Provided by: libgps-dev_3.4-2_i386 bug

NAME

       gpsd_json - gpsd request/response protol

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.
       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"     |
           +-------+---------+---------+------------------+
           |tag    | No      | string  | Type tag         |
           |       |         |         | associated with  |
           |       |         |         | this GPS         |
           |       |         |         | sentence; from   |
           |       |         |         | an NMEA          |
           |       |         |         |      device this |
           |       |         |         | is just the NMEA |
           |       |         |         | sentence type.   |
           +-------+---------+---------+------------------+
           |device | No      | string  | Name of          |
           |       |         |         | originating      |
           |       |         |         | device.          |
           +-------+---------+---------+------------------+
           |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        |
           |       |         |         | West/East.       |
           |       |         |         | Present          |
           |       |         |         |         when     |
           |       |         |         | mode is 2 or 3.  |
           +-------+---------+---------+------------------+
           |lon    | No      | numeric | Longitude in     |
           |       |         |         | degrees: +/-     |
           |       |         |         | signifies        |
           |       |         |         | North/South.     |
           |       |         |         | 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%     |
           |       |         |         | confifdence.     |
           +-------+---------+---------+------------------+
           |eps    | No      | numeric | Speed error      |
           |       |         |         | estinmate in     |
           |       |         |         | meters/sec, 95%  |
           |       |         |         | confifdence.     |
           +-------+---------+---------+------------------+
           |epc    | No      | numeric | Climb/sink error |
           |       |         |         | estinmate in     |
           |       |         |         | meters/sec, 95%  |
           |       |         |         | confifdence.     |
           +-------+---------+---------+------------------+
           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","tag":"MID2","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"     |
           +-----------+---------+---------+------------------+
           |tag        | No      | string  | Type tag         |
           |           |         |         | associated with  |
           |           |         |         | this GPS         |
           |           |         |         | sentence; from   |
           |           |         |         | an NMEA          |
           |           |         |         |      device this |
           |           |         |         | is just the NMEA |
           |           |         |         | sentence type.   |
           +-----------+---------+---------+------------------+
           |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.        |
           +-----------+---------+---------+------------------+
           |xdop       | No      | numeric | Longitudinal     |
           |           |         |         | 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","tag":"MID2","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"     |
           +-------+---------+---------+------------------+
           |tag    | No      | string  | Type tag         |
           |       |         |         | associated with  |
           |       |         |         | this GPS         |
           |       |         |         | sentence; from   |
           |       |         |         | an NMEA device   |
           |       |         |         | this is just the |
           |       |         |         | NMEA sentence    |
           |       |         |         | type.            |
           +-------+---------+---------+------------------+
           |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","tag":"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", "mode", and "tag" 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"     |
           +------------+---------+---------+------------------+
           |tag         | Yes     | string  | Type tag         |
           |            |         |         | associated with  |
           |            |         |         | this GPS         |
           |            |         |         | sentence; from   |
           |            |         |         | an NMEA          |
           |            |         |         |      device this |
           |            |         |         | is just the NMEA |
           |            |         |         | sentence type.   |
           +------------+---------+---------+------------------+
           |device      | Yes     | string  | Name of          |
           |            |         |         | originating      |
           |            |         |         | device           |
           +------------+---------+---------+------------------+
           |time        | Yes     | 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 | Roll in degrees. |
           +------------+---------+---------+------------------+
           |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.        |
           +------------+---------+---------+------------------+
           |mag_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.          |
           +------------+---------+---------+------------------+
           |temperature | 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","tag":"PTNTHTM","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,"temperature":0.000,"depth":0.000}

       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. Applies   |
           |       |         |         | only to AIS and  |
           |       |         |         | Subframe         |
           |       |         |         |      reports.    |
           +-------+---------+---------+------------------+
           |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, or have been specified on the GPSD command
           line together with an -n option.

           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.         |
           +---------+---------+------------+------------------+
           |fixes    | Yes     | JSON array | Comma-separated  |
           |         |         |            | list of TPV      |
           |         |         |            | objects.         |
           +---------+---------+------------+------------------+
           |skyviews | 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","tag":"MID41","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","tag":"MID41","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
           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 11. CONFIGCHAN 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    |         | device returned.   |
           |          | probe for        |         |                    |
           |          |      subtype or  |         |                    |
           |          | firmware-version |         |                    |
           |          | information.     |         |                    |
           +----------+------------------+---------+--------------------+
           |bps       | No               | integer | Device speed in    |
           |          |                  |         | bits per second.   |
           +----------+------------------+---------+--------------------+
           |parity    | Yes              | 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               |
           |          |                  |         |      ?CONFIGDEV    |
           |          |                  |         | when (and only     |
           |          |                  |         | when) the rate is  |
           |          |                  |         | switchable. It is  |
           |          |                  |         |      read-only and |
           |          |                  |         | not settable.      |
           +----------+------------------+---------+--------------------+
           The serial parameters will be omitted in a response describing a
           TCP/IP source such as an Ntrip, DGPSIP, or AIS feed.

           The contents of the flags field should be interpreted as follows:

           Table 12. 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 13. 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 14. 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 15. 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 16. 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 17. 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 18. 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 19. 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     |
           |            |         |            Teue 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 20. 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 21. 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 22. 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 23. 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 24. 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 25. 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. Navigation status and positioning-system type are dumped as text
       strings rather than AIS numeric codes. 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 GPS 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