What is RAIM?

RAIM (Receiver Autonomous Integrity Monitoring) is a technology that is used in GPS receivers to assess the integrity of the GPS signals that are being received at any given time. It is particularly applicable to receivers intended for safety-critical applications and in aviation or marine navigation. RAIM has been used to support horizontal guidance for aircraft during en-route flight using GPS L1 only. However, the number of GNSS has been increasing with the full deployment of Russia’s GLONASS constellation, as well as the strengthening of Europe’s Galileo constellation. This has piqued interest in using Advanced RAIM (ARAIM), which employs redundant, dual-frequency measurements from multiple GNSS, to support vertical aircraft guidance (ARAIM has been broadly explained in another article).

From time to time, GPS satellites can broadcast slightly incorrect information that can cause 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 several GPS position fixes and then compares them to figure out if there are any inconsistencies. A statistical function determines whether or not a fault can be associated with any of the signals. RAIM utilizes redundant pseudorange (is the pseudo distance between a satellite and a navigation satellite receiver) error measurements to cross-check for individual measurements that disagree from the rest sufficiently to create a potential for unaccepted errors. It requires pseudorange measurements from a minimum of 5 satellites to detect a faulty measurement and a minimum of 6 satellites to detect and exclude a faulty pseudorange measurement.

Methodology 

RAIM can also be defined as a user algorithm that determines the accuracy of the GPS solution. The RAIM algorithm compares the smoothed pseudorange measurements among themselves to ensure that they are all consistent. If the pseudorange data in any of the signals is different from the position computed from the other signals it may indicate a fault in that satellite, such as a clock error. Alternatively, the error may be due to unexpected atmospheric conditions. By comparing the distance measurements of several satellites, the RAIM function can identify a satellite failure and issue an alert the user of the GNSS receiver. Without RAIM capability, the user has no assurance of the accuracy of the GPS position. So lets take the example of an aircraft, RAIM can warn the pilot that there might be an issue with the GNSS signal received. The pilot can then verify this information.

Fault Detection and Exclusion

• Traditional RAIM uses fault detection (FD) only, however, newer GNSS receivers incorporate fault detection and exclusion (FDE) which enables them to continue to operate in the presence of a GPS failure. 
• 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. 
• An enhanced version of RAIM employed in some receivers is known as fault detection and exclusion (FDE). It uses a minimum of six 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 GPS navigation to continue without interruption. 
• The 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.

Limitation of RAIM

The major drawback of RAIM is that it is not considered adequate for LPV-200 operations. LPV-200 (Localizer Performance with Vertical guidance) delivers accurate information on an aircraft’s approach to a runway with the use of GNSS positioning technology. The result is lateral and angular vertical guidance without the need for visual contact with the ground until an aircraft is 200 feet above the runway.

The main reasons for which conventional RAIM techniques are not considered appropriate for LPV-200 operations are:

• Design assurance level required (severe-major/hazardous for LPV-200 and only major for LNAV (azimuth/lateral navigation without vertical guidance)),
• Alert limits for LPV-200 are smaller when compared to LNAV’s,
• Multiple faults should now be considered for LPV-200,
• LPV-200 operations have more stringent accuracy requirements.

In a 2010 report, the GNSS Evolutionary Architecture Study (GEAS) panel recommended a path to achieve worldwide LPV-200 capability for air navigation.

For this reason (along with other discussed above), GEAS proposed a modified RAIM algorithm called Advanced RAIM (ARAIM) to guarantee LPV-200 operation worldwide, up to 2030 timeframe.

Click here for more information on RAIM in this published paper, entitled, GNSS receiver autonomous integrity monitoring (RAIM) performance analysis.