Onboard Autonomous Integrity Monitoring using Intersatellite Links

Robert Wolf

Abstract:	The integrity of the broadcast ephemeris and clock parameters plays an important role in the frame of the GNSS 2 design. In order to monitor integrity measurements have to be taken, statistically evaluated and compared to some decision thresholds. One important aspect, driving false alarm rate and misdetection (besides measurement accuracy) is the observation geometry. A way to enhance geometry is using ranging measurements between spacecrafts (intersatellite links). Although these intersatellite links could be also processed on ground, they have a great potential for autonomous onboard processing. A conceptual design for an onboard orbit estimator has been proposed and implemented. The integrity processing algorithms have been embedded in a medium complex simulation environment. To evaluate performance of the design and algorithms, several types of non-integrity cases like unintended orbit manoeuvres or excessive clock drift have been simulated. Although integrity is processed autonomously from ground, the onboard processors of each satellite are far from being independent. Induced by the intersatellite links, there is a coupling between the onboard processors of the satellites, additionally driven by the false alarm rate. In fact, as a major issue in a complete autonomous scenario it has been found that there is a threshold for the allowed simultaneous alarms, which - if exceeded – can cause the complete constellation to be "locked" at unhealthy. This paper describes the integrity algorithms, presents results of a simulation scenario consisting of a complete constellation of autonomous monitored spacecrafts and points out problems and solutions.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	1572 - 1581
Cite this article:	Wolf, Robert, "Onboard Autonomous Integrity Monitoring using Intersatellite Links," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 1572-1581.
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