DGPS directional signal strength meter

The U. S. Coast Guard currently operates a maritime differential GPS service consisting of two control centers and over 85 remote broadcast sites. This service broadcasts on marine radiobeacon frequencies using minimum shift keying (MSK) as the modulation method. For some time, the Coast Guard Academy has been developing hardware/software tools to assess the performance of this system. The tool described in this paper performs DGPS beacon field strength measurements using an electronically steerable antenna array. The steerable nature of this sensor provides the ability to perform spatial filtering while analyzing the DGPS beacon spectrum. This tool is useful for validation of DGPS coverage prediction software, for site certification, and reception troubleshooting: Consider two DGPS transmitters that are transmitting on the same frequency. This tool would provide the capability to make field strength measurements at a location half way between the two transmitters, in the direction of one transmitter, while nulling out the transmission from the other. Such a tool would be invaluable in validating propagation prediction coverage algorithms and software. The mandated coverage required by the National Differential Global Positioning System (NDGPS) makes such propagation prediction tools critical. The combination of this tool with a standard DGPS receiver would provide great insight while troubleshooting receiver reception problems. This combination could provide the ability to correlate directional signal strengths from DGPS beacons with periods of non-reception. The receiver consists of an antenna array (an H-field crossed loop antenna and an E-field whip antenna), amplifiers and filters, an analog-to-digital converter, and a MATLAB®-based, GUI driven, digital back end