source: ntrip/trunk/BNC/bnchelp.html@ 212

<h3>BKG Ntrip Client (BNC)</h3>

<p>
The BKG Ntrip Client (BNC) is a program for simultaneously retrieving real-time GNSS data streams from NTRIP broadcasters like <u>http://www.euref-ip.net/home</u> or <u>http://www.igs-ip.net/home</u>.
</p>
<p>
BNC has been developed for the Federal Agency for Cartography and Geodesy (BKG) within the framework of the EUREF-IP Pilot Project (EUREF-IP) and the Real-Time IGS Working Group (RTIGS).
</p>
<p>
BNC is written under GNU General Public License (GPL). Binaries for BNC are available for Windows, Linux, and Solaris systems. It is likely that BNC can be compiled on other systems where a GNU compiler and Qt Version 4 are available.
</p>
<h3>Contents</h3>
<p>
<h4>
<a href=#purpose>A - Purpose</a><br>
<a href=#options>B - Options</a><br>
<a href=#limits>C - Limitations</a><br>
<a href=#authors>D - Authors</a><br>
<a href=#links>E - Links</a><br>
<a href=#annex>F - Annex</a><br>
</h4>
</p>
<br>

<a name="purpose">
<p><h3>A - Purpose</h3></p>

<p> The purpose of BNC is to

<ul>
<li>Retrieve real-time GNSS data streams available through NTRIP transport protocol,</li> 
<li>Generate high-rate RINEX files to support near real-time GNSS post-processing applications, and/or</li> 
<li>Output synchronize observations through an IP port to support real-time GNSS engines.</li> 
</ul>
<p>
BNC decodes and converts GNSS data streams carrying phase data coming in 
</p>
<ul>
<li>RTCM Version 2.x format containing message types 18 and 19, </li> 
<li>RTCM Version 3 format containing message types 1001, 1002, 1003, and 1004,</li>
<li>RTIGS format.</li> 
</ul>
</p>
<a name="options">
<p><h3>B - Options</h3></p>
<p>
B - 1. <a href=#file>File</a><br>
B - 2. <a href=#help>Help</a><br>
B - 3. <a href=#proxy>Proxy</a><br>
B - 4. <a href=#output>Synchronized Output</a><br>
&nbsp; &nbsp; &nbsp; B - 4.1. <a href=#wait>Wait for Full Epoch</a><br>
&nbsp; &nbsp; &nbsp; B - 4.2. <a href=#ascii>ASCII Output File</a><br>
&nbsp; &nbsp; &nbsp; B - 4.3. <a href=#binary>Port for Binary Output</a><br>
B - 5. <a href=#rinex>RINEX</a><br>
&nbsp; &nbsp; &nbsp; B - 5.1. <a href=#rnxpath>RINEX Directory</a><br>
&nbsp; &nbsp; &nbsp; B - 5.2. <a href=#rnxscript>RINEX Script</a><br>
&nbsp; &nbsp; &nbsp; B - 5.3. <a href=#rnxinterval>RINEX File Interval</a><br>
&nbsp; &nbsp; &nbsp; B - 5.4. <a href=#rnxsample>RINEX Sampling</a><br>
&nbsp; &nbsp; &nbsp; B - 5.5. <a href=#rnxskeleton>RINEX Skeleton Extension</a><br>
B - 6. <a href=#mountpoints>Mountpoints</a><br>
&nbsp; &nbsp; &nbsp; B - 6.1. <a href=#AddMounts>Add Mountpoints</a><br>
&nbsp; &nbsp; &nbsp; B - 6.2. <a href=#HostPort>Broadcaster Host and Port</a><br>
&nbsp; &nbsp; &nbsp; B - 6.3. <a href=#account>Broadcaster User and Password</a><br>
&nbsp; &nbsp; &nbsp; B - 6.4. <a href=#GetTable>Get Table</a><br>
&nbsp; &nbsp; &nbsp; B - 6.5. <a href=#delete>Delete Mountpoints</a><br>
&nbsp; &nbsp; &nbsp; B - 6.6. <a href=#edit>Edit Mountpoints</a><br>
B - 7. <a href=#log>Log File</a><br>
B - 8. <a href=#start>Start</a><br>
B - 9. <a href=#stop>Stop</a><br>
B - 10. <a href=#nw>No Window</a>
</p>

<a name="file">
<p><h4>B - 1. File</h4></p>

<p>
The 'File' button lets you
<ul>
<li>
Select an appropriate font.<br>
Use a small font size if the BNC main window exceeds the size of your screen.
</li> 
<li> Save selected options.<br>
Note that on Windows systems options are saved in register BKG_NTRIP_Client. On Unix/Linux systems options are saved in ${HOME}/.config/BKG/BKG_NTRIP_Client.conf.
</li>
<li>
Quit the execution of BNC.
</li> 
</ul>
</p>

<a name="help">
<p><h4>B - 2. Help</h4></p>

<p>
The 'Help' button provides access to
<ul>
<li>
General information about BNC.<br>
Close the 'About BNC' window to continue with BNC.
</li> 
<li>
Help contents.<br>
You may keep the 'Help Contents' window open while setting BNC options.
</li> 
</ul>
</p>

<a name="proxy">
<p><h4>B - 3. Proxy - mandatory if BNC is operated in a protected LAN</h4></p>

<p>
You may like to run BNC in a Local Area Network (LAN). LAN's are often protected by a proxy server. Enter your proxy server IP and port number in case one is operated in front of BNC. If you don't know the IP and port of your proxy server, check out the proxy server settings of your Windows browser or ask your network administrator.</p>
<p>
Note that IP streaming may be generally denied in a LAN. In such a case you need to request an appropriate modification of the security policy from your network administrator or ask for the installation of a TCP relay to involved NTRIP broadcasters. If that doesn't work out, run BNC on a host that is connected to the Internet through an Internet Service Provider (ISP).
</p>

<a name="output"> <p><h4>B - 4. Synchronized Output</h4></p>
<p>
BNC lets you output synchronized observations epoch by epoch. This output is made available in ASCII format and in a binary format. The output comprises the following observations if available:</p>
StatID, SVPRN, GPSWeek, GPSWeeks, sec, C1 or P1, P2, L1, L2, SNR1, SNR2, pCodeIndicator, cumuLossOfCont.
</p>

<a name="wait">
<p><h4>B - 4.1 Wait for Full Epoch - optional</h4></p>
<p>
When feeding a real-time GNSS engine waiting for input epoch by epoch, BNC ignores whatever is received later then 'Wait for full epoch' seconds. A value of 2 to 5 seconds may be an appropriate choice for that, depending on the delay you can accept for your real-time GNSS product. Default value for 'Wait for full ecpch' is 1 second.
</p>
<p>
Note that 'Wait for full epoch' does not influence the RINEX file contents. Observations received later than 'Wait for full epoch' seconds will still be included in the RINEX files.
</p> 

<a name="ascii">
<p><h4>B - 4.2 ASCII Output File - optional</h4></p>
<p>
Enter the full path for a file to save synchronized observations in a plain ASCII format. Default value is an empty option field, meaning that no ASCII output file is created.
</p>

<a name="binary">
<p><h4>B - 4.3 Port for Binary Output - optional</h4></p>
<p>
BNC makes synchronized observations available in a binary format on your local host IP 127.0.0.1 through an IP port. Enter an IP port number to activate this function. Default is an empty option field, meaning that no binary output is generated.</p>
<p>The binary output is provided as a continuous stream in the order</p>
<pre>
begEpoch
begObs  
Observation
begObs  
Observation
begObs  
Observation
...
endEpoch
begEpoch
...
</pre>
<p>where the corresponding structures are defined as</p>
<pre>
const char begEpoch = 'A';
const char begObs   = 'B';
const char endEpoch = 'C';
struct Observation {
  char     StatID[5+1];    // Station ID
  char     SVPRN;          // Satellite PRN
  short    GPSWeek;        // Week of GPS-Time
  int      GPSWeeks;       // Second of Week (GPS-Time)
  double   sec;            // second 
  double   C1;             // CA or P1-code pseudorange (meters)
  double   P2;             // P2-code pseudorange (meters)
  double   L1;             // L1-carrier phase (meters)
  double   L2;             // L2-carrier phase (meters)
  short    SNR1;           // Signal-to noise ratio (0.1 dB)
  short    SNR2;           // Signal-to noise ratio (0.1 dB)
  int      pCodeIndicator; // 0 ... CA Code, 1 ... P Code
  unsigned cumuLossOfCont; // Loss of lock indicator (0 to 31)
} ;
</pre>

<a name="rinex">
<p><h4>B - 5. RINEX</h4></p>
<p>
Observations are converted to RINEX Version 2.1. RINEX file names are derived by BNC from the first 4 characters of the corresponding mountpoint (4Char Station ID) while truncating the residual part of the mountpoint string. Thus, retrieving data from mountpoints FRANKFURT and WETTZELL leads to RINEX files named FRAN*.* and WETT*.*.</p>
<p>
If you retrieve streams that show mountpoints with an identical 4Char Station ID (same first 4 characters), the mountpoint string is split in two sub-strings and both become part of the RINEX file name. Example: When simultaneously retrieving data from mountpoints FRANKFURT and FRANCE, there RINEX file names are defined as FRAN*_KFURT.* and FRAN*_CE.*.
</p>
<p>
Note that RINEX file names for all intervals less than 1 hour are created following the file name convention for 15 minute RINEX files.
</p>

<a name="rnxpath">
<p><h4>B - 5.1 RINEX Directory - optional</h4></p>
<p>
Enter a path for saving the RINEX files in a directory. If this directory does not exist, BNC will not create RINEX files. Default value for 'RINEX directory' is an empty option field, meaning that streams are not converted to RINEX.
</p> 

<a name="rnxscript">
<p><h4>B - 5.2 RINEX Script - optional</h4></p>
<p>
Whenever a RINEX file is generated, you may like to compress, copy, or upload it immediately via FTP. For that you enter the full path of a script or batch file carrying out these operations. The RINEX file path will be passed to the script as a command line parameter (%1 on Windows systems, $1 on Unix/Linux systems).
</p> 

<a name="rnxinterval">
<p><h4>B - 5.3 RINEX File Interval - mandatory if 'RINEX directory' set</h4></p>
<p>
Select the interval for RINEX file generation. Default for 'RINEX file interval' is 15 minutes.
</p> 

<a name="rnxsample">
<p><h4>B - 5.4 RINEX Sampling - mandatory if 'RINEX directory' set </h4></p>
<p>
Select the RINEX sample interval in seconds. Zero '0' stands for converting all incoming epochs to RINEX. Default for RINEX 'Sampling' is '0'.
</p> 

<a name="rnxskeleton">
<p><h4>B - 5.5 RINEX Skeleton Extension - optional</h4></p>
<p>
BNC lets you introduce RINEX header skeleton files that contain the header records you would like to see instead of a default header. You may like to derive RINEX skeleton files from the information given in sitelogs. A file in the 'RINEX directory' with the 'RINEX skeleton extension' is understood by BNC as a RINEX header skeleton file for the affected stream.
</p>
<p>
Example: Mountpoint FRAN0 leads to the generation of RINEX file FRAN*.*. The header part of this file would be overwritten by the content of an existing skeleton file named FRAN.skl if 'RINEX skeleton extension' is set to 'skl' .
</p> 
<p> 
Note the following conditions regarding RINEX header skeleton files.
<ul>
<li>They should contain empty header records of type:</li>
<br>&nbsp; &nbsp; PGM / RUN BY / DATE
<br>&nbsp; &nbsp; # / TYPES OF OBSERVATIONS
<br>&nbsp; &nbsp; TIME OF FIRST OBS
<br>The existence of these empty records will force BNC to include such lines in the final RINEX file header together with an additional COMMENT line mentioning the source of the stream.
<li>They must contain an empty header record of type:</li>
<br>&nbsp; &nbsp; END OF HEADER
<li>They may contain any other complete header record as defined in the RINEX Version 2.1 documentation. Its contents may be derived from sitelog files.</li>
</ul>
<p> 

<a name="mountpoints">
<p><h4>B - 6. Mountpoints</h4></p>
<p>
Each stream on an NTRIP broadcaster is defined through a unique source ID called mountpoint. An NTRIP client like BNC can access the data of a desired stream by its mountpoint. Information about mountpoints is available through the sourcetable maintained by the NTRIP broadcaster.
</p> 

<a name="AddMounts">
<p><h4>B - 6.1 Add Mountpoints</h4></p>
<p>
Button 'Add Mountpoints' opens a window that lets you select data streams from an NTRIP broadcaster by their mountpoints.
</p> 

<a name="HostPort">
<p><h4>B - 6.2 Broadcaster Host and Port - mandatory</h4></p>
<p>
Enter the NTRIP broadcaster host IP and port number. <u>http://www.rtcm-ntrip.org/home</u> provides information about known NTRIP broadcaster installations. Note that EUREF and IGS operate NTRIP broadcasters at <u>http://www.euref-ip.net/home</u> and <u>http://www.igs-ip.net/home</u>.
</p> 

<a name="account">
<p><h4>B - 6.3 Broadcaster User and Password - mandatory for protected streams</h4></p>
<p>
Streams on NTRIP broadcasters may be password protected. Enter a valid User ID and Password for access to protected NTRIP broadcaster streams. Accounts are usually provided per NTRIP broadcaster through a registration procedure. Register through <u>http://igs.bkg.bund.de/index_ntrip_reg.htm</u> for access to protected streams on <u>www.euref-ip.net</u> and <u>www.igs-ip.net</u>.
</p> 

<a name="GetTable">
<p><h4>B - 6.4 Get Table</h4></p>
<p>
Hit button 'Get Table' to download the sourcetable from the NTRIP broadcaster. Pay attention to data fields 'format' and 'format-details'. Have in mind that BNC can only decode and convert streams that come in RTCM 2.x, RTCM 3, or RTIGS format. RTCM 2.x streams must contain message types 18 and 19 while RTCM 3 streams must contain message types 1001 or 1003, better 1003 or 1004, see data field 'format-details' for available message types and their repetition rates in brackets. Select your streams line by line, use +Shift and +Ctrl when necessary.
</p> 
<p>
Hit 'OK' to return to the main window. You may like to 'Add Mountpoints' from another NTRIP broadcaster when necessary.
</p> 

<a name="delete">
<p><h4>B - 6.5 Delete Mountpoints</h4></p>
<p>
To delete a stream shown under 'Mountpoints' in the main window select it by mouse click and hit 'Delete Mountpoints'. For simultaneous deletion of several streams single them out through using +Shift and +Ctrl.</p>

<a name="edit">
<p><h4>B - 6.6 Edit Mountpoints</h4></p>
<p>
BNC automatically selects one out of several internal decoders for a stream based on its 'format' and 'format-details' as given in the sourcetable. It may happen that you need to overrule the automated decoder selection because of sourcetable setup deficiencies. Therefore BNC allows to edit the decoder string (double-click) for each stream shown under 'Mountpoints'. Accepted decoder strings allowed to be introduced are 'RTCM_2.x', 'RTCM_3', and 'RTIGS'.
</p> 

<a name="log">
<p><h4>B - 7. Log File - optional</h4></p>
<p>
BNC comments its activities in the 'Log file' section on the main windows. Comments can be saved and concatenated in a file when entering a full path for 'Log file'. Information is given about the communication between BNC and the NTRIP broadcaster as well as about problems that occur concerning communication link, stream availability, stream delay, stream conversion etc. Default value for 'Log file' is an empty option field, meaning that BNC comments are not saved in a file.
</p>

<a name="start">
<p><h4>B - 8. Start</h4></p>
<p>
Hit 'Start' to start retrieving, decoding, and converting GNSS data streams in real-time.
</p> 

<a name="stop">
<p><h4>B - 9. Stop</h4></p>
<p>
Hit the 'Stop' button to stop BNC. 
</p> 

<a name="nw">
<p><h4>B - 10. No Window - optional</h4></p>
<p>
You can use BNC in batch mode with the command line option -nw. BNC then runs in 'no window' mode, reading options from the configuration file ${HOME}/.config/BKG/BNC_NTRIP_Client.conf (Unix/Linux) or from the registers (Windows).
</p>
<p>
Note that the self-explaining contents of the configuration file or the Windows registers can easily be edited.
</p> 
<br>
<a name="limits">
<p><h3>C - Limitations</h3></p>
<ul>
<li>
The connection to an NTRIP broadcaster may break or a stream requested may be temporarily unavailable. Furthermore, a connection is interpreted by BNC to be broken if no data is coming in for a period of 20 seconds. When this happens, a reconnect is tried with decreasing frequency. BNC first tries to reconnect with ~1 second delay, if unsuccessful, tries again in ~2 seconds from the last attempt, if still unsuccessful tries with ~4 seconds from the last attempt etc. Each attempt doubles the delay from the previous attempt. The maximum delay between attempts is limited to ~128 seconds. The reconnection process is documented in 'Log file'.
</li> 
<li>
So far BNC only handles GPS data while ignoring GLONASS and Galileo observations. Furthermore, its function is limited today to processing L1, L2, P1, and P2 observations only.
</li>
<li>
Concerning RTCM Version 2.x, BNC handles only message types 18 and 19. Concerning RTCM Version 3, BNC handles only message types 1001, 1002, 1003, and 1004.
</li>
<li>
If the ambiguity field is not set in RTCM Version 3 streams, the BNC output will be no valid RINEX. All values will be stored modulo 299792.458. A COMMENT line will tell you, when this happens. It would be necessary to approximately calculate the range to fix the ambiguity (needing ephemeris/almanac and easy positioning algorithm).
</li>
<li>
The generated RINEX is somewhat limited due to the streaming character of the conversion.
<ul>
<li>
Header records can only represent data, which is known after receiving the very first epoch. Data rate, position, number of observations and any such additional fields cannot be provided.
</li>
<li>
The number of observables cannot change during the program runtime. Only the observables, which exist in the first epoch are outputted. If there are new observables later on, these are ignored.
</li>
</ul>
</li>
<li>
BNC's 'Get Table' function only informs about the STR records of a sourcetable. Note that you can use an Internet browser to download the full sourcetable contents of any NTRIP broadcaster by simply entering its URL plus a dummy string in the form http://host:port/dummy. Data field number 8 of the NET records that may show up may provide information about where to register for an NTRIP broadcaster account.
</li>
<li>
EUREF as well as IGS follow an open data policy. Streams are made available through NTRIP broadcasters at <u>www.euref-ip.net</u> and <u>www.igs-ip.net</u> free of charge to anybody for any purpose. It is not clear today how many users will have to be supported simultaneously. The situation may develop in a way that it becomes difficult to serve all registered users at any time.  In case limited dissemination resources on the NTRIP broadcaster side (software restrictions, bandwidth limitation etc.) make it necessary, first priority in stream provision will be given to stream providers, re-broadcasting activities, and real-time analysis centres while access for others may be temporarily denied.
</li>
<br>
</ul>
<a name="authors">
<p><h3>D - Authors</h3></p>
<p>
The BKG Ntrip Client (BNC) Qt Graphic User Interface (GUI) has been developed for the Federal Agency for Cartography and Geodesy (BKG) by Leos Mervart, Czech Technical University Prague, Department of Advanced Geodesy. BNC integrates the following GNU GPL software components: 
<ul>
<li> RTCM 2.x decoder, written by Oliver Montenbruck, German Space Operations Center, DLR, Oberpfaffenhofen, with contributions from Leos Mervart</li> 
<li> RTCM 3 decoder, written for BKG by Dirk Stoecker, Euronik GmbH, Schoenefeld</li> 
<li> RTIGS decoder, written by Ken MacLeod, Natural Resources Canada, NRCan</li> 
</ul>
</p>
<p>
Note that this is a betta version of BNC provided for test and evaluation. Make sure you installed the latest version available from <u>http://igs.bkg.bund.de/index_ntrip_down.htm</u>. We are still working on the program and would appreciate if you could send your comments, suggestions, or bug reports to:
</p>
<p>
Georg Weber<br>
Federal Agency for Cartography and Geodesy (BKG)<br>
Frankfurt, Germany<br>
euref-ip@bkg.bund.de 
</p>
<br>

<a name="links">
<p><h3>E - Links</h3></p>
<table>
<tr></tr>
<tr><td>NTRIP &nbsp;</td><td><u>http://igs.bkg.bund.de/index_ntrip.htm</u></td></tr>
<tr><td>EUREF-IP NTRIP broadcaster &nbsp;</td><td><u>http://www.euref-ip.net/home</u></td></tr>
<tr><td>IGS-IP NTRIP broadcaster &nbsp;</td><td><u>http://www.igs-ip.net/home</u></td></tr>
<tr><td>NTRIP broadcaster overview &nbsp;</td><td><u>http://www.rtcm-ntrip.org/home</u></td></tr>
<tr><td>EUREF-IP Pilot Project &nbsp;</td><td><u>http://www.epncb.oma.be/euref_IP</u></td></tr>
<tr><td>Real-Time IGS Working Group &nbsp;</td><td><u>http://igscb.jpl.nasa.gov/projects/rtwg/index.html</u>
</table>
<br>

<a name="annex">
<p><h3>F - Annex</h3></p>
<p>
F - 1. <a href=#ntrip>NTRIP</a><br>
&nbsp; &nbsp; &nbsp; E - 1.1 <a href=#source>Sourcetable</a><br>
F - 2. <a href=#rtcm>RTCM</a><br>
&nbsp; &nbsp; &nbsp; F - 2.1 RTCM <a href=#rtcm2> Version 2.x</a><br>
&nbsp; &nbsp; &nbsp; F - 2.2 RTCM <a href=#rtcm3> Version 3</a><br>
F - 3. <a href=#rtigs>RTIGS</a><br>
&nbsp; &nbsp; &nbsp; F - 3.1 <a href=#soc>SOC</a>
</p>
<br>

<a name="ntrip">
<p><h4>F - 1. NTRIP</h4></p>

<p>
'Networked Transport of RTCM via Internet Protocol' Version 1.0 (NTRIP) stands for an application-level protocol streaming Global Navigation Satellite System (GNSS) data over the Internet. NTRIP is a generic, stateless protocol based on the Hypertext Transfer Protocol HTTP/1.1. The HTTP objects are enhanced to GNSS data streams.
</p>

<p>
NTRIP Version 1.0 is an RTCM standard designed for disseminating differential correction data (e.g in the RTCM-104 format) or other kinds of GNSS streaming data to stationary or mobile users over the Internet, allowing simultaneous PC, Laptop, PDA, or receiver connections to a broadcasting host. NTRIP supports wireless Internet access through Mobile IP Networks like GSM, GPRS, EDGE, or UMTS.
</p>

<p>
NTRIP is implemented in three system software components: NTRIP clients, NTRIP servers and NTRIP broadcasters. The NTRIP broadcaster is the actual HTTP server program whereas NTRIP client and NTRIP server are acting as HTTP clients.
</p>

<p>
NTRIP is an open none-proprietary protocol. Major characteristics of NTRIP's dissemination technique are:
<ul>
<li>Based on the popular HTTP streaming standard; comparatively easy to implement when having limited client and server platform resources available.</li>
<li>Application not limited to one particular plain or coded stream content; ability to distribute any kind of GNSS data.</li>
<li>Potential to support mass usage; disseminating hundreds of streams simultaneously for up to thousand users possible when applying modified Internet Radio broadcasting software.</li>
<li>Considering security needs; stream providers and users don't necessarily get into contact, streams often not blocked by firewalls or proxy servers protecting Local Area Networks.</li>
<li>Enables streaming over mobile IP networks because of using TCP/IP.</li>
</ul>
</p>

<a name="source">
<p><h4>F - 1.1 Sourcetable</h4></p>

<p>
The NTRIP broadcaster maintains a sourcetable containing information on available NTRIP streams, networks of NTRIP streams, and NTRIP broadcasters. The sourcetable is sent to an NTRIP client on request. Sourcetable records are dedicated to one of the following: Data Streams (record type STR), Casters (record type CAS), or Networks of streams (record type NET).
</p>

<p>
Sourcetable records of type STR contain the following data fields: 'mountpoint', 'identifier', 'format', 'format-details',  'carrier', 'nav-system', 'network', 'country',  'latitude', 'longitude', 'nmea', 'solution', 'generator', 'compr-encryp', 'authentication', 'fee', 'bitrate', 'misc'.
</p>
<p>
Sourcetable records of type NET contain the following data fields: 'identifiey', 'operator', 'authentication', 'fee', 'web-net', 'web-str', 'web-reg', 'misc'.
</p>
<p>
Sourcetable records of type CAS contain the following data fields: 'host', 'port', 'identifier', 'operator', 'nmea', 'country', 'latitude', 'longitude', 'misc'.
</p>

<a name="rtcm">
<p><h4>F - 2. RTCM</h4></p>

<p>
The Radio Technical Commission for Maritime Services (RTCM) is an international non-profit scientific, professional and educational organization. Special Committees provide a forum in which government and non-government members work together to develop technical standards and consensus recommendations in regard to issues of particular concern. RTCM is engaged in the development of international standards for maritime radionavigation and radiocommunication systems. The output documents and reports prepared by RTCM Committees are published as RTCM Recommended Standards. Topics concerning Differential Global Navigation Satellite Systems (DGNSS) and handled by the Special Committee SC 104.
<p>
Copies of RTCM Recommended Standards can be ordered through <u>http://www.rtcm.org/orderinfo.php</u>.
</p>
<br>

<a name="rtcm2">
<p><h4>F - 2.1 RTCM Version 2.x</h4></p>
<p>
Transmitting GNSS carrier phase data can be done through RTCM Version 2.x messages. Messages that may be of interest here are types 1, 2, 3, 6, 9, 16,18/19, 20/21, and 22.
</p>

<ul>
<li>
Type 1 message is the range correction message and is the primay message in code-phase differential positioning (DGPS). It is computed in the base receiver by computing the error in the range measurement for each tracked SV.
</li>
<li>
Type 2 message is automatically generated when a new set of satellite ephemeris is downloaded to the base receiver. It is the computed difference between the old ephemeris and the new ephemeris. Type 2 messages are used when the base station is transmitting Type 1 messages.
</li>
<li>
Type 3 and 22 messages are the base station position and the antenna offset. Type 3 and 22 are used in RTK processing to perform antenna reduction.
</li>
<li>
Type 6 message is a null frame filler message that is provided for data links that require continuous transmission of data, even if there are no corrections to send. As many Type 6 messages are sent as required to fill in the gap between two correction messages (type 1). Message 6 is not sent in burst mode.
</li>
<li>
Type 9 message serves the same purpose as Type 1, but does not require a complete satellite set. As a result, Type 9 messages require a more stable clock than a station transmitting Type 1 's, because the satellite corrections have different time references.
</li>
<li>
Type 16 message is simply a text message entered by the user that is transmitted from the base station to the rover. It is used with code-phase differential.
</li>
<li>
Type 18 and 20 messages are RTK uncorrected carrier phase data and carrier phase corrections.
</li>
<li>
Type 19 and 21 messages are the uncorrected pseudo-range measurements and pseudo-range corrections used in RTK.
</li>
</ul>

<a name="rtcm3">
<p><h4>F - 2.2 RTCM Version 3</h4></p>
<p>
RTCM Version 3 has been developed  as a more efficient alternative to RTCM 2.x.  Service providers and vendors have asked for a new standard that would be more efficient, easy to use, and more easily adaptable to new situations.  The main complaint was that the Version 2 parity scheme was wasteful of bandwidth. Another complaint was that the parity is not independent from word to word. Still another was that even with so many bits devoted to parity, the actual integrity of the message was not as high as it should be. Plus, 30-bit words are awkward to handle. The new standard, Version 3, is intended to correct these weaknesses.
</p>
<p>
RTCM Version 3 defines a number of message types. Messages that may be of interest here are:
</p>
<ul>
<li>Tpye 1001, GPS L1 code and phase.</li>
<li>Type 1002, GPS L1 code and phase and ambiguities and carrier to noise ratio.</li>
<li>Type 1003, GPS L1 and L2 code and phase.</li>
<li>Type 1004, GPS L1 and L2 code and phase and ambiguities and carrier to noise ratio.</li>
<li>Type 1005, Station coordinates XZY for antenna reference point.</li>
<li>Type 1006, Station coordinates XYZ for antenna reference point and antenna height.</li>
<li>Type 1007, Antenna descriptor and ID.</li>
<li>Type 1008, Antenna serial number.</li>
<li>Type 1009, GLONASS L1 code and phase.</li>
<li>Type 1010, GLONASS L1 code and phase and ambiguities and carrier to noise ratio.</li>
<li>Type 1011, GLONASS L1 and L2 code and phase.</li>
<li>Type 1012, GLONASS L1 and L2 code and phase and ambiguities and carrier to noise ratio.</li>
<li>Type 1013, Modified julian date, leap second, configured message types and interval.</li>
<li>Type 1014 and 1017, Network RTK (MAK) messages (under development).</li>
<li>Type 1019, GPS Ephemeris (under development).</li>
<li>Type 1020, GLONASS Ephemeris (under development).</li>
<li>Type 4088 and 4095, Proprietary messages (under development).
</li>
</ul>

<a name="rtigs">
<p><h4>F - 3. RTIGS</h4></p>
<p>
Input from Ken?
</p>

<a name="soc">
<p><h4>F - 3.1 SOC</h4></p>
<p>
The SOC format has been designed in July 1999 by the Jet Propulsion Laboratory (JPL) and the California Institute of Technology (CalTech) to transport 1Hz GPS data with minimal bandwidth over the open Internet. SOC follows the 'little-endian' byte order meaning that the low-order byte of a number is stored in memory at the lowest address, and the high-order byte at the highest address. (The little end comes first.) Because the transport layer is UDP, the format does not include sync bits, a checksum or cyclic redundancy checksum (CRC). SOC allows to transport the GPS observable CA, P1, P2, L1, and L2, efficiently compressed down to 14 bytes with 1 mm range resolution and 0.02 mm phase resolution. SOC contains epochs for cycle slips, a stand-alone time-tag per epoch, a minimum representation of the receiver's clock solution, 3 SNR numbers, a unique site id, a modulo 12 hour sequence number and flags for receiver type and GPS health. SOC's simple structure comprises an 8 byte header, a 9 byte overhead for timetag, number of gps, etc., plus 21 data bytes per gps.
</p>
<p>
Visit <u>http://gipsy.jpl.nasa.gov/igdg/papers/SOC_FORMAT.ppt</u> for further details.
</p>
<p>
</p>