Time Calibration Using the INMARSAT Geostationary Overlay Signal From the AOR-W Satellite

James LaMance, Alison Brown, Bruce Haines, Willy Bertiger, and Sien Wu

Abstract:	INMARSAT has designed a GPS (Ll) 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. NAVSY S 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, synchronized to UTC (NIST) time, were set up at the National Institute of Standards and Technology (NIST) in Boulder, CO, and at the COMSAT Earth Station (ESTA) in Southbury, CT, to measure pseudo-range and carrier phase to the satellite. A preliminary orbit adjustment based on the pseudo-range measurements demonstrates an orbit accuracy which is presently at the level of 4 m radial, 9 m cross-track, and 9 m along-track. With appropriate monitoring station distribution, group and atmospheric delay calibrations, and use of carrier phase measurements, orbit accuracies at the few meter level is anticipated, supporting time transfer at the few nano-second level.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	1175 - 1181
Cite this article:	LaMance, James, Brown, Alison, Haines, Bruce, Bertiger, Willy, Wu, Sien, "Time Calibration Using the INMARSAT Geostationary Overlay Signal From the AOR-W Satellite," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 1175-1181.
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