GPS and Galileo - Interoperability for Civil Aviation Applications

Peter Fyfe, Ken Davis, Isaac Jeng, Clifford Kelley and Charles Mosley

Abstract:	GPS provides positioning and timing services worldwide for a wide variety of applications, and for the majority of them the present GPS performance is more than sufficient. The Galileo system currently being defined will provide a universally available service with similar performance to GPS plus different capabilities to satisfy more demanding applications such as civil aviation. Availability of the two systems provides the opportunity to enhance user navigation system key aspects of civil aviation applications as well as develop new ones. Both systems will operate simultaneously, and so it is paramount that they do not interfere with each other. This paper reviews and evaluates the top-level constraints for civil aviation compatibility and interoperability. Constellation performance analyses are performed to determine the performance improvements for the combined constellations over individual constellation performance. The analyses evaluate the availability of accuracy and integrity (in terms of attainable protection limits using fault detection and exclusion [FDE]) for complete constellations and satellite outage scenarios for phases of flight from oceanic to Category I precision approach (PA). This paper also takes a new approach to evaluating signal interference of the Galileo signals on the GPS signal set. Previous interference computations have been carried out to determine the signal to noise ratio degradation of the GPS signal due to an overlay of Galileo signals. These computations have evaluated the equivalent carrier-to- noise ratio as the result of the Galileo signal addition compared to the carrier to noise prior to the addition. The carrier to noise ratio, C/N0, degradation is treated as the basis of a performance criterion, which implies that pseudo-range and range rate measurements will be subject to an equivalent C/N0 performance degradation. While the resulting expression yields an effective carrier to noise density ratio, the interference spectrum of the proposed Galileo signal is not white. Thus, although the spectral density level is calculated as the inner product of the interference power spectral density and the power spectral density of the desired signal, the results are presumably defined in a manner in which the same level of white noise would degrade performance by the same degree. Since the interfering signals are in fact non-white, it may be expected that the actual degradation will be different from that previously computed using this method. It has been shown that code tracking accuracy degradation is in fact not only a function of the receiver’s front-end bandwidth and early-late spacing but also of that of the specific spectrum shape of the interfering signal [1]. This paper evaluates via analysis the effect of the proposed Galileo signal on time of arrival (TOA) estimation and on the code-tracking loop using the relationships developed in [1].
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	289 - 302
Cite this article:	Fyfe, Peter, Davis, Ken, Jeng, Isaac, Kelley, Clifford, Mosley, Charles, "GPS and Galileo - Interoperability for Civil Aviation Applications," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 289-302.
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