Problems and Solutions for GPS Use Beyond the 12-Hour Orbit

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 Orbital Transfer Vehicle (OTV) current design uses an aero-assist phase of flight where the OTV briefly enters the upper atmosphere upon return from geosynchronous orbit (GEO) to thermally dissipate excess energy, and then emerges from the atmosphere for transfer to a low earth orbit. Fuel and weight savings incurred by this approach are substantial enough to make heavy payload GEO missions feasible. The entry point into the atmosphere (for a given flight path angle) has to be precise with an altitude tolerance in the order of ±740M for OTV. This altitude entry accuracy is only achievable with the GPS level of navigation update (also requires an attitude update) at GEO prior to the GEO de-orbit burn. Since GPS has been designed for low space orbit operation, its performance for GEO operation on OTV and recoverable Centaur needs to be determined. GPS operation at GEO is a key requirement for OTV and recoverable Centaur. It is anticipated that there will be other GEO users once GEO GPS operation has been flight tested. There have been discussions of modifying the GPS satellite with a backside antenna for servicing high-altitude space users. However, this would not occur until the 2000s, too late for initial OTV usage. The Block IIR GPS satellite is scheduled to go into use in 1994 at the earliest, and it does not have a backside antenna. An orbital simulation was conducted of the OTV or recoverable Centaur and the up-to-18 GPS satellites to determine the number of GPS satellites available for navigation update on the on-board GPS processor at any instant of orbital time. Acquisition of four or more GPS satellites at the time of nav update is desirable to give a complete state definition of all GPS variables. The simulation also computes the accuracy of the nav update, if it is available, and the antenna gain requirements. Since the OTV at GEO would not be in the normal viewing orientation of GPS satellites, the simulation has to include the effects of GPS satellite transmitting antenna offset look angles. The GPS simulation pointed out the need to increase the availability of GPS satellites for OTV operation beyond GPS orbital altitude. The availability of GPS satellites is severely limited to three or fewer at GEO with a standard GPS receiving antenna. The GPS satellite availability is strongly influenced by the right ascension and orbital timing relationships between the GPS satellites and the user vehicle. Eight different technological approaches were evaluated for improving availability of GPS satellites at GEO. The most promising of these were analyzed and simulated for greater depth of analysis. The performance improvement results and implementation are presented.
Published in:	Proceedings of the 1988 National Technical Meeting of The Institute of Navigation January 26 - 29, 1988 Santa Barbara, CA
Pages:	187 - 193
Cite this article:	Maki, Stanley C., "Problems and Solutions for GPS Use Beyond the 12-Hour Orbit," Proceedings of the 1988 National Technical Meeting of The Institute of Navigation, Santa Barbara, CA, January 1988, pp. 187-193.
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