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6 Message Formats
38 MiLLennium Command Descriptions Manual
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.
RTCM Standard Commands
RTCMRULE
The RTCM standard states that all equipment shall support the use of the "6 of 8" format (data bits a
1
through a
6
where bits a
1
through a
6
are valid data bits and bit a
7
is set to mark and bit a
8
is set to space).
The GPSCard
RTCMRULE command allows for flexibility in the use of the bit rule to accommodate compatibility
with equipment that does not strictly adhere to the
RTCM stated rule.
Syntax:
Example:
rtcmrule 6cr
RTCM16T
This is a NovAtel GPSCard command which relates to the RTCM Type 16
This command allows the GPSCard user to set an ASCII text string. Once set, the text string can be transmitted as
standard format
RTCM Type 16 data (see the RTCM16 command, Appendix C). The text string entered is limited to a
maximum of 90 ASCII characters. This message is useful for a reference station wanting to transmit special
messages to remote users.
The text string set here can be verified by observing the
RCCA command configuration log. As well, the message
Message Frame Header Data Bits
Word 1 – Message frame preamble for synchronization
– Fram/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
RTCMRULE rule
Syntax Range Value Description Default
RTCMRULE - Command
rule 6CR 6CR is for 6 bits of valid data per byte. Each frame is followed by a <CR> character. 6CR
6SP 6SP (6 bit special); the RTCM decoder of the remote receiver will ignore the two MSB of the
data and hence all 6 bit data will be accepted. This allows users with non-conforming 6 bit rule
data to use the NovAtel receiver to accept their RTCM data. The user will not be allowed to
enter extra control data such as CR/LF, as this will be treated as RTCM data and cause the
parity to fail. This option does not affect RTCM generation. The output will be exactly the same
as if the RTCMRULE 6 option was chosen. The upper two bits are always encoded as per
RTCM specification.
6 6 is for 6 bits of valid data per byte
8 8 is for 8 bits of valid data per byte