ARAIM for LPV-200: The Ideal Protection Level

C. Milner, W. Ochieng

Abstract:	Receiver Autonomous Integrity Monitoring (RAIM) has been used successfully for horizontal phases of flight up to Non-Precision Approach. However, the use of single frequency measurements from the current GPS constellation is unable to meet the stringent requirements for vertical guidance. In the coming years, the expected availability of protected multiple frequency signals for civil mission critical applications and increase in the number of satellites available as a result of modernized and new systems (GALILEO, GLONASS, Compass), have raised the possibility of using RAIM for more demanding phases of flight. LPV-200 operations are currently the focus of this endeavor within the U.S under the GNSS Evolutionary Architecture Study and within Europe. Three architectures have been identified as having the potential to meet LPV-200 on a global scale, a GNSS Integrity Channel, Relative RAIM and Absolute RAIM (ARAIM). This paper focuses on using the concept of an ideal Protection Level (PL) developed by the authors of this abstract (Milner and Ochieng, 2009) within the ARAIM framework (Blanch et al, 2007; Lee and McLaughlin, 2007). The ideal PL has been developed as previous protection levels employed within slope based RAIM have either been overly conservative in their approximation or fail to ensure sufficient integrity in all cases. The ideal PL uses a two step iterative search for the protection level and worst case bias which match the integrity risk requirement exactly. The approach is rigorous but includes a number of checks to accelerate the processing and provide sufficient efficiency to be a plausible solution both in an offline and real-time implementation. Previous processing algorithms for the ideal PL have focused on slope based RAIM which uses a chi-squared test statistic for a single failure hypothesis. The proposed algorithms for ARAIM are based on the historical Solution Separation method applied to a Multiple Hypothesis formulation (MHSS). The recent definition of failure in the GPS SPS Performance Standard on a per satellite basis allows the optimization of the MHSS by apportioning the integrity risk freely between modes. In this paper, the link between the MHSS technique and traditional slope based RAIM is strengthened and the ideal PL developed for slope based RAIM is applied within the ARAIM format and threat model. The ideal PLs computed are approximately 10-20% lower than the protection levels for MHSS when nominal biases are overbounded whilst also satisfying the necessary proofs of safety and service reliability. When the worst case nominal biases on all satellites are considered, the performance matches that of existing ARAIM as the worst case directions of biases must be chosen at all stages. However, proposals on how this additional conservative assumption may be avoided in the presented algorithm are considered with the potential for lower protection levels than those currently computed under ARAIM. The use of the ideal PL in place of existing methods enables a higher proportion of samples to be available. This has the potential to relax the strict ARAIM requirements on the constellation size needed to meet LPV-200 and more stringent applications under a multi-constellation environment.
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	3191 - 3198
Cite this article:	Milner, C., Ochieng, W., "ARAIM for LPV-200: The Ideal Protection Level," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 3191-3198.
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