92 MPC User Manual Rev 0D
Appendix B GPS Overview
B.5.2.1 The Reference Station
The nucleus of the differential network is the reference station. To function as a reference station, the
GPS receiver antenna must be positioned at a control point whose position is precisely known in the
GPS reference frame. Typically, the fixed position will be that of a geodetic marker or a pre-surveyed
point of known accuracy.
The reference receiver must then be initialized to fix its position to agree with the latitude, longitude,
and height of the phase centre of the reference station GPS receiver antenna. Of course, the antenna
offset position from the marker must be accurately accounted for.
Because the reference station’s position is fixed at a known location, it can now compute the range of
its known position to the satellite. The reference station now has two range measurements with which
to work: computed pseudoranges based on its known position relative to the satellite, and measured
pseudoranges which assumes the receiver position is unknown. Now, the reference station’s measured
pseudorange (unknown position) is differenced against the computed range (based on known position)
to derive the differential correction which represents the difference between known and unknown
solutions for the same antenna. This difference between the two ranges represents the combined
pseudorange measurement errors resulting from receiver clock errors, atmospheric delays, satellite
clock error, orbital errors, and SA.
The reference station will derive pseudorange corrections for each satellite being tracked. These
corrections can now be transmitted over a data link to one or more remote stations. It is important to
ensure that the reference station’s FIX POSITION setting be as accurate as possible, as any errors here
will directly bias the pseudorange corrections computed, and can cause unpredictable results
depending on the application and the size of the reference station position errors. As well, the
reference station’s pseudorange measurements may be biased by multipath reception.
B.5.2.2 The Remote Station
A remote station is generally any receiver whose position is of unknown accuracy, but has ties to a
reference station through an established data link. If the remote station is not receiving differential
corrections from the reference station, it is essentially utilizing single-point positioning measurements
for its position solutions, thus is subject to the various GPS system biases. However, when the remote
GPS receiver is receiving a pseudorange correction from the reference station, this correction is
applied to the local receiver’s measured pseudorange, effectively cancelling the effects of orbital and
atmospheric errors (assuming baselines < 50 km), as well as eliminating satellite clock error.
The remote station must be tracking the same satellites as the reference station in order for the
corrections to take effect. Thus, only common satellite pseudoranges will utilize the differential
corrections. When the remote is able to compute its positions based on pseudorange corrections from
the reference station, its position accuracies will approach that of the reference station. Remember, the
computed position solutions are always that of the GPS receiving antenna phase centre.
