Availability versus Availability - What Does It Really Mean?

Trent Skidmore

Abstract:	Availability is perhaps the most misunderstood of the required navigation performance (RNP) parameters. Everyone associated with GPS for aircraft applications has an immediate intuitive “feel” for what is meant by accuracy, integrity, and continuity, but one’s intuition often becomes confused when addressing av&&/i@. This confusion over the definition and interpretation of availability is due in part to the proliferation of technical works addressing the availability of various functions. For example, papers have been published concerning the availability of GPS to provide specific accuracies, the availability of a given dilution of precision (DOP), the availability of ground-station integrity monitoring in local-area DGPS applications, the availability of the Wide Area Augmentation System (WAAS) including geostationary satellites, and the availability of receiver autonomous integrity monitoring (RAIM). To further complicate matters, all of the above- stated GPS “availabilities” are based on widely varying assumptions about factors such as satellite constellation make-up, satellite failures, elevation (mask) angles, and desired user location or service volume. This paper address the various interpretations of availability and attempts to put them into perspective using both analogies and examples based on extensive simulation results using the Ohio University Satellite Coverage Research Analysis Model (SCRAM). It is the goal of this paper to provide readers with a foundation for interpreting the exact meaning and functional limitations of a given availability analysis. This paper is of particular importance to two groups: 1) policy makers and government officials who must interpret and make potentially far-reaching decisions based on often confusing GPS availability concepts, and 2) engineers/scientists performing availability analyses who should take great care in clearly stating assumptions and limitations of a particular analysis or simulation.
Published in:	Proceedings of the 1995 National Technical Meeting of The Institute of Navigation January 18 - 20, 1995 Disneyland Hotel Anaheim, CA
Pages:	175 - 180
Cite this article:	Skidmore, Trent, "Availability versus Availability - What Does It Really Mean?," Proceedings of the 1995 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 1995, pp. 175-180.
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