Dual Tone Signaling to Mitigate Self-Skywave Interference to DGPS Broadcasts from Radiobeacons

David Young and Bruce Butler

Abstract:	A medium frequency (MF) radio system is being deployed worldwide for the broadcast of differential corrections to users of the Global Positioning System (GPS). This network of DGPS/radiobeacom is expanding rapidly, because the radiobeacons are a cost effective way to broadcast corrections over relatively large areas to mobile marine users. To maximize the benefit to cost ratio, the coverage area of an individual DGPS/radiobeacon is generally maximized by radiating the greatest power commensurate with co-channel interference considerations. Typically, DGPS/radiobeacons radiate a few Watts and provide coverage for hundreds of kilometers over seawater. At these ranges, the differential corrections are still valid and valuable. However at ranges over 100 kilometers, the radiobeacon signal may reflect of the ionosphere (skywave) and destructively interfer with the desired signal which follows the surface of the earth (groundwave). This paper shows that only two frequencies, separated by 2500 Hz, are needed to obviate this problematic fading of the desired groundwave signal by the skywave signal. The paper presents field measurements at ranges of 100 and 220 kilometers from the transmitter. All results strongly support the theory described herein. These results indicate that self-skywave interference is not a meaningful limitation to DGPS/radiobeacon coverage (and/or availability) in those cases where spectrum for two signals is available. Such spectrum should become increasingly available since direction finding beacons are being decommissioned and larger DGPS/radiobeacons are preferred, For these reasons, we believer that two-tone diversity will help maximize the benefit to cost ratio for future radiobeacon networks. A pending patent also addresses the novel features of this signaling technique and the best mode receiver.
Published in:	Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997) September 16 - 19, 1997 Kansas City, MO
Pages:	995 - 1004
Cite this article:	Young, David, Butler, Bruce, "Dual Tone Signaling to Mitigate Self-Skywave Interference to DGPS Broadcasts from Radiobeacons," Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997), Kansas City, MO, September 1997, pp. 995-1004.
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