CSI Wireless DGPS MAX, Reference Manual

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DGPS MAX Reference Manual
4
1.4 Factors Affecting Positioning Accuracy
Many factors affect the positioning accuracy that a user may expect from a
DGPS system. The most significant of these influences include:
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Proximity of the remote user to the reference station
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Age of the received differential corrections
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Atmospheric conditions at the beacon and remote user locations
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Satellite geometry, often expressed as a Dilution of Precision (DOP)
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Magnitude of multipath present at the remote station
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Quality of the GPS receiver being used at both the reference and remote
stations.
The distance between a remote user and the reference station is often
considerable when using 300 kHz DGPS radiobeacons. Broadcast ranges
may be as great as 450 km (280 miles) or more, depending primarily upon
transmission power and surface conductivity. Consequently, some of the
errors associated with GPS at the base station differ somewhat from those
at the remote user’s location. This spatial decorrelation of errors can
result in a relative position offset from the absolute coordinates of the
remote receiver. This offset may be as much as one meter for every 100
km (62 miles) between the base station and remote receiver. The WAAS
and OmniSTAR services are not as susceptible to this error, as it they use a
wide-area correction format. However, when operating away from the
reference stations in this network, accuracy may degrade, as correction
data may not model errors in your geographic area as thoroughly.
The latency of differential corrections to a lesser extent affects the
achievable positioning accuracy at the remote receiver since the magnitude
of SA was turned to zero in year 2000. Latency is a function of the
following:
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The time it takes the base station to calculate corrections
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The data rate of the radio link
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The time it takes the signal to reach the user
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The time required for the remote differential receiver to demodulate the
signal and communicate it to the GPS receiver.