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     

Performance Criteria for Spread Spectrum Communications

The criteria for antijam (AJ) and anti-intercept (AI) systems are described, in terms of the appropriate action taken by the jammer or interceptor. Avoiding pseudonoise (PN) sequences which can be partially or totally predicted is a foremost criteria, especially for AJ, and use of nonlinear feedforward logic (NFFL) with long-period linear maximal sequences appears attractive. A frequency-hopping (FH) system must anticipate a multitone jamming signal, and an error-control code is necessary. A novel method for generating a multitone signal using repeating maximal sequences is described. PN systems must anticipate a tone jammer, and now an errorcontrol code may be needed to assure that sporadic PN sequence correlation with the tone does not reduce the processing gain. Any AI system must anticipate that the interceptor may, if advantageous, integrate his decision energy over many communicator symbols (up to the message length). Now spreading both in frequency and in time may be valuable. Finally, the use of an automatic adaptive data rate is suggested to realize flexibly either AJ or AI objectives.