Availability Characteristics of GPS and Augmentation Alternatives

Woody S. Phlong and Bryant D. Elrod

Abstract:	Acceptance of GPS as a sole means navigation aid in the National Airspace System (NAS) will necessitate meeting stringent availability and continuity of service requirements which may be unachievable for GPS used alone. The objective of this paper is to present the results of current efforts to evaluate the availability of GPS and GPS augmented by bare-altimeter aiding and/or geostationary satellites (GSs) with a GPS-like ranging capability. The availability measure adopted is the probability of achieving a specified level of geometric performance in terms of equivalent horizontal/vertical dilution of precision parameters (I-IDOP/VDOP). Availability distributions are determined for CONUS users based on the planned operational 21 primary + 3 spares GPS constellation and various GS deployments with full consideration of operational probabilities for both GSs and GPS satellites. The impact of baro-aiding is included by parameterizing barn-altitude accuracy relative to GPS/GS pseudorange accuracy. To assess navigation performance in terms of satisfying accuracy/integrity protection limit requirements, relationships are identified between HDOP/VDOP characteristics and parameters for estimating bounds on user navigation error at a specified availability. This enables convenient comparison of GPS augmentation alternatives as a function of GPS/GS pseudorange error and positioning requirements.
Published in:	Proceedings of the 1993 National Technical Meeting of The Institute of Navigation January 20 - 22, 1993 Parc 55 Hotel San Francisco, CA
Pages:	69 - 79
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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