RAIM

RAIM stands for Receiver Autonomous Integrity Monitoring, a technique used in GNSS receivers to assess the integrity of navigation solutions without external monitors. The goal is to detect and exclude faulty satellite signals to ensure safe and reliable positioning, particularly in safety-critical operations such as aviation.

Conceptually, RAIM relies on redundant satellite measurements. A GNSS receiver needs at least four pseudorange observations to solve for the receiver’s X, Y, Z coordinates and its clock bias; with more satellites there is redundancy that can be used for integrity checks. RAIM analyzes the residuals of a least-squares position solution to identify outliers. If a satellite’s residual exceeds a threshold, that satellite is suspected of fault. Depending on the method, the algorithm may perform forward RAIM, backward RAIM, or a combination to confirm the fault and exclude the satellite. This process is often called fault detection and exclusion (FDE). Variants such as weighted RAIM (wRAIM) weight measurements to improve sensitivity, and multi-constellation RAIM uses signals from GPS, GLONASS, Galileo, and others. Some systems integrate RAIM with augmentation networks like SBAS to provide higher integrity guarantees.

Limitations and considerations include the need for adequate satellite visibility; in urban canyons, canyons, or at high latitudes, there may not be enough satellites to perform a reliable integrity check. RAIM may fail to detect multiple simultaneous faults or be affected by large measurement noise. Its integrity assurances are often complemented by augmentation systems or external integrity monitoring.

Applications span civil aviation for IFR navigation and approach procedures, including LPV, as well as other fields where GNSS integrity is critical, such as timing, surveying, and autonomous systems.