Trimble Outdoors Part Number 45005-00-ENG manual Position Accuracy, Selective Availability S/A

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).

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