RAIM
Encyclopedia
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
.
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 requirements for the probability of false alarm (Pfa) and the probability of missed detection (Pmd)
RAIM availability is an important issue when using such kind of algorithm in safety-critical applications (as the aeronautical ones); in fact, because of geometry and satellite service maintenance, RAIM is not always available at all, meaning that the receiver's antenna could have sometimes less than five satellites in view.
Availability is also a performance indicator of the RAIM algorithm, as it is, basically, a statistical method. Due to this it has a "power of test" which is a measure of the reliability of this test to detect a failure when it happens. This power of test is its availability and it is a function of the geometry of the constellation which is in view and of other environmental conditions. If availability is seen in this way it is clear that it is not an on–off feature meaning that the algorithm could be available but not with the required performance of detecting a failure when it happens. So availability is a performance factor of the algorithm and characterizes each one of the different kinds of RAIM algorithms and methodologies.
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.
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
(GPS) signals in a GPS receiver system. It is of special importance in safety-critical
Life-critical system
A life-critical system or safety-critical system is a system whose failure ormalfunction may result in:* death or serious injury to people, or* loss or severe damage to equipment or* environmental harm....
GPS applications, such as in aviation
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
or marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
.
General description
RAIM detects faults with redundant GPS pseudorangePseudorange
The pseudorange is the pseudo distance between a satellite and a navigation satellite receiver —for instance Global Positioning System receivers....
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
Outlier
In statistics, an outlier is an observation that is numerically distant from the rest of the data. Grubbs defined an outlier as: An outlying observation, or outlier, is one that appears to deviate markedly from other members of the sample in which it occurs....
) 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 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 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.RAIM availability is an important issue when using such kind of algorithm in safety-critical applications (as the aeronautical ones); in fact, because of geometry and satellite service maintenance, RAIM is not always available at all, meaning that the receiver's antenna could have sometimes less than five satellites in view.
Availability is also a performance indicator of the RAIM algorithm, as it is, basically, a statistical method. Due to this it has a "power of test" which is a measure of the reliability of this test to detect a failure when it happens. This power of test is its availability and it is a function of the geometry of the constellation which is in view and of other environmental conditions. If availability is seen in this way it is clear that it is not an on–off feature meaning that the algorithm could be available but not with the required performance of detecting a failure when it happens. So availability is a performance factor of the algorithm and characterizes each one of the different kinds of RAIM algorithms and methodologies.
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.
RAIM prediction websites
To enable pilots to quickly determine whether enroute and/or approach level RAIM will be available, the FAA and EUROCONTROL have created "dispatch level" websites that predict RAIM status to meet pre-flight check requirements.- The FAA's RAIM prediction website "AC 90-100" covers US territories in graphical map format (showing green for RAIM available and red for RAIM not available) and is located at http://www.raimprediction.net/ac90-100/
- EUROCONTROL provides international coverage for most waypoints in the worldwide aviation waypoint database and displays results in a "timeline" bar showing predictions of whether baro-aided or non-baro-aided RAIM will be available. The EUROCONTROL "AUGUR" website is located at http://augur.ecacnav.com/augur/app/home
- EUROCONTROL puts a disclaimer on its data (stating that USCG data takes precedence), whereas the FAA certifies its website as meeting regulatory requirements.
External links
- AC90-100A GPS RAIM PREDICTION: FAA RAIM prediction and flight planning.
- AUGUR: the Eurocontrol GPS RAIM prediction tool.