Operation 39
Operation
On initial power-up, or after issuing the $PASHS,RST command (restore
defaults), the G12’s setting for differential mode is OFF, and the setting for the
maximum age of an RTCM differential correction is 60 seconds, meaning that an
incoming correction whose age is greater than 60 seconds is not used. If
automatic differential GPS mode is not enabled ($PASHS,DIF,AUT), and if the
differential correction data is unavailable or is older than the maximum age
specified by the $PASHS,RTC,MAX set command, a G12 set as a remote
differential station will not output position data. If automatic differential mode is
enabled, a G12 set as a remote differential station will output uncorrected
positions if differential correction data is unavailable or if the age of correction
exceeds the maximum age setting.
RTCM 104 Format, Version 2.2
When the RTCM base option is enabled and the G12 is configured as a reference
station, it computes differential corrections for up to 12 satellites, converts those
corrections to RTCM format and outputs the converted messages through its
serial ports. The G12 generates message types 1, 2, 3, 6, 9, and 16, listed in
Table 3.1:
The G12 uses the six-of-eight format (data bits a1 through a6 of an eight-bit byte)
for differential corrections.
When the RTCM remote option [U] is installed and the G12 is set in differential
remote mode, it can decode RTCM message types 1, 2, 3, 6, 9, and 16, but uses
only types 1, 2, and 9 to correct its position calculations. When using radio-
modems for the communication link, the G12 in remote mode is able to recover bit
slippage.
Table 3.1. RTCM Format
Message
Type
Contents of message
1 Differential GPS corrections
2 Delta differential corrections
3 Reference station coordinates
6 Null frame
9 High-rate differential GPS corrections
16 Special Message
G12RevD.book Page 39 Tuesday, April 2, 2002 4:33 PM
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