RTCM data format

could someone tell me where I can get RTCM data format information?

It is for my personnal interest and I would like to know how it works,
I found nothing interseting up to now.

Thanks

 

I have been trying to find a free source for it for years. No way - you
HAVE to buy it. But it's not that expensive, about 80 US$ for the 2.0
spec as far as I remember.

I don't understand why it's possible to publish free pieces of
information on NMEA, but not on RTCM. Maybe so far no one bothered about
working it up in a way that the copyright is not infringed.

- Carsten

 

Well, it is a fine line for information on NMEA. I have been on both
sides of the line on my NMEA page so I know. Currently I seem to be
setting just on the line so they have been silent so far. I have not
been able to get any RTCM data either but I do know a little about how
it works. Some of the data in only may DGPS page. The best RTCM source
at the moment seems to be the Canadian DGPS web site. It tells a little
more than I have written about so far but does not specify the actual
sentence content. They do mention a bit about the sentence types that
are present.

Dale

 

Actually one of those RTCM Windows monitors (I have to find out exactly
which of those executables) with Internet-DGPS connectivity displays a
table of all individual msg types with content, so it will probably take
you further.

I think it's this here (windows version):

http://igs.ifag.de/root_ftp/software/euref-ip-rtcm-v101.zip

You may have to try various stations, most may be password protected or
behind firewalls. But even without a connection the table tells quite a
bit, even about the newer 2.1-2.3 msg types. If you find a working
station, you can even listen to the RTCM data on the COM port.


Some information can be extracted from the few D.I.Y DGPS projects. But
of course it's not really funny to reverse engineer that from a PIC
assembler source code file ...

This document: http://www.geopp.de/download/DGPS-datenformate2.pdf tells
quite a bit, but is not complete. Well, it's in german anyway. Maybe
some of it is usable without knowing german.

- Carsten

 

Ref: http://www.navcen.uscg.gov/ftp/dgps/dgeninfo/rtcm104.txt


RTCM RECOMMENDED STANDARDS
FOR
DIFFERENTIAL NAVSTAR GPS SERVICE

RTCM Paper 194-93/SC 104-STD

AUTHOR

Radio Technical Commission for Maritime Services

PREFACE (quoted by permission)

This recommended standards document has been developed by
RTCM SC-104 to replace the document entitled "RTCM Recommended
Standards for Differential Navstar GPS Service, Version 2.0" issued on
January 1, 1990. Experience has shown that the recommended data format
and message structure were generally sound, and that the 25% overhead required
for the parity coding guarantees a highly robust channel for the differential
GPS corrections.
The results of usage of the RTCM SC-104 standard have been have been
highly successful. While 8-10 meters (95%) was originally targeted for
shipboard applications, results have generally been better than 5 meters,
often achieving 1-3 meters.
These results have been obtained using the C/A code pseudorange
measurements, with varying amounts of integrated Doppler smoothing. Real-time
kinematic techniques will enable decimeter accuracy for those applications
amenable to them.
Governments have taken advantage of the SC-104 standard by prescribing it
as the format for publicly supported radiobeacon broadcasts of differential
GPS corrections. Scandinavian and North American countries have announced
plans to equip their coastal waters with these services. This medium is
highly attractive because of its inexpensiveness and accessibility.

The major revisions in Version 2.1 have been the following:

1. Update chapters 1-3, and add a section on GLONASS.
2. To allow service providers to define the meaning of the station health
field.
3. To fix some message types previously designated as tentative, and to add
new messages which support real-time kinematic applications.
4. Provide additional guidance material in Chapter 5 and Appendices I-IV.

There is additional work currently in progress to address messages to
support differential GLONASS, provide guidance for interfacing between
radiobeacon-based data links and GPSreceivers, and to provide standards
for ground-based radiobeacons differential GPS stations. Documentation
on these topics will be released as the work is completed.

EDITIONS

DOCUMENT: RTCM RECOMMENDED STANDARDS FOR DIFFERENTIAL
NAVSTAR GPS SERVICE, VERSION 2.1, RTCM PAPER 194-93/SC 104-STD

Version 2.1, dated January 25, 1994, is the current edition.

HOW TO ORDER (from RTCM memo dated January 25, 1994)

Price: The publication may be ordered at a prepaid price of $50.00
(U.S. Dollars). Prepaid order price includes handling and shipping charges for shipment via book-rate mail to U.S./Canada destinations and via
surface mail to destinations other than U.S./Canada.

Expedited Delivery: If expedited delivery is desired please add $4.00
(U.S. Dollars) per copy for shipment via first class mail to U.S./Canada
destinations and $10.00 (U.S. Dollars) per copy for shipment via air-mail to
destinations other than U.S./Canada.

ALSO STILL AVAILABLE

DOCUMENT: RTCM RECOMMENDED STANDARDS FOR DIFFERENTIAL
NAVSTAR GPS SERVICE, VERSION 2.0, RTCM PAPER 134-89/SC 104-68

Price/Delivery: The publication may be ordered at a prepaid price of $40.00
(U.S. Dollars), for shipment via book-rate mail to U.S./Canada destinations
other than U.S./ Canada. For expedited delivery please add $3.00
(U.S. Dollars) per copy for shipment via first class mail to U.S./Canada
air-mail to destinations other than U.S./Canada.

FOR ALL ORDERS

Payment Methods: RTCM documents are only shipped on a prepaid basis. We do not
accept purchase orders, COD or credit cards. Payment is required in U.S.
dollars by check or by money order. Bank transfers are not acceptable forms of
payment due to excessive bank charges.

Ordering Procedure: All orders must identify specific document being ordered
and number of copies. Orders, with payment as specified above, should be sent
to:
RTCM
655 Fifteenth Street, NW
SUITE 300
Washington, D.C. 20005, U.S.A

For additional information, contact the RTCM Office at the above address, or
telephone (202) 639-4006.

TABLE OF CONTENTS

INTRODUCTION 1-1 thru 1-10
THE NEED FOR DIFFERENTIAL GPS SERVICE 2-1 thru 2-10
EQUIPMENT CONFIGURATION AND DESIGN REQUIREMENTS 3-1 thru 3-14
RECOMMENDED DATA MESSAGE FORMAT 4-1 thru 4-48
USER EQUIPMENT INTERFACE 5-1 thru 5-4

 

Ref: http://www.epncb.oma.be/ftp/general/rtcmmessagetypes.txt

All real-time positioning is done by transmitting RTCM-format messages or
proprietary binary messages. Available RTCM message types are 1, 2, 3, 6,
9, 16,18/19, 20/21, and 22. These messages are described below.

TYPE 1 - RTCM type 1 message is the range correction message and is the
primay message in code-phase differential positioning (DGPS). Type 1 is
computed in the base receiver by computing the error in the range
measurement for each tracked SV.

TYPE 2 - messages are 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.

TYPE 3 and 22 - RTCM type 3 and 22 are the base station position and the
antenna offset. Type 3 and 22 are used in RTK processing to perform antenna
reduction.

TYPE 6 - 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.

TYPE 9 - 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.

TYPE 16 - RTCM type 16 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.

TYPE 18 and 20 - are RTK uncorrected carrier phase data and carrier phase
corrections.

Type 19 and 21 - are the uncorrected pseudo-range measurements and pseudo-
range corrections used in RTK.

 

Also: http://www.qgsl.com/gps_tutorial/rtcm.shtml