Novatel OM-20000041 用户手册

4

Message Formats

48

 MiLLennium GPSCard Software Version 4.50 Command Descriptions 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-104

2

 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.

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).

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 would
be identical to what was entered.

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)

Word 1

–

Message frame preamble for synchronization

–

Frame/message type ID

–

reference station ID

–

Parity

8
6
10
6

Word 2

–

Modified z-count (time tag)

–

Sequence number

–

Length of message frame

–

reference station health 

–

Parity

13
3
5
3
6