Fault detection and exclusion

An enhanced version of integrity monitoring 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 GNSS navigation to continue without interruption. The availability of 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 FDE.

Integrity monitoring 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 integrity monitoring 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.

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 integrity scheme (i.e. which meets the false alarm rate and missed detection rate requirements). The HPL is calculated as a function of the integrity monitoring threshold and the satellite geometry at the time of the measurements. The HPL is compared with the horizontal alarm limit (HAL) to determine if integrity monitoring is available.