Orbit and Ranging Analysis of the INMARSAT AOR-West Geostationary Satellite

Thomas M. Kelecy, Alison Brown, Willy Bertiger and Steven M. Lichten

Abstract:	INMARSAT has designed a GPS ILlI transponder that will be carried on their third generation satellites. This transponder will broadcast a pseudo- GPS signal that can be used for navigation and timing, and also for disseminating integrity data or differential corrections for the GPS satellites. Use of this INMARSAT Geostationary Overlay (IGO) broadcast service will require accurate knowledge of the satellite positions. Orbit accuracies on the order of 1 m or less are achievable, and would support nanosecond level precise time transfer and dissemination over the INMARSAT area of coverage. NAVSYS has built a ground station test-bed to generate the pseudo-GPS signal that is relayed via the IGO satellite transponder. The ground station includes a closed-loop control mechanism that precisely synchronizes the IGO broadcast signal to an external UTC time reference. This provides the capability for using the IGO signal to disseminate precise time on a global basis. Synchronizing the IGO signal to UTC and providing GPS differential corrections also improves the performance of the IGO service for aircraft navigation, as this technique eliminates signal degradation due to GPS selective availability. The INMARSAT-2 AOR-West geostationary satellite, which is presently stationed over the Western Hemisphere, carries a transponder that was used for testing the IGO. Data were collected at two ground monitoring stations during system tests to measure the L-band signal transmitted by the satellite transponder. Specially modified receivers were set up at the National Institute of Standards and Technology (NISTI in Boulder, CO, and at the COMSAT Earth Station (ESTA) in Southbury, CT, to measure pseudo-range and carrier phase to the satellite synchronized to UTC (NIST) time. A preliminary orbit adjustment based on the pseudo-range measurements demonstrates an orbit accuracy which is presently at the level of 5 m radial, 80 m cross-track, and 25 m along-track. With appropriate monitoring station distribution, group and atmospheric delay calibrations, and use of carrier phase measurements, orbit accuracies at the meter level or better are anticipated.
Published in:	Proceedings of the 50th Annual Meeting of The Institute of Navigation (1994) June 6 - 8, 1994 Antlers Doubletree Hotel Colorado Springs, CO
Pages:	259 - 266
Cite this article:	Kelecy, Thomas M., Brown, Alison, Bertiger, Willy, Lichten, Steven M., "Orbit and Ranging Analysis of the INMARSAT AOR-West Geostationary Satellite," Proceedings of the 50th Annual Meeting of The Institute of Navigation (1994), Colorado Springs, CO, June 1994, pp. 259-266.
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