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F-6 Acutime 2000 Synchronization Kit User Guide
Theory of Operation
F
F.3 Position Accuracy
GPS position accuracy is degraded by atmospheric distortion, satellite
and receiver clock errors, and Selective Availability (S/A). Effective
models for atmospheric distortion of satellite signals have been
developed to minimize the impact of tropospheric and ionospheric
effects. The impact of satellite clock errors is minimized by
incorporating the clock corrections transmitted by each satellite used
in the position solution. S/A is the most significant contributor to
position error and cannot be effectively counteracted except with
differential GPS.
F.3.1 Selective Availability (S/A)
The U.S. Department of Defense, through a program called Selective
Availability, intentionally degrades GPS accuracy for civilian users.
The S/A program creates position errors by modifying the apparent
position of each satellite and introducing random dither into each
satellite's clock.
In extreme cases, all sources of error (natural, PDOP, and S/A) can
combine to produce large position errors. The DOD's definition of
accuracy under S/A is 100 meters 2 dRMS (horizontal two-
dimensional, 95 percent of the time).