Provided by: gpsd_2.30-1ubuntu3_i386 bug

NAME

       rtcm - RTCM-104 dump format emitted by GPSD tools

OVERVIEW

       RTCM-104  is  a  serial  protocol  used  for  broadcasting  pseudorange
       corrections from differential-GPS reference stations. This manual  page
       describes some aspects of the RTCM protocol, mainly in order to explain
       the RTCM-104 dump format emitted by rtcmdecode(1).  It  describes  that
       dump format completely.

       The  applicable standard is RTCM Recommended Standards for Differential
       NAVSTAR GPS Service RTCM Paper 194-93/SC 104-STD. Ordering instructions
       are  accessible  from the website of the Radio Technical Commission for
       Maritime Services: http://www.rtcm.org/ under "Publications".

RTCM WIRE FORMAT

       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 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.)

       An RTCM message consists of a sequence of 30-bit  words.  The  24  most
       significant  bits  are  data  and  the  six  least significant bits are
       parity. The parity algorithm used is the  same  as  that  used  on  GPS
       satellite downlinks.

       Each message consists of two header words followed by zero or more data
       words, depending upon message type.

RTCM DUMP FORMAT

       For each message, the header is listed first, followed by zero or  more
       lines containing the specific data for that message. The general format
       is a line beginning with a capital letter, followed by a tab,  followed
       by  the  fields  of  the  message  separated  by  tabs, terminated by a
       newline.

   Header message (H)
       H <message type> <reference station id> <modified z_count> <sequence no>
         <message length> <station health> [T <useful length>]

       Here is an example:

              H    9    687  337.2     4    5    0

       <message type> is one of

       1      full corrections - one message containing  corrections  for  all
              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 satellites in view.

       16     special message -- a text message from the beacon operator.

       <reference  station id> is the id of the GPS reference receiver. The LF
       transmitters also have (different) id numbers.

       <modified z_count> is the reference time  of  the  corrections  in  the
       message in seconds within the current hour. Note that it is the current
       hour in GPS time,  which  is  several  seconds  ahead  of  UTC  (13  in
       1999-2005).

       <sequence  no> is a number which increments, modulo 8, for each message
       transmitted.

       <message length> is the number of words after the header that  comprise
       the message.

       <station  health>  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.

       If  the message contains a parity error after the header but before the
       end of the message, then the  extra  fields  [T  <useful  length>]  are
       appended to indicate a truncated message.

       Here is an example:

              H    9    687  331.8     1    5    0    T    4

       <useful  length> indicates the number of useful words before the parity
       error. Depending on the message type, useful information may  still  be
       extracted.

   Correction data (S)
       One  or  more of these follow the header for type 1 or type 9 messages.
       Here is the format:

       S <satellite> <udre> <iod> <modified z_count> <range error>
         <range error rate>

       Here is an example:

              S    7    0    199  331.8     -12.160   0.288

       <satellite> is the PRN number  of  the  satellite  for  which  this  is
       correction data.

       <udre> is User Differential Range Error with the following values:

       0    1-sigma error  <= 1m
       1    1-sigma error  <= 4m
       2    1-sigma error  <= 8m
       3    1-sigma error  >  8m

       <iod>  is  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.

       <modified z_count> is just a copy of the same field from the header.

       <range  error> is the pseudorange error in metres for this satellite as
       measured by the beacon reference receiver at  the  epoch  indicated  by
       <modified z_count>

       <range  error  rate>  is  the  rate  of  change of pseudorange error in
       metres/sec for this satellite  as  measured  by  the  beacon  reference
       receiver  at the epoch indicated by <modified z_count>. This is used to
       calculate pseudorange errors at other epochs, if required  by  the  GPS
       receiver.

   Reference Station Parameters (R)
       Here is the format:

       R <X-coordinate> <Y-coordinate> <Z-coordinate>

       Here is an example:

              R    3746729.40     -5086.23  5144450.67

       The  coordinates  are  the  position  of  the station, in metres 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.

   Datum (D)
       Here is the format:

       D <dgnss type> <dat> <datum name> [ <dx> <dy> <dz> ]

       Here is an (ertificial) example:

              D    GPS  0    ABC12     25.8 30.5 33.0

       <dgnss type> is either GPS or GLONASS.

       <dat> is 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> is a standard name for the datum.

       <dx> <dy> <dz> are offsets to convert from local datum to GNSS datum or
       vice versa. These fields are optional.

   Constellation Health (C)
       One or more of these follow the header for type 5 messages --  one  for
       each satellite.

       Here is the format:

       C <sat> <iodl> <health> <snr> <hlth en> <new data> <los warning>
         <time to unhealthy>

       Here is an example:

              C    29   0  0 53   0  0  0    0

       <sat> is the PRN number of the satellite.

       <iodl>  is  1  bit.  0  indicates  that this information relates to the
       satellite information in an accompanying type 1 or type 9 message.

       <health> 0 indicates that the satellite is  healthy.  Any  other  value
       indicates a problem (coding is not known).

       <snr> gives the carrier/noise ratio of the received signal in the range
       25 to 55 dB(Hz).

       <health en> is 1 bit. If set to 1 it indicates that  the  satellite  is
       healthy even if the satellite navigation data says it is unhealthy.

       <new data> is 1 bit. a 1 indicates that the IOD for this satellite will
       soon be updated in type 1 or 9 messages.

       <los warning> is 1 bit. a 1 indicates that the satellite  will  shortly
       go  unhealthy.  The  healthy  time  remaining  is given in the <time to
       unhealthy> field.

   Radio Beacon Almanac (A)
       Here is the format:

       A <latitude> <longitude> <range> <frequency> <health> <station id>
         <bitrate>

       Here is an example:

              A    54.1176   -0.0714   100  302.5     0    447  2

       <latitude> and <longitude> give the position, in  degrees,  of  the  LF
       transmitter antenna for the station for which this is an almanac. North
       and East are positive.

       <range> is the published range of the station in km.

       <frequency> is the broadcast frequency in kHz.

       <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> is 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> indicates the transmitted bitrate.

   Special Message (T)
       Here is the format:

       T <text>

       Here is an example:

              T    THLS TRIAL SERVICE

       <text> is just a text message sent by the beacon operator.

   Null (N)
       This just indicates a null message. There are no fields.

   Unknown message (U)
       This 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:

       U <hex-literal>

       Here is an example:

              U    0x76423055

       The  <hex-literal>  will represent 32 bits of information, after parity
       checks and inversion. The high two bits should be ignored.

   Null (N)
       This just indicates a null message. There are no fields.

SEE ALSO

        gpsd(8), xgps(1), libgps(3), libgpsd(3), gpsprof(1), gpsfake(1).

AUTHOR

       Much  of   this   text   was   originally   written   by   John   Sager
       <john.sager@btinternet.com> in association with his RTCM decoder. Other
       material comes from the GPSD project. There is a project page for  gpsd
       here: http://gpsd.berlios.de/.

                                                                       RTCM(5)