An Optimized Multiple Hypothesis RAIM Algorithm for Vertical Guidance

Juan Blanch, Alex Ene, Todd Walter and Per Enge

Abstract:	For several years, Receiver Autonomous Integrity Monitoring (RAIM) has been used successfully for horizontal positioning in the phases of flight with Protection levels on the order of several hundreds of meters. In the next years, there will be many more navigation satellites (Galileo, a renewed GLONASS constellation, Compass), all expected to have signals in at least two frequencies. This has raised the possibility of using RAIM for much more demanding phases of flight (LPV 200 or Cat. I, for example), and perhaps ultimately replacing integrity providers such as SBAS and GBAS. However, it is not possible to apply the assumptions that are made for RAIM today in studies for vertical approach availability. Among others, the definition of failure needs to be changed: because the expected accuracy will be better, the threshold for failure will be reduced; as a consequence, the prior probability of failures could be larger than what is used now. This, together with the fact that there will be many more ranging sources makes it necessary to consider the possibility of multiple simultaneous failures. There are several RAIM algorithms treating multiple failures. However, most of them present certain disadvantages: either the calculations required to compute the Protection Levels are very complex, or the link between these Protection Levels and the Probability of Hazardously Misleading Information is problematic (often because several approximations are necessary). In this paper, we give a detailed explanation of an optimized Multiple Hypothesis Solution Separation algorithm for RAIM. There are several advantages in the Multiple Hypothesis approach. First, the link between threat model, Protection Level and PHMI is a very easy and straightforward one; second, the calculation of the Protection Level does not involve complex steps. We will show how these advantages stem mostly from the fact that the algorithm works by computing a Protection Level that meets the PHMI requirement, rather than computing a probability of misdetection. One of the key points in this algorithm is the allocation of the PHMI to the different failure modes. In previous papers, the allocation was made heuristically, in this one we will show how to compute an optimal allocation. As an example, the algorithm will be applied to a single dual frequency constellation (GPS or Galileo) and a dual constellation.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2924 - 2933
Cite this article:	Blanch, Juan, Ene, Alex, Walter, Todd, Enge, Per, "An Optimized Multiple Hypothesis RAIM Algorithm for Vertical Guidance," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2924-2933.
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