GPS Ephemeris Verification for Local Area Augmentation System (LAAS) Ground Stations

摘要

Certain types of satellite failures, such as broadcastrnephemeris messages that do not correspond to truernsatellite locations, can be difficult to detect by a LocalrnArea Augmentation System (LAAS) ground facility (LGF)rnbecause satellites are observed from reference antennasrnthat are very close together. Although ephemeris failuresrnlarge enough to threaten LAAS user integrity should bernvery rare, a combination of monitors in the LGF isrnneeded. This paper analyzes the combined effectivenessrnof several monitoring techniques and proposes solutionsrnfor all phases of LAAS operations.rnThe first solution is the combination of the GPSrnOperational Control Segment (OCS) and monitorsrnincluded in the existing Category I LGF prototypes.rnBuilt-in LGF checks confirm that broadcast range andrnrange-rate corrections have reasonable values and alsorncompare computed satellite positions based on the currentrnand previous ephemeris messages as well as the currentrnalmanac message. OCS monitoring by itself is insufficientrnfor LAAS because OCS alerts are not guaran-teed to meetrnthe minimum time-to-alert requirement of 6 seconds forrnprecision approaches. However, the combi-nation of OCSrnand existing LGF monitoring is sufficient to meet thernallocated per-approach probability of an undetectedrnephemeris error for Category I operations.rnFor Category II and III operations, the integrityrnrequirement is 200 times tighter than for Category I; thusrnthe monitors described above may be insufficient. Thisrnpaper introduces new algorithms for validatingrnephemerides in three dimensions that can provide thernneeded improvement. The Differential PseudorangernResidual method (DPR) is used to detect ephemeris errorsrnparallel to the LGF – satellite line of sight, and thernDouble Phase Difference with Ambiguity Search methodrn(DPDAS) is used to detect ephemeris errors perpendicularrnto the line of sight (and thus very difficult to observe).rnUnlike RAIM, these two methods need only one otherrnalready-approved GPS satellite in view. These two methods can detect all ephemeris failures in threerndimensions. This paper develops these algorithms inrndetail and presents experimental results from the StanfordrnLAAS Integrity Monitor Testbed (IMT).