NAME
rtcm-104 - RTCM-104 dump format emitted by GPSD tools
OVERVIEW
RTCM-104 is a family of serial protocols 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 formats emitted by gpsdecode(1). It
describes that dump format completely.
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 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.
SAGER RTCM2 DUMP FORMAT
For each message, the header is listed first, followed by zero or more
lines containing the specific data for that message, followed by a
trailing sentinel line containing a single dot. 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 number>
<message length> <station health> [T <useful length>]
Here is an example:
H 9 268 249.6 1 5 0
S 13 0 3 249.6 -26.120 0.068
S 2 0 73 249.6 1.220 -0.080
S 8 0 22 249.6 23.760 0.030
.
<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 in GPS
time, which is some seconds ahead of UTC (see the U.S. Naval
Observatory's table of leap second corrections[2]).
<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 meters 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
meters/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 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.
Datum (D)
Here is the format:
D <dgnss type> <dat> <datum name> [ <dx> <dy> <dz> ]
Here is an (artificial) 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.
JSON RTCM2 DUMP FORMAT
Fields are dumped in the order and with the conversions described
above, except that they are wrapped in a single JSON object per message
and appear as attributes of that object. Arrays of satellite, station,
and constellation statistics become arrays of JSON sub-objects.
(One exception: The zcount field included in the Sager-format
per-satellite reports in type 1 and 9 messages is omitted from the JSON
version, as it is redundant with the zcount attribute of the parent
object.)
RTCM3 DUMP FORMAT
The support for RTCM104v3 dumping is still incomplete and buggy. Anyone
interested in it should read the source code.
SEE ALSO
gpsd(8), gps(1), libgps(3), libgpsd(3), gpsprof(1), gpsfake(1).
COMPATIBILITY NOTE
In versions of the RTCM2 dump format prior to gpsd 2.28, there was no
trailing sentinel line after each stanza of the Sager-format dump.
AUTHOR
Much of the portion of this text describing RTCM2 was originally
written by John Sager john.sager@btinternet.com in association with his
RTCM2 decoder. Other material comes from the GPSD project. There is a
project page for gpsd here[3].
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
3. here
http://gpsd.berlios.de/