Augmenting the DGPS broadcast with emergency information — Potential coverage and data rate

Differential GPS, or DGPS, is a medium frequency (MF) radio system that is used worldwide for the broadcast of differential corrections to users to improve the accuracy and integrity of the GPS. This communications system works by digitally modulating radio signals broadcast from a network of marine radio beacons operating in the medium frequency 283.5-325 kHz radio band. The modulation scheme called Minimum Shift Keying (MSK) is used to transmit the correction data at typical data rates of between 50 and 200 bits per second (bps). The U.S. Coast Guard has pioneered the use of MSK for transmission of differential GPS corrections, and has provided over ten years of worthy service with the system. The U.S. DGPS installation is nation-wide, with over 85 transmitters providing double coverage to most of the CONUS. Today, the Coast Guard is re-examining the role of DGPS/radio beacons with the goal of optimizing service for the next ten years. Here we suggest that the DGPS system has significant capability for use beyond that of its current mandate; specifically, there exists the potential for concurrently transmitting a second information-bearing signal on the beacon signal. We believe that this simultaneous transmission of the current navigation correction information (the primary channel) and additional messaging (perhaps DHS emergency messaging or other relevant information) could be accomplished at very minimal cost, and with minimal impact on current users, using a technique we have called phase trellis overlay. This idea has been proposed in earlier work by these authors; several variations of the approach have been designed, analyzed, and tested with results presented at Institute of Navigation conferences. These previous presentations have focused on the technical details of the method; for example, design of the new communications signals, bandwidth of the resulting signal relative to the DGPS system requirements, implementation concerns at the transmitter, and its impact on legacy user performance were analyzed. Here we summarize these earlier results within the context of a potential DHS emergency messaging system. We re-examine the technical details of this approach as a simply parameterized FM (frequency modulation) overlay which yields mathematically tractable performance results. Sample results of this analysis highlight the tradeoffs between coverage expected for legacy users and coverage expected for the new DHS messaging system.