Novel Signal Designs for Improved Data Capacity from DGPS Radiobeacons

Richard Hartnett, Keith Gross, Michael McKaughan, Per Enge, Peter Swaszek, Gregory Johnson, Al Cleveland, and Mike Parsons

Abstract:	The U.S. Coast Guard and their worldwide counterparts operate maritime and nationwide differential GPS (DGPS) services. This service broadcasts GPS correction information on marine radiobeacon frequencies to improve the accuracy and integrity of GPS. The radiobeacons operate in the medium frequency (MF) portion of the radio spectrum, and minimum shift keying (MSK) is the modulation method. This system provides navigation aid to over 1.5 million users worldwide, and the U.S. system comprises two control centers and over 85 remote broadcast sites. The U.S. Coast Guard Command and Control Engineering Center (C2CEN) has initiated several engineering studies of this service to determine if it is possible to harvest available signal bandwidth to improve accuracy and reliability without diminished utility to the legacy user community. The reexamination contains two initiatives. First, new messages could be added to the existing Radio Technical Commission for Maritime Services (RTCM) message flow. The existing message flow could be rescheduled to make room for new messages without a performance penalty, because Selective Availability (SA) no longer drives the decorrelation time of the corrections. The new messages could carry atmospheric data to improve accuracy to DGPS users with dual frequency receivers at all ranges. In addition, this additional bandwidth may allow corrections from adjacent sites to be transmitted to improve signal reliability. Second, a new signal could be transmitted in addition to the legacy signal to provide a higher data capacity. The data rate goal for this high-rate signal is approximately 500 bps. This high-rate capability could support RTK operations, ionospheric corrections, network corrections, and other broadcast applications to support homeland security initiatives. This paper will focus on the high-rate signal designs. We will describe the signal candidates and assess their communications efficiency as well as the impact on legacy users. In addition, we will describe physical constraints imposed on the new signal by the system transmitting hardware. Specifically, the present antenna system bandwidth may impose limits on what can be accomplished theoretically, and we address this issue by providing analysis and measurements of typical DGPS transmitters/antennas.
Published in:	Proceedings of the 2004 National Technical Meeting of The Institute of Navigation January 26 - 28, 2004 The Catamaran Resort Hotel San Diego, CA
Pages:	674 - 688
Cite this article:	Hartnett, Richard, Gross, Keith, McKaughan, Michael, Enge, Per, Swaszek, Peter, Johnson, Gregory, Cleveland, Al, Parsons, Mike, "Novel Signal Designs for Improved Data Capacity from DGPS Radiobeacons," Proceedings of the 2004 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2004, pp. 674-688.
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