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The availability of a PNT system is the percentage of time that the services of the system are usable. Availability is an indication of the ability of the system to provide usable service within the specified coverage area. Signal availability is the percentage of time that PNT signals transmitted from external sources are available for use. Availability is a function of both the physical characteristics of the environment and the technical capabilities of the PNT service provider.
NISTIR 8323r1 from USG FRP (2021) - Appendix E, adapted