If the number of GPS satellites is 23 or fewer, RAIM availability must be checked using approved ground-based prediction software.
Among these is the WAAS system, which uses separate signals broadcast from different satellites to indicate these problems directly.
A pseudorange that differs significantly from the expected value (i.e., an outlier) may indicate a fault of the associated satellite or another signal integrity problem (e.g., ionospheric dispersion).
The test statistic is compared with a threshold value, which is determined based on the required probability of false alarm (Pfa).
RAIM has various kinds of implementations; one of them performs consistency checks between all position solutions obtained with various subsets of the visible satellites.
GNSS differs from traditional navigation systems because the satellites and areas of degraded coverage are in constant motion.
The location and duration of these outages can be predicted with the aid of computer analysis and reported to pilots during the pre-flight planning process.
The test statistic is compared with a threshold value, which is determined based on the requirements for the probability of false alarm (Pfa) and the expected measurement noise.
The horizontal integrity limit (HIL) or horizontal protection level (HPL) is a figure which represents the radius of a circle which is centered on the GPS position solution and is guaranteed to contain the true position of the receiver to within the specifications of the RAIM scheme (i.e. which meets the Pfa and Pmd).