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relates to a given epoch of position computation. The GST gives an indication of the
overall quality (precision) of the CPD position by displaying the RMS value of the
standard deviation of all the range inputs to the position solution. The GST message
also gives a real-time estimate of the actual error in the CPD position at a 1 sigma
probability by displaying the standard deviation of latitude, longitude and altitude.
The actual position error of the system will be less than the standard deviations
displayed in the GST approximately 68% of the time. If you multiply the standard
deviations by 2, the result is a conservative estimate of actual accuracy about 95% of
the time.
The quality of the GST estimates improve with increasing number of satellites. The
GST estimates may be very unreliable with only 4 satellites in view. Figure 5.4 plots
horizontal error estimates (from GST) and actual horizontal error (calculated using
known antenna position) for typical GST performance for 10 satellites in view.The
horizontal estimates are derived from:
2
2
(
)
(
)
GSTLatError
+
GSTLonError
Differential and RTK Operations
63
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