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|Year of Publication||2011|
The availability of a navigation system is the percentage of time that the services of the system are usable by the navigator. 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 navigation signals transmitted from external sources are available for use. It is a function of both the physical characteristics of the environment and the technical capabilities of the transmitter facilities[footnotes 1].
The availability is usually measured as percentage. Availability will then express the percentage of time that the system is usable by a receiver, user or application.
Particularizations of the availability concept can be made by considering the availability of a usable signal from a specific satellite (or slot[footnotes 2]) or by considering the availability of position, velocity and time (PVT) from the full constellation.
The availability of a usable signal from a specific satellite is related with the correct behavior of the satellite. The availability of usable signal for a satellite will only guarantee that the pseudorange to the satellite will be known. For the availability of a PVT it is required that a lock can be made on the signal of at least 3 satellites (for 2D positioning plus time).
When considering the availability of a PVT additional constraints can be added to define when the system is available. If a specific application requires that a PVT is only usable if the expected error is below a certain threshold it can be said that the system is available only when the error meets that requirement. This approach will only make sense if the system supports integrity or if there is a reference system to measure errors. Otherwise the application doesn't have any way to determine whether the threshold is being violated or not.
Availability can be influenced by several factors (see GNSS Performances for factors that influence the performance of GNSS) being the most important the constellation configuration and its visibility at user location and the surrounding environment (buildings and other obstacles) that might mask part or all of the satellites in the sky.
- ^ This definition was adapted from the 2008 US Federal Radionavigation Plan
- ^ The orbital planes of a constellation are usually divided in slots that should contain at least one operational satellite
- ^ US Federal Radionavigation Plan, DOT-VNTSC-RITA-08-02/DoD-4650.5, 2008
- ^ GPS Standard Positioning Service (SPS) Performance Standard, 4th Edition, September 2008.