Abstract: The secure communication keying system has a noise generator whose output a predetermined finite band of noise within, for example, the zero to eleven hundred cycle per second range. The noise generator may be a pseudorandom noise generator that is synchronized with a given clock frequency. The output of the system may have noise-like characteristics, but it is encoded with any type of intelligence desired to be transmitted. The output signals simulate the ambient noise occurring within the communication medium and has the same frequency spectrum regardless of the frequency of the modulating signals supplied thereto.

Abstract: A secure sonar communication system for transmitting and receiving intellnce signals within an aqueous medium comprising in combination means for generating an electrical pseudo random noise signal within a predetermined bandwidth, means connected to said pseudo random noise generating means for selectively driving same at one of a plurality of predetermined frequencies, keying oscillator means adapted to selectively produce an output of one of a plurality of predetermined frequency signals, a balanced modulator means coupled to said pseudo random noise signal generating means and said keying oscillator means for mixing the output signals therefrom, bandpass filter means connected to the output of said balanced modulator means for passing the upper sideband thereof, means for receiving an electrical pseudo random noise signal, transducer means, said transducer means being capable of broadcasting acoustical energy upon excitation by the upper sideband from the aforesaid bandpass filter means and in proportion