Operation 35
Operation
structure and content for all the above messages, refer to “Raw Data Commands”
section on page 113.
This section contains a general discussion of real-time differential operation,
including basic concepts, sources of error, G12 commands related to differential
GPS, plus format and content for the RTCM-SC104 (Sub-committee 104)
correction messages supported by the G12. Differential remote [U] and base [B]
operation are available as receiver options. Both options must be installed in order
for the G12 to be able to support both differential modes (base and rover).
When the G12 is set as a differential base or rover, the port which is designated to output or receive
differential corrections can no longer be used to communicate with the receiver. If you have set the
receiver to output RTCM corrections through port A, you can communicate with the receiver
through port B only. You must disable differential mode in order to resume communication with the
receiver through port A.
General
Real-time differential positioning involves a reference (base) station calculating
range corrections for each satellite it is tracking and transmitting them to the
remote (rover) stations through a real-time data communications link. Remote
receivers apply the corrections to their own range measurements and use the
corrected ranges to compute positions.
The base receiver determines range correction by subtracting the measured
range from the true range. A precise reference position must be entered in the
base receiver before true range can be calculated. The reference position must
have been previously surveyed using GPS or some other comparable technique.
RTCM type 1 corrections with a UDRE (User Differential Range Error) field set to 3 (one-sigma
differential error > 8 meters) are not used.
As a stand-alone receiver, the G12 can compute a position with ± 3 meter CEP
(Circular Error Probable) of accuracy (on average) with Selective Availability (SA)
off. Autonomous accuracy worsens to an average of ±100 meters with SA on. In
differential mode, a G12 in rover mode can achieve sub-meter accuracy.
DEFAULT SETTINGS
Output interval setting for raw data messages is 20 seconds.
G12RevD.book Page 35 Tuesday, April 2, 2002 4:33 PM
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