Trimble Outdoors 45005-00-ENG Manuel d'utilisateur télécharger pdf (Page 296)

F-6 Acutime 2000 Synchronization Kit User Guide

Theory of Operation

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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Trimble Outdoors 45005-00-ENG Manuel d'utilisateur Page 296