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Availability
| Fundamentals | |
|---|---|
| Title | Availability |
| Author(s) | Rui Barradas Pereira |
| Level | Basic |
| 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[nb 1].
Measuring Availability
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 by considering the availability of position, velocity and time (PVT) from the full constelation[2].
The availability of a usable signal from a specific satellite...
When considering the availability of a PVT additional contrains can be added to define when the system is available. If a specific application requires that a PVT is only usable if the 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 the system support 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.
Availabilty can be influenced by several factors (see GNSS Performances for factors that influence the performance of GNSS) being the most important the constelation 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.
Notes
- ^ This definition was adapted from the 2008 US Federal Radionavigation Plan[1]
References
- ^ US Federal Radionavigation Plan, DOT-VNTSC-RITA-08-02/DoD-4650.5, 2008
- ^ GPS Standard Positioning Service (SPS) Performance Standard, 4th Edition, September 2008.