Receiver Autonomous Integrity Monitoring

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 thanneeded to produce a position fix, the extra pseudoranges should all be consistent with the computed position. A pseudorange that differs significantlyfrom 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.

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)

RAIM

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 5 visible satellites with satisfactory geometry must be visible to it. The RAIM function performs consistency checks between position solutions obtained with various subsets of the visible satellites. The receiver provides an alert to the pilot if the consistency checks fail. Because of geometry and service maintenance RAIM is not always available.

Fault Detection and Exclusion

An enhanced version of RAIM employed in some receivers is known as Fault Detection and Exclusion (FDE). It uses a minimum of 6 satellites to not only detect a possible faulty satellite, but to exclude it from the navigation solution so the navigation function can continue without interruption. The goal of fault detection is to detect the presence of a positioning failure. Upon detection, proper fault exclusion determines and excludes the source of the failure (without necessarily identifying the individual source causing the problem), thereby allowing GNSS navigation to continue without interruption. Availability of RAIM and FDE will be slightly lower for mid-latitude operations and slightly higher for equatorial and high latitude regions due to the nature of the orbits. The use of satellites from multiple GNSS constellations or the use of SBAS satellites as additional ranging sources can improve the availability of RAIM and FDE.

RAIM Prediction

GNSS differs from traditional navigation systems because the satellites and areas of degraded coverage are in constant motion. Therefore, if a satellite fails or is taken out of service for maintenance, it is not immediately clear which areas of the airspace will be affected, if any. 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. This prediction process is, however, not fully representative of all RAIM implementations in the different models of receivers. Prediction tools are usually conservative and thus predict lower availability than that actually encountered in flight to provide protection for the lowest end receiver models.

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.