RAIM

RAIM is the abbreviation for Receiver Autonomous Integrity Monitoring, 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.

General Description

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 only (FD) however newer GPS receivers incorporate Fault Detection and Exclusion (FDE) which enables them to continue to operate in the presence of a GPS failure.

Because RAIM operates autonomously, that is without the assistance of external signals, it requires redundant pseudorange measurements. To obtain a 3D position solution, at least 4 measurements are required. To detect a fault, at least 5 measurements are required, and to isolate and exclude a fault, at least 6 measurements are required, however often more measurements are needed depending on the satellite geometry. Typically there are 7 to 12 satellites in view.

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 requirements for the probability of false alarm (Pfa) and the probability of missed detection (Pmd), and the expected measurement noise. In aviation systems, the Pfa is fixed at 1/15000 which allows for the best possible Pmd.

The Horizontal Integrity Limit (HIL) or Horizontal Protection Limit (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). The HPL is calculated as a function of the RAIM threshold and the satellite geometry at the time of the measurements. The HPL is compared with the Horizontal Alarm Limit (HAL) to determine if RAIM is available.

References

• Brown, R. G. (1992). "A Baseline RAIM Scheme and a Note on the Equivalence of Three RAIM Methods." Navigation: Journal of The Institute of Navigation 39 No. 3(Fall 1992): 301-316.

• Parkinson, B. W. and P. Axelrad (1988). "Autonomous GPS Integrity Monitoring Using the Pseudorange Residual." Navigation 35 No. 2: 255-274.

• Parkinson, B. W., J. J. Spilker Jr., et al., Eds. (1996). The Global Positioning System: Theory and Applications; Volume I & II. Progress in Astronautics and Astronautics. Washington, American Institute of Aeronautics and Astronautics, Inc.

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