Further Development of Galileo-GPS RAIM for Vertical Guidance

A. Ene

Abstract:	With the much anticipated deployment of Galileo, a new partner will rise on the sky of Global Navigation Satellite Systems (GNSS). Equally anticipated is the launch of the modernized block III GPS satellites, which will provide numerous enhancements to the existing system. It is expected that both Galileo and the modernized GPS will become fully operational within the next 10 years. As a consequence, efforts have been initiated at the global level in order to contrive ways in which to gain the full benefits of having two independent multifrequency systems available to the user. One of the hard problems that stand-alone GPS has been trying to address over the years is that of measurement integrity. Providing guidance during the landing approach phase of aircraft flight is one of the most challenging applications for satellite-based navigation because both high accuracy and user safety are required during the procedure. By combining two frequencies, users will be able to remove the ionospheric delay, which is currently the largest error, and thus increase the positioning accuracy by more than 50%. This reduction in nominal error bounds together with the presence of a larger number of satellites is going to increase the robustness against satellite failures or hazardous pseudorange errors. Previous studies [Ene et al. 2006] suggest that, using Receiver Autonomous Integrity Monitoring (RAIM), it might be possible to provide a 35m Vertical Alert Limit (VAL) worldwide, with a bound on the maximum error and without the need for additional augmentation, 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 Vertical Protection Level (VPL) values could be achieved with RAIM under conservative failure assumptions. Both the RAIM algorithm and the corresponding threat model presented in the previous paper [Ene et al. 2006] have been revised. The previously defined threat model is refined to include measurement biases, and the study on degraded operation modes is also extended to include partial GPS and Galileo constellations and to appreciate the impact of critical satellites. It was found that an unaided dual Galileo-GPS constellation yielded VPL values under 20m for nominal operation conditions, and that moderate biases or degenerate constellations can increase the VPL up to around 35m. These protection levels will likely enable APV-II landings at all runway ends in the world without the need for a SBAS or GBAS.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	2597 - 2607
Cite this article:	Ene, A., "Further Development of Galileo-GPS RAIM for Vertical Guidance," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 2597-2607.
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