Galileo-GPS RAIM for Vertical Guidance

A. Ene, J. Blanch, and T. Walter

Abstract:	In the next ten years the number of pseudorange sources and their quality is expected to increase dramatically: The United States is going to add two new civil frequencies (L5 and L2C) in the modernized GPS, and the European Union is planning to launch Galileo, which is planned to be fully operative before 2015, also with multiple frequencies. By combining two frequencies, users will be able to remove the ionospheric delay which is currently the largest error, thus reducing nominal error bounds by more than 50%. This reduction in nominal error bounds together with the large number of satellites is not only going to increase the accuracy of the positioning, but more importantly, it is going to increase the robustness against satellite failures (or other range errors), even without augmentation (e.g., Inertial Reference Unit (IRU), baro-altimeter). Preliminary studies suggest that, using Receiver Autonomous Integrity Monitoring (RAIM), it might be possible to provide a 50m Vertical Alert Limit (VAL) worldwide, with a bound on the maximum error, even in the event of one satellite failure, one constellation failure or a multiple satellite failure. The purpose of this work is to investigate which VALs could be achieved with RAIM under conservative failure assumptions. This paper also summarizes previous work concerning RAIM algorithms and compares their results against a common standard. First, in light of the experience with the Wide Area Augmentation System (WAAS), a threat space for a dual frequency Galileo-GPS constellation is defined. This threat space is necessary in order to achieve a low VAL, as it does not suffice to assume single failures only. Second, RAIM methodologies adapted to the threat space are compared, and the most practical one was found to be a multiple hypothesis approach. Finally, the performance results of the chosen RAIM scheme with a Galileo-GPS dual frequency constellation are presented. It was found that an unaided Galileo-GPS constellation yielded Vertical Protection Level (VPL) values under 20m for the combined dual system. This optimistic conclusion indicates that it will likely be possible to provide vertical guidance to aircraft without the need for any additional augmentation when the future GPS and Galileo constellations are operational.
Published in:	Proceedings of the 2006 National Technical Meeting of The Institute of Navigation January 18 - 20, 2006 Hyatt Regency Hotel Monterey, CA
Pages:	432 - 440
Cite this article:	Ene, A., Blanch, J., Walter, T., "Galileo-GPS RAIM for Vertical Guidance," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 432-440.
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