GPS Availability, Part II: Evaluation of State Probabilities for 21 Satellite and 24 Satellite Constellations

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Abstract
GPS will offer extremely high navigation performance and has a very promising future for most civil navigation needs. However, certain users consider that it may have insufficient integrity and limited availability. Sensitivity to these inadequacies varies according to size of constellation, type of user, navigation phase in which GPS is used, level of service expected, and complementary equipment operated. However, for a given constellation, GPS availability can be inferred from two fundamental factors: probability of constellation being in a given state (number of working satellites), and service availability when the constellation is in that state. This paper looks at the first of these factors by examining the consistency of the data currently being published by the civilian community on GPS system failures and availability. The paper also contains a plausible set of parameters based on a Markov chain model. These can be used to determine plausible state probabilities for a 24 satellite constellation. The analysis and results described in this paper are used in a previous paper (GPS Availability, Part I) published in the Summer 1990 issue of this journal.

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