Analysis of phase trellis overlay modulations for a DGPS secondary communications channel

The U.S. Coast Guard (USCG) currently operates a Nationwide Differential GPS (NDGPS) service consisting of over 85 broadcast sites operating in the MF band. This system broadcasts GPS correction information to improve the accuracy and integrity of GPS. The existing system provides differential correction data 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. Here we suggest that the Coast Guard\u27s NDGPS has significant capability for use beyond that of its current mandate, by transmitting a second information-bearing signal on the DGPS MSK carrier signal. The simultaneous transmission of navigation correction information (primary channel) and additional messaging (perhaps DHS emergency messaging) could be accomplished (at very minimal cost) using a technique called phase trellis overlay. This idea has been proposed in earlier work by the authors, and several variations have been analyzed and tested with results presented at previous ION conferences. For this method, which directly modified the phase trellis of the MSK transmission, relevant performance measures (bandwidth, impact on legacy performance, and new data channel error performance) were analyzed. Previously, no direct relationship between the proposed (piecewise linear) trellis changes and the resulting performance was discernable, hence it became very difficult to find the best signal set that maximized data throughput, minimized bandwidth, and minimized impact to the legacy DGPS correction channel. Here we propose a new class of trellises that lends itself to closed-form expressions for signal distances, and convenient relationships for bandwidth costs, thereby making signal set optimization a relatively straightforward exercise