Mining Publication: Field Test of a Post-Disaster Communication System

A new postdisaster communication and location system using through-the-earth electromagnetic wave propagation was tested at the Bureau of Mines Lake Lynn Experimental Mine. Successful detection of the signal, estimated to be at a signal-to-noise ratio (SNR) of -30 dB, was accomplished using a coherent integration time of 4.46 Min. The transmitted waveform consisted of a repeating pseudo-noise maximal length sequence phase modulating a 1,950-Hz carrier. The receiver used a sliding-window matched filter, optimal for signal known exactly except for phase and starting time, implemented via a special purpose processor. Parallel processing permits adaptive signaling where the detection (integration) time is determined by the actual input SNR encountered. The field test demonstrates a trapped miner detection system that is about 36 dB more sensitive than the current pulsed system. The increased sensitivity permits some desired combination of increased range, shorter detection time, and reduced transmitter power or antenna size.