Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Inter Satellite LinksSoftware Description5.6 Autonomous Integrity MonitoringAnother feature of the software package is the simulation of an onboard processor, usingonboard measurements from ground links and inter satellite links to evaluate the integrity ofits ephemeris and clock states. One investigated approach utilises a RAIM (ReceiverAutonomous Integrity Monitoring) algorithm. RAIM algorithms are well known in the GPSuser (receiver) domain. They basically work on the sample variance of the observationresiduals, as well as on the observation matrix, containing the unit vectors of the line-of-sightsand measurement variances. The following figure shows the flow chart of the integritymonitoring.Propagator=> Own PositionGround LinkLOS Unit Vectors,PredictedMeasurementsRAIM AlgorithmISLData Base=> Station PositionPropagator=> Target SatelliteNoAvailable ?Non Integrity CaseNoClock / EphemerisYesNo Integrity OKFaultYesFaultYesSingle Ground Link / ISLFigure 5-8 Integrity Processing CheckMost RAIM algorithms check the sample variance against a protection level, computed fromthe observation matrix. If the protection level is exceeded, one measurement after the other isremoved, and the check is repeated with n-1 observations. Therefore a faulty measurementcan be isolated, if there is enough redundancy in the measurements.The mathematical formulation of the fault detection (FD) / and isolation (FDI) problem forsatellite based integrity monitoring is generally given as follows. If it is assumed that no moreR. Wolf Page 89