MBX-4 User Guide
11
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. In the
past when SA was “on,” latency was a concern if lock on the differential
signal was lost for ten seconds or more. Without the effects of SA, the
age of the differential corrections is not as significant, but care should
be taken to ensure that their age is kept below a couple minutes by
ensuring consistent beacon receiver lock.
To account for latency, 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
observed over large distances between base station and remote
receivers
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