"Differential GPS, or DGPS [ Differential Global Positioning Satellite], is a medium frequency 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 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. 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. In this paper 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."
2011 Naval Postgraduate School (U.S.). Center for Homeland Defense and Security. Posted here with permission. Documents are for personal use only and not for commercial profit.
Homeland Security Affairs Journal: http://www.hsaj.org
Homeland Security Affairs (March 2011), Supplement no.3