Satellite Integrity Monitoring Concepts for GPS/Galileo Augmentation Systems

Sam Pullen and Per Enge

Abstract:	The future fielding of the Galileo satellite constellation will, when combined with the existing GPS constellation, provide users with more than 50 navigation satellites, of which 10 or more will almost always be in view for users with a good view of the sky. The key question studied in this paper is the optimal way to exploit the additional redundancy provided by a combined GPS / Galileo constellation in future Ground Based Augmentation Systems (GBAS) and Space Based Augmentation Systems (SBAS). Our hope is that GPS and Galileo, when combined with other enhancements such as dual-frequency reference and user receivers, will allow GBAS and SBAS to provide “end-state” availabilities of 0.99999 or greater for all categories of precision approach. Simulations of combined GPS/Galileo satellite constellations (with 27 GPS and 27 Galileo satellites) show that, when two of the 54 combined GPS and Galileo satellites are known to be unhealthy, a minimum of 11 satellites are in view of a user with a 5-degree elevation mask angle, and a minimum of 10 satellites are in view of a user with a 10-degree elevation mask angle (higher mask angles are of interest because the help mitigate multipath and RF interference coming from the ground). Using standard error models for single-frequency GBAS, the availability of CAT III precision approach under these conditions is always 1.0 (no unavailable geometries are present) as long as the required vertical alert limit (VAL) is 5.3 meters or greater. In addition, no critical satellites exist when the VAL is 5.3 meters or greater and the mask angle is 5 degrees (this also applies for higher mask angles as the VAL increases). Where it applies, the complete absence of critical satellites may be of significant value for “end state” GBAS. It means that no loss of a single satellite could cause a loss of continuity (by increasing the vertical protection level or VPL above the VAL). This, in turn, means that GBAS satellite exclusions can be much more frequent without impacting continuity – faulty satellites can be detected earlier and with lower fault magnitudes. This is a significant aid to GBAS ground system integrity monitoring, and its implications for the design of GBAS are laid out in this paper.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	1674 - 1682
Cite this article:	Pullen, Sam, Enge, Per, "Satellite Integrity Monitoring Concepts for GPS/Galileo Augmentation Systems," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 1674-1682.
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