EVT-SIAM: A Tool Based on Extreme-Value Theory for the Assessment of SBAS Accuracy and Integrity

Guillaume Buscarlet, Jean-Marc Azaïs, Sébastien Gadat, Norbert Suard

Abstract:	Following the start of WAAS extensions in Canada, in Alaska and in Mexico, as well as the start of WAAS system evolutions between 2005-2008, and considering the noticed improvements of performances and procedures, the FAA suggested widening the WAAS services up to LPV200 (aka. category I approaches), with a vertical alert limit (VAL) of 35m. This objective has also been assigned to EGNOS in Europe, and ICAO Annex 10, Volume 1 has been amended in that direction (Amendment #85). Though most of the specifications of LPV200 are identical to those for APV1 service level (where VAL = 50m) in terms of integrity, horizontal alert limit, availability and continuity, different or additional specifications have been introduced in the field of positioning accuracy. Indeed, the specification of APV1 accuracy – that the 95th centile of the horizontal error (H-NSE) be below 16 m, and the 95th centile of the vertical error (V-NSE) be below 20 m – becomes for LPV200 in the vertical domain that - V-NSE_95 % < 4m - Proba(VPE_1sec > 10m) < 1E-7 in the absence of failure everywhere the operation is to be approved; - Proba(VPE_1sec > 15m) < 1E-5 in case of failure. The specification of APV1 and LPV200 integrity is Proba(VPE > VAL or HPE > HAL) < 2E-7 per 150 s where VAL (resp. HAL) the vertical (resp. horizontal) alert limits values are fixed by the International Civil Aviation Organisation for different flight phases. The failures to be taken into account are the ones that affect the used basic constellations and GNSS augmentation systems. This latter probability results from an allocation that takes into account the probability that a given failure occurs and of the probability of detection. Verifying such low probabilities with standard statistical methods require several months or years of observation data, even with a 1-second accuracy sampling. This is not realistic and compatible with industrial constraints that require system qualification time scales. For instance, such rare “events” are generally not observed within the data used to qualify a SBAS release because of their scarcity.
Published in:	Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012) September 17 - 21, 2012 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3488 - 3495
Cite this article:	Buscarlet, Guillaume, Azaïs, Jean-Marc, Gadat, Sébastien, Suard, Norbert, "EVT-SIAM: A Tool Based on Extreme-Value Theory for the Assessment of SBAS Accuracy and Integrity," Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 3488-3495.
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