.\"Generated by db2man.xsl. Don't modify this, modify the source. .de Sh \" Subsection .br .if t .Sp .ne 5 .PP \fB\\$1\fR .PP .. .de Sp \" Vertical space (when we can't use .PP) .if t .sp .5v .if n .sp .. .de Ip \" List item .br .ie \\n(.$>=3 .ne \\$3 .el .ne 3 .IP "\\$1" \\$2 .. .TH "RTCM" 5 "" "" "" .SH NAME rtcm \- RTCM-104 dump format emitted by GPSD tools .SH "OVERVIEW" .PP 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 \fBrtcmdecode\fR(1)\&. It describes that dump format completely\&. .PP 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: \fIhttp://www.rtcm.org/\fR under "Publications"\&. .SH "RTCM WIRE FORMAT" .PP 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\&. .PP 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\&.) .PP 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\&. .PP Each message consists of two header words followed by zero or more data words, depending upon message type\&. .SH "RTCM DUMP FORMAT" .PP 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\&. .SS "Header message (H)" .nf H [T ] .fi .PP Here is an example: .IP .nf H 9 687 337\&.2 4 5 0 .fi .PP is one of .TP 1 full corrections \- one message containing corrections for all satellites in view\&. This is not common\&. .TP 3 reference station parameters \- the position of the reference station GPS antenna\&. .TP 4 datum -- the datum to which the DGPS data is referred\&. .TP 5 constellation health -- information about the satellites the beacon can see .TP 6 null message -- just a filler\&. .TP 7 radio beacon almanac -- information about this or other beacons\&. .TP 9 subset corrections -- a message containing corrections for only a subset of the satellites in view\&. .TP 16 special message -- a text message from the beacon operator\&. .PP is the id of the GPS reference receiver\&. The LF transmitters also have (different) id numbers\&. .PP 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)\&. .PP is a number which increments, modulo 8, for each message transmitted\&. .PP is the number of words after the header that comprise the message\&. .PP 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\&. .PP If the message contains a parity error after the header but before the end of the message, then the extra fields [T ] are appended to indicate a truncated message\&. .PP Here is an example: .IP .nf H 9 687 331\&.8 1 5 0 T 4 .fi .PP indicates the number of useful words before the parity error\&. Depending on the message type, useful information may still be extracted\&. .SS "Correction data (S)" .PP One or more of these follow the header for type 1 or type 9 messages\&. Here is the format: .nf S .fi .PP Here is an example: .IP .nf S 7 0 199 331\&.8 \-12\&.160 0\&.288 .fi .PP is the PRN number of the satellite for which this is correction data\&. .PP is User Differential Range Error with the following values: .nf 0 1\-sigma error <= 1m 1 1\-sigma error <= 4m 2 1\-sigma error <= 8m 3 1\-sigma error > 8m .fi .PP 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\&. .PP is just a copy of the same field from the header\&. .PP is the pseudorange error in metres for this satellite as measured by the beacon reference receiver at the epoch indicated by .PP 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 \&. This is used to calculate pseudorange errors at other epochs, if required by the GPS receiver\&. .SS "Reference Station Parameters (R)" .PP Here is the format: .nf R .fi .PP Here is an example: .IP .nf R 3746729\&.40 \-5086\&.23 5144450\&.67 .fi .PP 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\&. .SS "Datum (D)" .PP Here is the format: .nf D [ ] .fi .PP Here is an (ertificial) example: .IP .nf D GPS 0 ABC12 25\&.8 30\&.5 33\&.0 .fi .PP is either GPS or GLONASS\&. .PP 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\&. .PP is a standard name for the datum\&. .PP are offsets to convert from local datum to GNSS datum or vice versa\&. These fields are optional\&. .SS "Constellation Health (C)" .PP One or more of these follow the header for type 5 messages -- one for each satellite\&. .PP Here is the format: .nf C