RAIM stands for receiver autonomous integrity monitoring, a technology used in GNSS receivers to assess the integrity of the GNSS signals that are being received at any given time. It is particularly applicable to receivers intended for safety-critical applications, and in particular in aviation applications.
The RAIM concept makes use of redundant satellite signals – i.e., any that are available above and beyond those needed to produce a position fix. If the pseudorange data in any of the signals is at odds with the position computed from the other signals it may indicate a fault in that satellite, such as a clock error. Alternatively, the error may be due to unexpected atmospheric conditions.
In general, to obtain a 3D positional fix at least four satellite signals are required. To detect a fault, at least five signals are required, and to isolate and exclude a fault, at least six satellite signals are required. However more signals are often needed, depending on the geometry of the satellite constellation, and so RAIM is not always available.
In aviation applications, pilots can check whether RAIM will be operational on any given route and/or approach by checking one or other of the RAIM prediction websites operated by the Federal Aviation Authority and Eurocontrol. These sites allow pilots to predict RAIM status during pre-flight checks.
Applicable GNSS receivers can be tested for RAIM performance using Spirent's SimGEN™ software, which incorporates a pseusorange ramp feature that changes the simulated position of a satellite in a controlled (but abnormal) manner. This deviation is not declared in the navigation message, but any receiver with a RAIM algorithm should be able to detect the anomaly.