Receiver autonomous integrity monitoring

Receiver autonomous integrity monitoring (RAIM) is a technology developed to assess the integrity of global positioning system (GPS) signals in a GPS receiver system. It is of special importance in safety-critical GPS applications, such as in aviation or marine navigation.

GPS does not include any internal information about the integrity of its signals. It is possible for a GPS satellite to broadcast slightly incorrect information that will cause navigation information to be incorrect, but there is no way for the receiver to determine this using the standard techniques. RAIM uses redundant signals to produce several GPS pseudoranges and compare them, and a statistical function determines whether or not a fault can be associated with any of the signals. RAIM is considered available if 24 GPS satellites or more are operative. If the number of GPS satellites is 23 or less, RAIM availability must be checked using approved ground-based prediction software.

Several GPS-related systems also provide integrity signals separate from GPS. Among these is the WAAS system, which uses separate signals broadcast from different satellites to indicate these problems directly.

RAIM detects faults with redundant GPS pseudorange measurements. That is, when more satellites are available than needed to produce a position fix, the extra pseudoranges should all be consistent with the computed position. 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). Traditional RAIM uses fault detection (FD) only, however newer GPS receivers incorporate fault detection and exclusion (FDE) which enables them to continue to operate in the presence of a GPS failure.

The test statistic used is a function of the pseudorange measurement residual (the difference between the expected measurement and the observed measurement) and the amount of redundancy. The test statistic is compared with a threshold value, which is determined based on the requirement probability of false alarm (Pfa).

Receiver autonomous integrity monitoring (RAIM) provides integrity monitoring of GPS for aviation applications. In order for a GPS receiver to perform RAIM or fault detection (FD) function, a minimum of five visible satellites with satisfactory geometry must be visible to it. RAIM has various kind of implementations; one of them performs consistency checks between all position solutions obtained with various subsets of the visible satellites. The receiver provides an alert to the pilot if the consistency checks fail.

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