GPS Constellation Transition to Three Planes And Drifting Ground Tracks

J.H. Drake

Abstract:	The GPS constellation evolved initially from 18 satellites where 6 planes were optimal, but has since grown to 27 satellites where 3 planes are shown to have several advantages. Orbits with drifting ground tracks have also been suggested to virtually eliminate need for periodic adjustment of orbits to correct for earth’s gravitational perturbations. Orbit adjustments require removing affected satellites from service for several hours. Both constellation transitions are relatively difficult while maintaining operational GPS service without significant degradation or increasing constellation size and cost. A zero cost plan is proposed meeting both transition objectives. It begins with minor satellite deployment changes in 6 planes, which significantly improves near term maximum global position dilution of precision performance. Improvements compared to the present constellation provide necessary robustness to effect transition. The enabling constellation has alternating planes with 6 and 3 satellites each. Resulting locations of satellites permit a subsequent 1 for 1 replenishment strategy for depopulating planes initially with 3 satellites while populating planes initially with 6 satellites. The strategy ensures good performance for all possible states resulting from any failure and replenishment order for launch on need with constellation size fixed at 27. After achieving three-plane transition, relocations of satellites in these planes are still needed for optimal performance. We show this is easily accomplished while changing constellation altitude to achieve drifting ground tracks. Constellation insensitivity to phasing of planes and minimal performance degradation when selected sets of 3 satellites in a single plane are temporarily out of service enable this process. Degradation is comparable to one satellite out during re-phasing for station keeping. These attributes of 3-plane transition permit a 3 satellite at a time, one plane at a time re-phasing plus drift initiation process. The resulting constellation has superior performance and availability attributes sufficient to suggest the lengthy transition is worthwhile. Three plane constellations generally provide improved availability of vertical performance needed for aviation CAT 1 precision approach and achieve good performance at higher elevation mask angles.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	494 - 503
Cite this article:	Drake, J.H., "GPS Constellation Transition to Three Planes And Drifting Ground Tracks," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 494-503.
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