rtcm-104.xml

gpsd, a popular GPS daemon.

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<refentry id='rtcm104.5'>
<refmeta>
<refentrytitle>rtcm-104</refentrytitle>
<manvolnum>5</manvolnum>
<refmiscinfo class='date'>12 Jul 2005</refmiscinfo>
</refmeta>
<refnamediv id='name'>
<refname>rtcm-104</refname>
<refpurpose>RTCM-104 dump format emitted by GPSD tools</refpurpose>
</refnamediv>

<refsect1 id='overview'><title>OVERVIEW</title>

<para>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
<citerefentry><refentrytitle>rtcmdecode</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
It describes that dump format completely.</para>

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

</refsect1>
<refsect1 id='wire-format'><title>RTCM WIRE FORMAT</title>

<para>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.</para>

<para>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.)</para>

<para>An RTCM 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 as that used on GPS satellite downlinks.</para>

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

</refsect1>
<refsect1 id='dump-format'><title>RTCM DUMP FORMAT</title>

<para>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.</para>

<refsect2><title>Header message (H)</title>

<literallayout>
H &lt;message type&gt; &lt;reference station id&gt; &lt;modified z_count&gt; &lt;sequence no&gt;
  &lt;message length&gt; &lt;station health&gt; [T &lt;useful length&gt;]
</literallayout>

<para>Here is an example:</para>

<informalexample><literallayout>
H	9	687	337.2	4	5	0
</literallayout></informalexample>

<para>&lt;message type&gt; is one of</para>

<variablelist>
<varlistentry>
<term>1</term>
<listitem><para>full corrections - one message containing corrections for
all satellites in view. This is not common.</para></listitem>
</varlistentry>

<varlistentry>
<term>3</term>
<listitem><para>reference station parameters - the position of the
reference station GPS antenna.</para></listitem>
</varlistentry>

<varlistentry>
<term>4</term>
<listitem><para>datum &mdash; the datum to which the DGPS data is
referred.</para></listitem>
</varlistentry>

<varlistentry>
<term>5</term>
<listitem><para>constellation health &mdash; information about the
satellites the beacon can see</para></listitem>
</varlistentry>

<varlistentry>
<term>6</term>
<listitem><para>null message &mdash; just a filler.</para></listitem>
</varlistentry>

<varlistentry>
<term>7</term>
<listitem><para>radio beacon almanac &mdash; information about this or other beacons.</para></listitem>
</varlistentry>

<varlistentry>
<term>9</term>
<listitem><para>subset corrections &mdash; a message containing corrections
for only a subset of the satellites in view.</para></listitem>
</varlistentry>

<varlistentry>
<term>16</term>
<listitem><para>special message &mdash; a text message from the beacon
operator.</para></listitem>
</varlistentry>
</variablelist>

<para>&lt;reference station id&gt; is the id of the GPS reference receiver. The
LF transmitters also have (different) id numbers.</para>

<para>&lt;modified z_count&gt; 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 (14 as of January 2006).</para>

<para>&lt;sequence no&gt; is a number which increments, modulo 8, for each
message transmitted.</para>

<para>&lt;message length&gt; is the number of words after the header that
comprise the message.</para>

<para>&lt;station health&gt; 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.
</para>

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

<para>Here is an example:</para>

<informalexample><literallayout>
H	9	687	331.8	1	5	0	T	4
</literallayout></informalexample>

<para>&lt;useful length&gt; indicates the number of useful words before the
parity error. Depending on the message type, useful information
may still be extracted.</para>

</refsect2>
<refsect2><title>Correction data (S)</title>

<para>One or more of these follow the header for type 1 or type 9
messages. Here is the format:</para>

<literallayout>
S &lt;satellite&gt; &lt;udre&gt; &lt;iod&gt; &lt;modified z_count&gt; &lt;range error&gt;
  &lt;range error rate&gt;
</literallayout>

<para>Here is an example:</para>

<informalexample><literallayout>
S	7	0	199	331.8	-12.160	0.288
</literallayout></informalexample>

<para>&lt;satellite&gt; is the PRN number of the satellite for which this is
correction data.</para>

<para>&lt;udre&gt; is User Differential Range Error with the following
values:</para>

<literallayout>
0	1-sigma error	&lt;= 1m
1	1-sigma error	&lt;= 4m
2	1-sigma error	&lt;= 8m
3	1-sigma error	&gt;  8m
</literallayout>

<para>&lt;iod&gt; 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.</para>

<para>&lt;modified z_count&gt; is just a copy of the same field from
the header.</para>

<para>&lt;range error&gt; is the pseudorange error in meters for this satellite
as measured by the beacon reference receiver at the epoch indicated
by &lt;modified z_count&gt;</para>

<para>&lt;range error rate&gt; 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 &lt;modified z_count&gt;. This is
used to calculate pseudorange errors at other epochs, if
required by the GPS receiver.</para>

</refsect2>
<refsect2><title>Reference Station Parameters (R)</title>

<para>Here is the format:</para>

<literallayout>
R &lt;X-coordinate&gt; &lt;Y-coordinate&gt; &lt;Z-coordinate&gt;
</literallayout>

<para>Here is an example:</para>

<informalexample><literallayout>
R	3746729.40	-5086.23	5144450.67
</literallayout></informalexample>