Optimization of a GPS User Experiment to GEO

Stanley C. Maki

Abstract:	The NAVSTAR Global Positioning System (GPS) was originally designed for us­ers Located on the earth, for aircraft and other atmospheric flight users, and for Low-orbit space users. The Space Transfer Vehicle (SIV) and recoverable Centaur current designs utilize an aeroassist phase of flight where the vehicle briefly enters the upper atmosphere upon return from geosynchronous or Mars/lunar return orbits to thermally dissipate excess energy, repeats entry from two intermediate orbits, and then finally emerges from the atmosphere for transfer to a low Earth orbit. Fuel and weight savings resulting from this approach are substantial enough to make heavy-payload, high-orbit missions feasible. The entry point into the atmosphere for a given flight-path angle must be precise, with an altitude tolerance in the order of± 280M for a flexible aero brake. This altitude entry accuracy is achievable economically with the GPS level of navigation update, which also requires an attitude update, at or beyond geosynchronous Earth orbit (GEO) prior to the GEO deorbit or correction burn. General Dynamics Space Systems Division conducted an orbital simulation of the SIV or recoverable Centaur and the up-to-18 GPS satellites to determine the number of GPS satellites available for navigation update to the on-board GPS processor at any instant of orbital time. This GPS simulation pointed out the need for increasing the availability of GPS satellites for STV operation beyond GPS orbital altitude. The availability of GPS satellites is severely limited to three or less at GEO with a standard GPS receiving antenna. Eight different technological approaches were evaluated for improving the availability of G PS satellites at GEO, with three approaches receiving additional considerations. The three most promising approaches are filter optimization, high-gain antenna systems, and Earth-based GPS transmitters. Filter optimization analysis has two aspects: the improvement of the receiver filter loops and tracking; and the modification of the Kalman GPSJINS filters for one-, two·, three-, or four satellite operation. Performance and operational considerations were assessed. In this paper, in order to verify the performance of GPS at all orbital altitudes up to and beyond GEO, a GPS avionics flight experiment incorporating the selected GPS improvements is described. An operational STV or recoverable Centaur avionics and GPS user
Published in:	Proceedings of the International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1988) September 19 - 23, 1988 The Broadmoor Hotel Colorado Spring, CO
Pages:	203 - 210
Cite this article:	Maki, Stanley C., "Optimization of a GPS User Experiment to GEO," Proceedings of the International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1988), Colorado Spring, CO, September 1988, pp. 203-210.
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