Geostationary Complement to GPS: Problem Analysis and Main Results

D.Flament, J.M. Durand

Abstract:	Within few years, the Global Positioning Systm (GPS) seems to offer a very high perfonmnce navigation service so as to be viewed by the Civil Aviation as a possible sole means air navigation systm for n-cost aeronautical flight phases. fkwaer,GPS keeps three disadvantages which don't fit with this prospect: lack of integrity, limited availability and a deliberate spatio- temporal degradation ("Selective Availability") of the perfonrances. Various solutions to these problem have yet been put forwamd and med. The mst hopeful approach appears to be the concept of an autoncamus external system using one or several geostationary satellites. First, each of these satellites would broadcast an indication of the health of the GPS constellation in order to ensureabetterintegrity. Second, itwouldhave the sane function as a classical GPS satellite and then *mvetheGPS availability. Our purpose in this ammnication is to analyse the problems involved by the use of geostationaq satel- lites as Navstars. Within the context of this external system, for cost and simplicity reasons, the ultrastable clock stays on the gmund. Consequently, it is necessary to perform scma ground-segmsnt operations which will be presented here and which allcw: 1. A synchmnisation between the Geostationary massage and the GPS reference time. A good knowlsdge of the GPS time will be obtained.with the help of a pemanent observation of theGPS contellation by the ground network. The precision of this knowledge will bs in-hezntly dependent on the guality of the GPS signals (affected by the S.A. or not). 2. A sanra massage datation principle as GPS. It means that eventually, the geostationary massage must ciyz to the user as if it was emitted from the satel- Thereafter, the ground-segment must elaborate Geostationaq's navigation data whose content and ac- curacy agree with the GPS massage structure and will ensure the user a same positioning guality using a sama oanputation. At first, the geostationaq orbit has to be detemined as precisely as the Navstar orbit is by GPS. The hypothesis and the most significant results of a study conducted by simulation on this subject will bs given here. A special attention was focused on the measuresant error n-cdel in order to satisfy with the error budget introducxd by the specific operations mentioned before. Mo?xover, the Geostationary dynamical rrrxlel error levels have been previously tested by fitting tracking real data of the geostationary satellite TDRSl. The orbit determina tion accuracy finally achievexi is consistent with the GPS specifications and little degraded bySelectiveAvai.lability. At last, the use of the GPS message structure to transmit the Gsostationary navigation data has been studied. The procedm employed and the User Range Fxror it produces, are the final concerns of this -Communication.
Published in:	Proceedings of the 3rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1990) September 19 - 21, 1990 The Broadmoor Hotel Colorado Spring, CO
Pages:	297 - 303
Cite this article:	D.Flament,, Durand, J.M., "Geostationary Complement to GPS: Problem Analysis and Main Results," Proceedings of the 3rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1990), Colorado Spring, CO, September 1990, pp. 297-303.
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