Weighted RAIM for APV: An Optimised Protection Level

C.D. Milner

Abstract:	International standards (ICAO SARPS, 2006) require the use of a weighted least-squares approach to on-board Receiver Autonomous Integrity Monitoring (RAIM). However, the protection levels which have been developed to determine if the conditions exist to perform a measurement check, are not specified. Various protection levels exist, the most current being those of Brown and Chin (1998) and Walter and Enge (1995). These protection levels are essentially approximations to the complex problem of computing the worst case missed detection probability under a weighted system. In this paper, a novel approach to determining this probability at the worst case measurement bias is presented. The missed detection probabilities are then iteratively solved against the integrity risk requirement in order to derive an optimal protection level for the operation. At the end of the paper, the benefits of this technique are shown for Vertical Alert Limits (VAL) of 50m and 20m which relate to Approaches with Vertical Guidance APV-I and APV-II approaches, and it is shown that availability is significantly improved (˜30%) whilst staying safe. Given that the computations are exact, it is unnecessary to add an additional safety buffer to this ideal protection level as is done in other solutions (Hwang and Brown, 2005). The results presented here are in comparison to the baseline weighted RAIM (WRAIM) (Walter and Enge, 1995) and do not optimise integrity at the cost of accuracy. Hence, the potential for combining the technique with recent advances such as NIORAIM exists and may further alleviate the unnecessary dips in RAIM coverage for certain applications.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	1645 - 1651
Cite this article:	Milner, C.D., "Weighted RAIM for APV: An Optimised Protection Level," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 1645-1651.
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