The World after Selective Availability

Rob Conley, John W. Lavrakas

Abstract:	In March 1996, President Clinton issued a Presidential De-cision Directive (PDD) that directed the Department of De-fense (DoD) to discontinue the use of Selective Availability (SA) by the year 2006, with annual reviews of the DoD’s status in preparing for the event beginning in 2000. Unless the DoD presents compelling evidence to counter the PDD, SA will transition to an “off” state within the next few years. The purpose of this paper is to explore the ramifications of removing SA from the Standard Positioning Service (SPS). The paper begins with a discussion of GPS orbit and clock error behaviors in the absence of SA, and how those errors map into Signal-in-Space (SIS) User Range Errors (UREs). This data is then used to generate instantaneous position solution errors on a global basis, using the current constella-tion. This analysis serves to demonstrate the core contribu-tion of the GPS space and control segments to SPS accu-racy. The GPS Modernization Program being executed by the GPS Joint Program Office (JPO) includes a new civil signal that will provide SPS users with a two-frequency ionosphere delay correction capability. However, it will be several years before this capability is available, so single-frequency model errors will still be part of the SPS user’s error budget. The paper details an analysis of measured errors in the single-frequency model based upon correlations between the single frequency model and two-frequency measurements taken over an extended period of time from the Federal Aviation Administration’s (FAA) National Satellite Test Bed (NSTB). This error information is then added to our SIS URE dataset, and instantaneous position solutions including the single frequency error are generated. The paper closes with a discussion of the implications our findings have for the global Positioning/Navigation/Timing (PNT) infrastructure. The paper provides a summary of how well GPS will support a wide assortment of user communi-ties, and discusses the policy considerations implicit in the decision to discontinue the use of SA.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	1353 - 1362
Cite this article:	Conley, Rob, Lavrakas, John W., "The World after Selective Availability," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 1353-1362.
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