rtcm-104.xml

gpsd, a popular GPS daemon.

<para>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.</para>

</refsect2>
<refsect2><title>Datum (D)</title>

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

<literallayout>
D &lt;dgnss type&gt; &lt;dat&gt; &lt;datum name&gt; [ &lt;dx&gt; &lt;dy&gt; &lt;dz&gt; ]
</literallayout>

<para>Here is an (ertificial) example:</para>

<informalexample><literallayout>
D	GPS	0	ABC12	25.8	30.5	33.0
</literallayout></informalexample>

<para>&lt;dgnss type&gt; is either GPS or GLONASS.</para>

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

<para>&lt;datum name&gt; is a standard name for the datum.</para>

<para>&lt;dx&gt; &lt;dy&gt; &lt;dz&gt; are offsets to convert from
local datum to GNSS datum or vice versa. These fields are
optional.</para>

</refsect2>
<refsect2><title>Constellation Health (C)</title>

<para>One or more of these follow the header for type 5 messages &mdash; one
for each satellite.</para>

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

<literallayout>
C &lt;sat&gt; &lt;iodl&gt; &lt;health&gt; &lt;snr&gt; &lt;hlth en&gt; &lt;new data&gt; &lt;los warning&gt;
  &lt;time to unhealthy&gt;
</literallayout>

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

<informalexample><literallayout>
C	29	0  0	53	0  0  0	 0
</literallayout></informalexample>

<para>&lt;sat&gt; is the PRN number of the satellite.</para>

<para>&lt;iodl&gt; is 1 bit. 0 indicates that this information relates to the
satellite information in an accompanying type 1 or type 9 message.</para>

<para>&lt;health&gt; 0 indicates that the satellite is healthy. Any other value
indicates a problem (coding is not known).</para>

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

<para>&lt;health en&gt; is 1 bit. If set to 1 it indicates that the
satellite is healthy even if the satellite navigation data says it is
unhealthy.</para>

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

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

</refsect2>
<refsect2><title>Radio Beacon Almanac (A)</title>

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

<literallayout>
A &lt;latitude&gt; &lt;longitude&gt; &lt;range&gt; &lt;frequency&gt; &lt;health&gt; &lt;station id&gt;
  &lt;bitrate&gt;
</literallayout>

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

<informalexample><literallayout>
A	54.1176	-0.0714	100	302.5	0	447	2
</literallayout></informalexample>

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

<para>&lt;range&gt; is the published range of the station in km.</para>

<para>&lt;frequency&gt; is the broadcast frequency in kHz.</para>

<para>&lt;health&gt; 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 &lt;health&gt; field and even about its bit width.
<!--
From itu p.9 just under the type7 msg figure:
 
    *** Radiobeacon health:
                  00     (0) Radiobeacon operation normal
                  01     (1) No integrity monitor operating
                  10     (2) No information available
                  11     (3) Do not use this radiobeacon
RTCM104, in the other hand, makes it 3 bits wide.

The Sager documentation said health has the same meaning as in the header.
but this cannot be true unless the field is 3 bits wide.
-->
</para>

<para>&lt;station id&gt; 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. 
<!-- John Sanger noted: "However I know of at least one station
that gets it wrong." --></para>

<para>&lt;bitrate&gt; indicates the transmitted bitrate.</para>

</refsect2>
<refsect2><title>Special Message (T)</title>

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

<literallayout>
T &lt;text&gt;
</literallayout>

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

<informalexample><literallayout>
T	THLS TRIAL SERVICE
</literallayout></informalexample>

<para>&lt;text&gt; is just a text message sent by the beacon operator.</para>

</refsect2>
<refsect2><title>Null (N)</title>

<para>This just indicates a null message. There are no fields.</para>
</refsect2>
<refsect2><title>Unknown message (U)</title>
<!-- The Sager decoder didn't have this -->

<para>This is used to dump message words in hexadecimal when the
message type field doesn't match any of the known ones.</para>

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

<literallayout>
U &lt;hex-literal&gt;
</literallayout>

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

<informalexample><literallayout>
U	0x76423055
</literallayout></informalexample>

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

</refsect2>
<refsect2><title>Null (N)</title>

<para>This just indicates a null message. There are no fields.</para>
</refsect2>

<!--
Decoder Status Messages (M)

format:

M &lt;type&gt;: &lt;information&gt;

examples:

M	state change: NO_SYNC -&gt; WORD_SYNCING
M	sync_bit: 5

&lt;type&gt; indicates textually the type of message. There are
only the two types shown above.

&lt;information&gt;
For &lt;type&gt; = state change it describes the internal state
transition of the decoder when it changes state as a result
of the incoming data.

For &lt;type&gt; = sync_bit this indicates the bit position in the
serial data stream which is a word boundary.
-->

</refsect1>
<refsect1 id='see_also'><title>SEE ALSO</title>
<para>
<citerefentry><refentrytitle>gpsd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gps</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>libgps</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>libgpsd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpsprof</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpsfake</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para>
</refsect1>

<refsect1 id='maintainer'><title>AUTHOR</title> 

<para>Much of this text was originally written by John Sager
<email>[e-mail]</email> in association with his RTCM
decoder. Other material comes from the GPSD project.  There is a
project page for <application>gpsd</application> <ulink
url="http://gpsd.berlios.de/">here</ulink>.</para>

</refsect1>

</refentry>