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Aviation Words — RAIM
By Ian Brown, Editor
June 2019 - Receiver autonomous integrity monitoring (RAIM) is a method of ensuring that a GPS is accurate on a real-time basis.
Your GPS has no way of knowing whether the signals it receives are accurate, and if a specific satellite were to transmit slightly incorrect information, you would want to know. RAIM uses signals not otherwise being used to produce several position fixes independently and compare them. If it finds an outlier, it will exclude that one from the position estimation.
RAIM needs 24 or more satellites to operate, although normally only five are needed for position estimation.
Older GPS units will simply report a GPS fail, but newer ones are able to recover. A newer version of RAIM, FDE (fault detection and exclusion) will allow the GPS to function in more cases and with fewer satellites. Factors such as ionospheric interference, antenna interference, or a particular satellite being taken out of service for maintenance can cause this.
At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time). The position of each satellite is very precisely known so the time it takes for each signal to reach the GPS is used to compute your position. Any extra signals can be used for RAIM and FDE. There may be 24 or more GPS satellites visible to your aircraft at one time.