Availability: What is Availability? Availability of What?

“Availability” is ubiquitous in the evaluation of the performance of satellite constellations. However, the definition of Availability and its targets have evolved over the years. In the early days of GPS, availability of a simple requirement (minimum of 4 visible satellites or maximum PDOP of 6) of an ideal constellation (with 18 or 21 or 24 satellites) was adequate. Then the availability of same requirement was applied to degraded constellations with outages of one or more satellites. Today, availability has become more complex involving satellite’s Markov state probability with different satellite failure and restoration models. For navigation, availability of continuity of accuracy, not accuracy itself, is the dominant requirement. The system performance requirement has also evolved from a simple number of visible satellites to Dilution of Precision (where satellites’ errors are assumed to be identical) to more complex horizontal and vertical user navigation accuracy with satellites’ errors changing with time as the satellites’ position changes. This paper provides a short review of all those definitions and requirements with emphasis on precise mathematical expression for four types of availability - ideal, degraded, expected and with continuity - and different categories of requirements - visibility, GDOP, accuracy and integrity . A unified formulation for availability that encompasses all four types of availability is presented. The various types of availability combined with different requirements have been a constant source of confusion and sometimes frustration (even for researchers working in this field). There have been suggestions from the international navigation community for clarifications and standardization’s on the various definitions of availability. This paper serves as a beginning for such efforts.