gpsd_json - Man Page

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

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}

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

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

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}]}

A GST object is a pseudorange noise report.

Table 4. GST object

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}

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

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

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}

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.

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

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}

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

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}

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

Here’s an example:

{"class":"OSC","running":true,"device":"/dev/ttyUSB0",
    "reference":true,"disciplined":true,"delta":67}

Returns an object with the following attributes:

Table 10. VERSION object

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
}

Returns a device list object with the following elements:

Table 11. DEVICES object

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.

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

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}

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

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}
]}]}

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

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

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.

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"}

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

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.

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.)

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.

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.

Table 17. Sky object

<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.

One or more satellite objects follow the header for type 1 or type 9 messages. Here is the format:

Table 18. Satellite object

User Differential Range Error values are as follows:

Table 19. UDRE values

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}
]}

Here are the payload members of a type 3 (Reference Station Parameters) message:

Table 20. Reference Station Parameters

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
}

Here are the payload members of a type 4 (Datum) message:

Table 21. Datum

<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.

One or more of these follow the header for type 5 messages — one for each satellite.

Here is the format:

Table 22. Constellation health

This just indicates a null message. There are no payload fields.

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

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.

Here is the format:

Table 24. Constellation health

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}
]}

Here are the payload members of a type 13 (Groumf Tramitter Parameters) message:

Table 25. Ground Transmitter Parameters

This message type replaces message type 3 (Reference Station Parameters) in RTCM 2.3.

Here are the payload members of a type 14 (GPS Time of Week) message:

Table 26. Reference Station Parameters

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}

Table 27. Special Message

One or more GLONASS satellite objects follow the header for type 1 or type 9 messages. Here is the format:

Table 28. Satellite object

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}
]}