────────────────┴─────────┴─────────────────────┘
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.
Here's an example:
{"class":"DEVICE","path":"/dev/pts1","activated":0}
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[1] 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 own 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 the 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 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 │
│ │ │ corrections[2]). │
├───────────────┼─────────┼─────────────────────────────┤
│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 <= 1m │
├──┼──────────────────────┤
│1 │ 1-sigma error <= 4m │
├──┼──────────────────────┤
│2 │ 1-sigma error <= 8m │
├──┼──────────────────────┤
│3 │ 1-sigma error > 8m │
└──┴──────────────────────┘
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 to 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)..PP │
├────────────┼─────────┼──────────────────────────┤
│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..PP │
├────────────┼─────────┼──────────────────────────┤
│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. Contellation 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..PP │
├───────────┼─────────┼───────────────────────────┤
│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. Grund 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-200E document; each message field table may be directly interpreted as a
specification for the members of the corresponding JSON object type.
SEE ALSO
gpsd(8), libgps(3),
AUTHOR
The protocol was designed and documented by Eric S. Raymond.
NOTES
1. Radio Technical Commission for Maritime Services
http://www.rtcm.org/
2. table of leap second corrections
ftp://maia.usno.navy.mil/ser7/tai-utc.dat