User's Manual Part 1

Positioning Modes of Operation Chapter 5
OEMV Family Installation and Operation User Manual Rev 5B 83
5.3.3 Dual Station Differential Positioning
It is the objective of operating in differential mode to either eliminate or greatly reduce most of the
errors introduced by the system biases discussed in Section 5.1.1, GPS System Errors starting on Page
77. Pseudorange differential positioning is quite effective in removing most of the biases caused by
satellite clock error, ionospheric and tropospheric delays (for baselines less than 50 km), and
ephemeris prediction errors. However, the biases caused by multipath reception and receiver clock
offset are uncorrelated between receivers and thus cannot be cancelled by "between receiver single
differencing" operation.
Differential operation requires that stations operate in pairs. Each pair consists of a base station
and a
rover station
. A differential network could also be established when there is more than one rover
station linked to a single base station.
In order for the differential pair to be effective, see Figure 26, Typical Differential Configuration on
Page 84, differential positioning requires that both base and rover station receivers track and collect
satellite data simultaneously from common satellites. When the two stations are in relatively close
proximity (< 50 km), the pseudorange bias errors are considered to be nearly the same and can be
effectively cancelled by the differential corrections. However, if the baseline becomes excessively
long, the bias errors begin to decorrelate, thus reducing the accuracy or effectiveness of the
differential corrections.
The Base Station
The nucleus of the differential network is the base station. To function as a base 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 is that of a geodetic marker or a pre-surveyed point of
known accuracy.
The base receiver must then be initialized to fix its position to agree with the latitude, longitude, and
height of the phase centre of the base station GPS receiver antenna. Of course, the antenna offset
position from the marker must be accurately accounted for.
Because the base station’s position is fixed at a known location, it can now compute the range of its
known position to the satellite. The base 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 base 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, and orbital errors.
The base station derives pseudorange corrections for each satellite being tracked. These corrections
can now be transmitted over a data link to one or more rover stations. It is important to ensure that the
base station’s FIX POSITION setting be as accurate as possible, as any errors here directly bias the
pseudorange corrections computed, and can cause unpredictable results depending on the application
and the size of the base station position errors. As well, the base station’s pseudorange measurements
may be biased by multipath reception.