The Effect of Selective Availability on Differential GPS Corrections

Gregory T. Kremer, Rudolph M. Kalafus, Peter V.W. Loomis, James C. Reynolds

Abstract:	The Block II GPS satellites have the capability of controlling the accuracy of the Standard Positioning Service signals by a combination of "dithering' the satellite clock and data manipulation of the ephemeris. This capability is called Selective Availability (SA). The errors in both position and velocity are dominated by this effect. the errors of ionosphere, troposphere, receiver noise, etc. being much smaller. Differential GPS operation greatly reduces the effects of SA. ionosphere, troposphere and other slowly-varying errors for users in a local area around a reference station. The Radio Technical Commission for Maritime Services established Special Committee 104 to develop standards for differential GPS operation. One of the major technical issues is the required frequency of update of the corrections, as this affects data link bandwidth and systems costs. Working with only some sparse statistical data on the SA dithering, the required minimum data update period was projected to be about once every ten seconds for 5-10 meter service accuracy. However, until the recent launch of the new Block II satellites, there has not been .firm evidence that this data rate would be adequate. In fact. it has not been certain that differential service could be provided without interruptions. nimble Navigation has been collecting data on all GPS satellites, including PRN 14 and PRN 2, and' 'dynamic behavior of the signal has been observed having the characteristics expected of Selective Availability. The dynamic data of PRN's 2 and 14 was analyzed for their statistical character: position and velocity variations, and correlation times. Pseudorange corrections generated by differential reference station equipment using these signals were applied to obtain estimates of the effect of the dynamic behavior on differential GPS performance.
Published in:	Proceedings of the 2nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1989) September 27 - 29, 1989 The Broadmoor Hotel Colorado Spring, CO
Pages:	307 - 312
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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