Patent ID: 8139442

Claim:
An underwater communication system, comprising: an acoustic signal transmitter; and a remotely positioned acoustic signal receiver: wherein the acoustic signal transmitter is arranged to transmit data encoded as a plurality of symbols, each symbol of the plurality of symbols having two components one of which comprises a distinctive bit code and the other of which appertains to the frequency of a carrier signal on which the symbol is transmitted: wherein the frequency of the carrier signal of successive symbols is arranged to be stepped through a predetermined continuously repeating sequence of distinctive steps each of which occurs once in the sequence: and the signal receiver is arranged to be operated synchronously with the signal Transmitter: and further comprising; a correlator unit arranged to be responsive both to the bit code and to the frequency of the carrier signal of received signals for effecting demodulation and having a plurality of outputs one for each symbol, so that as each symbol is detected a signal on the output to which it corresponds predominates; and an amplitude detector arranged to be responsive to the outputs from the correlators unit for providing an output signal corresponding to the data transmitted.