Performance Of A Low Earth Orbit Navigation System: Error Model, Impact Of Doppler Measurements, Comparison With GPS And Application To Civil Aviation

Bruno Lobert, Christophe Bourga, Stéphane Journo

Abstract:	A new navigation satellite concept, INES (Innovative Navigation European System), is currently being proposed to prepare the transition between GNSS-1 and GNSS-2. This system is based on a Low Earth Orbit (LEO) constellation (around 1500 km altitude), completed by geosynchronous satellites of GNSS-1 (e.g. EGNOS). There are many applications for such a system: civil aviation, geodesy, geophysics¼ To reach a performance of positioning similar to that of GPS, it is worthwhile to take advantage of all characteristics of the constellation. The signals emitted by LEO satellites have large Doppler shifts. This may be an advantage if the pseudo-range measurements and the pseudo-velocity measurements (Doppler shifts converted to velocities) are combined to compute the position instead of being separately processed. The performance of this processing can only be estimated through a good knowledge of the error budget for pseudo-ranges and pseudo-velocities. Therefore, a precise error model has been developed to characterize the statistics of each error term in function of the satellite-receiver geometry. The navigation algorithm is then described: it combines both raw measurements to estimate the user position and velocity, and calculates the associated precision. Numerical and graphical results are presented: they show that accuracy is enhanced when compared to positioning with pseudo-ranges only. The improvement is the strongest: 1) when the satellite geometry is poor; 2) for vertical accuracy, which is the critical point for applications in general aviation. The same algorithm has been applied to GPS data (without Selective Availability). Results show that for Medium Earth Orbit constellations, the impact of Doppler is negligible. Finally, raw INES measurements for a receiver located on a landing aircraft are simulated. A Kalman filtering is applied to the estimated position and velocity. The results show that significant improvement may be reached thanks to precise estimation of user’s velocity.Numerical and graphical results are presented: they show that accuracy is enhanced when compared to positioning with pseudo-ranges only. The improvement is the strongest: 1) when the satellite geometry is poor; 2) for vertical accuracy, which is the critical point for applications in general aviation. The same algorithm has been applied to GPS data (without Selective Availability). Results show that for Medium Earth Orbit constellations, the impact of Doppler is negligible. Finally, raw INES measurements for a receiver located on a landing aircraft are simulated. A Kalman filtering is applied to the estimated position and velocity. The results show that significant improvement may be reached thanks to precise estimation of user’s velocity.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	2031 - 2041
Cite this article:	Lobert, Bruno, Bourga, Christophe, Journo, Stéphane, "Performance Of A Low Earth Orbit Navigation System: Error Model, Impact Of Doppler Measurements, Comparison With GPS And Application To Civil Aviation," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 2031-2041.
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