Part 2: GPS – RAIM (Receiver Autonomous Integrity Monitoring) and FDE (Fault Detection and Exclusion)

RAIM is capability of GPS receiver to perform integrity monitoring. It ensures the availability of satellite signals meet the integrity requirement for given phase of flight. This fault detection is critical for performance-based navigation(PBN). GPS control segment can experience a delay of up to two hours before an erroneous satellite transmission is detected and corrected.

FDE – Fault Detection and Exclusion can be considered as sub set of RAIM. It is capability of RAIM system to isolate corrupt satellite signal, remove it from position solution and still provide integrity assured position.

  • GPS receivers with RAIM only feature require 5 satellite
  • GPS receivers with RAIM and FDE features require 6 satellite
Selective Availability (SA)
With the use of SA GPS accuracy is intentionally degraded. This was aimed as avoiding hostile use of precise GPS positioning data. SA was discontinued on 1st May 2000.

Keynotes on RAIM:

  • RAIM outage may occur due to insufficient number of satellites or due to unsuitable satellite geometry
  • If RAIM is not available, integrity of GPS is compromised and alternate mean of navigation must be available
  • On longer flights RAIM prediction for destination should be reassured in flight to avoid last moment surprises
  • If RAIM failure occurs prior to final approach waypoint (FAWP) approach must be terminated
  • If RAIM failure occurs after FAWP, missed approach must be initiated

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