Performance of Signal Designs for Improved Data Capacity from DGPS Radio Beacons

Peter Swaszek, Richard Hartnett, Keith Gross, Per Enge and Gregory Johnson

Abstract:	The Coast Guard currently operates a maritime differential GPS service consisting of two control centers and over 85 remote broadcast sites. This service broadcasts GPS correction information on marine radio beacon frequencies to improve the accuracy and integrity of GPS. The existing system provides differential corrections over a medium frequency carrier using minimum shift keying (MSK) as the modulation method. MSK is a version of the Continuous Phase Frequency Shift Keying (CPFSK) modulation technique that is ""spectrally compact,"" meaning that it is a narrow band modulation scheme. In a binary signaling channel, the two instantaneous frequencies for this modulation method are chosen in such a way so as to produce orthogonal signaling with a minimum modulation index. Current DGPS corrections are transmitted at a relatively low data rate, with message structures designed in an era when Selective Availability was in full operation. Greater demands for accuracy coupled with current operations in a ""post SA"" environment have prompted a reexamination of the DGPS data and signal structure, with an eye towards improving information rate while minimizing legacy user impact. A two-phased plan for a new generation of DGPS capability can be envisioned. In the first phase (near-term) new ionospheric messages would be introduced to allow greater DGPS accuracy at larger distances from the beacons. This capability could support both double (LI/L2) and triple (L1/L2/L5) frequency operation. This phase requires only the definition of the new message type(s) and the commitment of receiver manufacturers to implement the usage of the new data. In the second phase (intermediate future) a new signal would come on line to support RTK using two and three frequencies and homeland security messaging. This signal would have the capacity to send 500 bps or so without disrupting the legacy signal or legacy receiver performance. This new signal could be one of the new modulation techniques that we have been investigating; phase trellis overlay and discrete-multi-tone. A preliminary examination (ION NTM Jan. 2004) of both of the techniques has shown the potential for increased bandwidth usage. This paper describes investigations into the performance of these new modulations and to identify optimal implementations. Specifically, a software DGPS modulator test-bed has been created and has been used to implement the new modulations to examine their impacts on legacy receiver performance.
Published in:	Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004) June 7 - 9, 2004 Dayton Marriott Hotel Dayton, OH
Pages:	698 - 708
Cite this article:	Swaszek, Peter, Hartnett, Richard, Gross, Keith, Enge, Per, Johnson, Gregory, "Performance of Signal Designs for Improved Data Capacity from DGPS Radio Beacons," Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004), Dayton, OH, June 2004, pp. 698-708.
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