Algorithms and Performance of the EGNOS CPF Independent Check Set

Wolfgang Werner, Udo Rossbach and Robert Wolf

Abstract:	One of the main objectives of the European Geostationary Navigation Overlay System (EGNOS) is the protection of the user by offering not only position accuracy but also service integrity. For this reason the Central Processing Facility (CPF) is divided into two sets: the Processing Set (PS) and the Independent Check Set (CS). While the task of the PS is to generate ionosphere as well as ephemeris and clock corrections with corresponding error bounds, the CS is in charge of verifying the correctness of this information by checking the whole set of NOF (Navigation Overlay Frame) messages independently. Two main issues have to be addressed by the CS to ensure the user’s integrity: the integrity of satellite corrections on the one hand and of the ionospheric corrections on the other hand. For each of these two types of corrections error bounds are transmitted to the user. These are the User Differential Range Error (UDRE) for satellite corrections and the Grid Ionospheric Vertical Error (GIVE) for ionospheric corrections. Two key requirements are conflicting with each other: integrity and continuity of service. Concerning the Check Set, the continuity of service corresponds to the false alarm rate and implicitly determines decision thresholds, while the integrity requirement corresponds to misdetection probabilities in given error situations. When the PS- provided error bounds are close to the true minimum error bounds, the CS false-alarm probability will increase, thus leading to a probable “don’t use” for a satellite or ionospheric correction and hence to a continuity of service problem. However, when the error bounds are conservative (thus raising integrity), the continuity of service at user level might be endangered through his protection level equations for a given phase of flight. The achievable overall performance depends on a wide variety of key parameters, like measurement accuracy, number of monitoring stations and observability of the ionosphere. This paper describes the CS algorithms that have been developed within the EGNOS project and presents some results concerning their performance figures in terms of integrity and continuity of service.
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:	219 - 229
Cite this article:	Werner, Wolfgang, Rossbach, Udo, Wolf, Robert, "Algorithms and Performance of the EGNOS CPF Independent Check Set," 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. 219-229.
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