4 Message Formats
48 MiLLennium GPSCard Software Version 4.50 Command Descr iptions Manual Rev 1
The standards recommended by the Radio Technical Commission for Maritime Services Special Committee 104,
Differential GPS Service (RTCM SC-104,Washington, D.C.), have been adopted by NovAtel for implementation
into the GPSCard. Because the GPSCard is capable of utilizing RTCM formats, it can easily be integrated into
positioning systems around the globe.
As it is beyond the scope of this manual to provide in-depth descriptions of the RTCM data formats, it is
recommended that anyone requiring explicit descriptions of such, should obtain a copy of the published RTCM
specifications. See Appendix F, Page 233 for reference information.
RTCM SC-1042 Type 3 & 59 messages can be used for reference station transmissions in differential systems.
However, since these messages do not include information on the L2 component of the GPS signal, they cannot be
used with RT-2 positioning. Regardless of whether single or dual-frequency receivers are used, the RT-20
positioning algorithm would be used. This is for a system in which both the reference and remote stations utilize
NovAtel receivers.
Note that the error-detection capability of an RTCM-format message is less than that of an RTCA-format message.
The communications equipment that you use may have an error-detection capability of its own to supplement that
of the RTCM message, although at a penalty of a higher overhead (see the discussion at the beginning of this
chapter, Page 45). Consult the vendor’s documentation for further information.
• RTCM Type 3 Reference Station Position
A Type 3 message contains reference station position information. This message must be sent at least once every
30 seconds, although it is recommended that it be sent once every 10 seconds. It uses four RTCM data words
following the two-word header, for a total frame length of six 30-bit words (180 bits).
• RTCM Type 59 NovAtel Proprietary (RT-20)
A Type 59N message contains reference station satellite observation information, and should be sent once every 2
seconds. It is variable in size, and can be up to thirty three 30-bit words (990 bits) long.
If RTCM-format messaging is being used, the optional station id field that is entered using the FIX POSITION
command can be any number within the range of 0 - 1023 (e.g. 119). The representation in the log message wou l d
be identical to what was entered.
RTCM General Message Format
All GPSCard RTCM standard format logs adhere to the structure recommended by RTCM SC-104. Thus, all RTCM
message are composed of 30 bit words. Each word contains 24 data bits and 6 parity bits. All RTCM messages
contain a 2-word header followed by 0 to 31 data words for a maximum of 33 words (990 bits) per message
The remainder of this section will provide further information concerning GPSCard commands and logs that utilize
the RTCM data formats.
2. For further information on RTCM SC-104 messages, you may wish to refer to:
RTCM Recommended Standards for Differential Navstar GPS Service, Version 2.1, RTCM Paper 194-
93/SC104-STD (January 3, 1994)
Message Frame Header Data Bits
Word 1 – Message fr ame preamble for synchronization
– Frame/m essage type ID
– ref erence station ID
–Parity
8
6
10
6
Word 2 – Modifi ed z-count (time tag)
– Sequen ce number
– Length of mes sage frame
– ref erence station health
–Parity
13
3
5
3
6