If a receiver autonomous integrity monitoring (RAIM) capability is lost in-flight, the pilot should take immediate action to ensure the safety of the aircraft. Depending on the situation, the pilot may need to switch to a different navigation system or use a different navigation aid to ensure the accuracy of the aircraft’s position. The pilot should also contact air traffic control to inform them of the situation and to request any additional assistance. Additionally, the pilot should ensure that the aircraft is flying in a safe and legal manner, and that all navigational aids are being used correctly.

Receiver autonomous integrity monitoring

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Receiver standalone health monitoring (RAIM) is a technology developed to assess the integrity of Global Positioning System (GPS) into a GPS receiver system. It is of particular importance in security 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 transmit slightly incorrect information which will cause the navigation information to be incorrect, but there is no way for the receiver to determine this using standard techniques. RAIM uses redundant signals to produce multiple GPS position fixes and compare them, and a statistical function determines whether or not a failure can be associated with any of the signals. RAIM is considered available if 24 or more GPS satellites are operational. If the number of GPS satellites is 23 or less, RAIM availability must be verified using approved ground-based forecasting software.

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

General description

RAIM detects failures with redundant GPS pseudorange Measurements. That is, when more satellites are available than are needed to produce a fixed position, the extra pseudoranges must all be consistent with the calculated position. A pseudorange that differs significantly from the expected value (that is, a point outside the curve) may indicate a failure of the associated satellite or another signal integrity issue (e.g. ionospheric scattering). Traditional RAIM uses fault detection (FD) only, however newer GPS receivers incorporate fault detection and deletion (FDE) which allows them to continue to operate in the presence of a GPS fault.

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 required false alarm probability (Pfa).

RAIM

Autonomous receiver health monitoring (RAIM) provides GPS health monitoring for aviation applications. In order for a GPS receiver to perform RAIM or fault detection (FD), a minimum of five visible satellites with satisfactory geometry must be visible to it. RAIM has several types 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 consistency checks fail.

RAIM availability is an important issue when using this type of algorithm in security critical applications (such as aeronautics); in fact, due to the geometry and maintenance of the satellite service, RAIM is not always available, which means that the receiver’s antenna can sometimes have fewer than five satellites in view.

Availability is also a performance indicator of the RAIM algorithm. Availability is a function of the geometry of the constellation that is in view and other environmental conditions. If availability is viewed this way, it is clear that it is not an on-off resource, which means that the algorithm may be available, but not with the necessary performance to detect a failure when it occurs. Therefore, availability is an algorithm performance factor and characterizes each of the different types of RAIM algorithms and methodologies.

Fault detection and deletion

An improved version of RAIM employed in some receivers is known as fault detection and exclusion (FDE). It uses a minimum of six measurements that can be achieved with 6 satellites or 5 baro-assisted satellites to not only detect a possible failing satellite, but to exclude it from the navigation solution so that the navigation function can continue without interruption. . The purpose of fault detection is to detect the presence of a positioning fault. Upon detection, proper fault exclusion determines and excludes the source of the fault (without necessarily identifying the individual source causing the problem), thus allowing GNSS navigation to continue without interruption. RAIM and FDE availability will be slightly lower for mid-latitude operations and slightly higher for equatorial and high-latitude regions due to the nature of the orbits. Using satellites from various GNSS constellations or using SBAS satellites as additional sources of range can improve RAIM and FDE availability.

RAIM forecast

GNSS differs from traditional navigation systems because satellites and degraded coverage areas 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 airspace, if any, will be affected. The location and duration of these interruptions can be predicted with the aid of computer analysis and reported to pilots during the pre-flight planning process. This prediction process is not, however, fully representative of all RAIM implementations across different receiver models. Prediction tools are generally conservative and therefore predict lower availability than actually encountered in flight to provide protection for the lowest end receiver models.

As RAIM operates autonomously, i.e. without the help of external signals, it requires redundant pseudorange measurements. To obtain a 3D position solution, at least four measurements are required. To detect a fault, at least 5 measurements are needed, and to isolate and exclude a fault, at least six measurements are needed, but many times more measurements are needed, depending on the geometry of the satellite. Typically, there are seven 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 to a threshold value, which is determined based on false alarm probability (Pfa) requirements and expected measurement noise. In aviation systems, Pfa is fixed at 1/15000.

The horizontal integrity limit (HIL) or horizontal protection level (HPL) is a figure representing the radius of a circle centered on the GPS position solution and guaranteed to contain the actual position of the receiver within the specifications of the RAIM scheme (ie, meeting Pfa and Pmd). The HPL is calculated as a function of the RAIM threshold and the geometry of the satellite at the time of measurements. The HPL is compared with the horizontal alarm limit (HAL) to determine if RAIM is available.

RAIM forecast sites

To allow pilots to quickly determine whether route-level or approach-level RAIM will be available, the FAA and EUROCONTROL created “dispatch-level” websites that predict RAIM status to meet pre-flight verification requirements.

  • The FAA RAIM forecast website “AC 90–100” covers US territories in graphical map format (showing green for available RAIM and red for unavailable RAIM)
  • EUROCONTROL provides international coverage for most waypoints in the worldwide aviation waypoint database and displays the results in a “timeline” bar showing predictions of whether RAIM with or without baro-assisted will be available.
    • EUROCONTROL places a notice on your data (stating that USCG data takes precedence), while the FAA certifies your site as meeting regulatory requirements.
    • As of July 1, 2012, AUGUR coverage was limited to ECAC airspace only.
  • Since 2006, the N-RAIM Prediction Service, hosted by NAVBLUE, has provided worldwide coverage for all PBN applications including RNP 10, RNAV 5, RNAV 2, RNAV 1, RNP 4, RNP 1, RNP Approach and RNP AR Approach to 0.1 NM. The online tool is an alternative to the automated service integrated directly into the flight planning software. It is kept up-to-date in accordance with new editions of the ICAO PBN Manual and any specific worldwide regulations.
  • The SPACEKEYS RAIM prediction and prediction system developed and hosted by FLIGHTKEYS offers worldwide coverage for any type of RAIM prediction and covers all integrity levels from RNP10 (Enroute) to RNP Approach and RNP AR Approach (up to 0.1NM). The online tool enables RAIM predictions for locations and complete trajectories (routes) as well as area-based RAIM predictions. REST and SOAP APIs are also available for third-party system integrations.

External Links


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