Integrity Monitoring
The integrity of a navigation system is a measure of its ability to provide a timely warning when the system fails to meet its stated accuracy and should no longer be used. The GPS systems itself can not provide integrity monitoring which satisfies aviation requirements. Integrity monitoring within certified aviation receivers is currently provided by Receiver Autonomous Integrity Monitoring (RAIM). RAIM is a method which examines the internal consistency of a set of redundant measurements, within the GPS receiver, to detect, and perhaps remove, a faulty measurement. In the case of GPS, this amounts to detecting, and perhaps removing, a faulty satellite from the set used navigation. When GPS is used as a supplemental navigation system, fault detection is all that is required. When GPS is used as a primary means of navigation, both fault detection and fault exclusion (FDE) is required.
Integrity work at the Volpe Center is done in conjunction with RTCA Special Committee 159 and is sponsored by the FAA and by other organizations. The Volpe Center has performed integrity coverage and availability studies for all phases of flight from oceanic through precision approach. Our most recent work includes the use of differential corrections, as with the FAA Wide Area Augmentation System (WAAS). It has been extended to include the case where multiple measurement failures are possible. The Volpe Center also has evaluated integrity and continuity for the FAA Local Area Augmentation System (LAAS).
For more information contact John Kraemer at kraemer@volpe.dot.gov.
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