A new post-disaster mine communication system

A post-disaster signalling system from mine to surface, using through-the-earth electromagnetic propagation of a 1950 Hz carrier, phase modulated by a repeating 255 bit maximal length sequence, is discussed. Coherent integration up to 4.46 min allows output pulse detection on a CRT at input signal-to-noise ratios down up -30 dB, which is about 30 dB less than required for the present pulse transmitter system. The system offers vastly increased reliability along with some combination of increased range, signal processing delay time, smaller transmitter antenna, and reduced power. It avoids a manual signal search period and permits more accurate automated or manual null-seeking emitter location. After location, it can be used with pulse position modulation (PPM) for communication. It should also be valuable for measurements to determine geological properties