Time Transfer Test Results Using the INMARSAT Geostationary Overlay

Alison Brown, Thomas M. Kelecy, Dick Davis, Rick Walton

Abstract:	INMARSAT has designed a GPS (Ll) transponder that will be included in their third generation satellites. This transponder will broadcast a pseudo- GPS signal that can be used for navigation and also for disseminating integrity data or differential corrections for the GPS satellites. This INMARSAT Geostationary Overlay (IGO) service will be used to enhance the performance of the GPS navigation service for civil aviation and other users. NAVSYS has developed a ground station for the IGO satellite to generate the pseudo-GPS signal that is relayed through the transponder. The ground station includes a closed-loop control mechanism that allows the IGO signal broadcast by the satellite to be precisely synchronized to an external UTC time reference. This capability allows the IGO signal to be used for disseminating precise time on a global basis. By synchronizing the IGO signal to UTC, the navigation performance of the IGO service is also improved by eliminating any degradation from GPS selective availability errors on the IGO signal. The IGO service has the capability to provide better timing accuracy than any of the existing broadcast time services. Since the IGO signal is compatible with GPS, conventional GPS timing receivers can easily be modified to utilize the proposed service. The redundant global coverage provided by the INMARSAT-III satellites will provide the capability for disseminating a precise UTC time reference worldwide. A test program is being conducted with the cooperation of the National Institute of Standards and Technology (NIST) in Boulder, CO. The IGO signal broadcast by the FAA test-bed is synchronized to UTC using an HP 5071A cesium standard. The pseudo-range to the satellite is monitored at NIST using a GPS receiver that is synchronized to a UTC primary frequency standard. Data collected at the IGO Earth Station in Southbury, CT, is processed with the monitor station data to demonstrate the timing precision possible. Test results from this experiment are included in the paper.
Published in:	Proceedings of the 1994 National Technical Meeting of The Institute of Navigation January 24 - 26, 1994 Catamaran Resort Hotel San Diego, CA
Pages:	117 - 123
Cite this article:	Brown, Alison, Kelecy, Thomas M., Davis, Dick, Walton, Rick, "Time Transfer Test Results Using the INMARSAT Geostationary Overlay," Proceedings of the 1994 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1994, pp. 117-123.
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