Chapter 4. Differential Operation - ashtech DG16 Reference Manual

The DG16 can operate as either a Remote Station or Base Station for
differential GPS, and it can be set to operate as a combined base and remote
station. The DG16 also makes use of multiple correction data streams and
offers a multi-base differential solution.
This chapter contains a general discussion of real-time differential operation,
including available options, basic concepts, sources of error, DG16
commands related to differential GPS.Differential remote [U] and base [B]
operation are available as receiver options. Both options must be installed in
order for the DG16 to be able to support both differential modes (base and
rover).
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.
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 DG16 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
Differential Operation 53