CSI Wireless GBX, Reference Manual

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GBX Reference Manual
8
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 latency of differential corrections also affects the achievable
positioning accuracy at the remote receiver. Latency is a function of
the following:
•
The time it takes the base station to calculate corrections
•
The data rate of the radio link
•
The time it takes the signal to reach the user
•
The time required for the remote differential receiver to demodulate the
signal and communicate it to the GPS receiver.
•
Any data loss that occurs through reception problems
Most of these delays require less than a second, though in some
instances, depending upon the amount of information being transferred,
overall delays of three to five seconds may occur. Latency can
become a concern if lock on the differential signal is lost for ten
seconds or more.
To account for latency and the rapidly changing SA error, a GPS
receiver can calculate approximate corrections until new corrections
are available. Calculating the differential correction for a new epoch,
using old corrections, leads to inaccuracy that grows with time.
Accuracy is restored when new corrections become available.
Although ionospheric errors are normally removed through differential
positioning, the state of the ionosphere can differ between the base
station and remote user over large distances. As the base station
calculates corrections based on local ionospheric conditions, they may
not completely account for the errors observed at the remote user’s
location. This causes part of the spatial decorrelation that may be