Robust underwater communication system

1. 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.

2. The underwater communication system of claim 1 , wherein the sequence used comprises sufficient step changes for enough time to have elapsed to allow all significant multi-path components to decay to insignificance before it is repeated.

3. The underwater communication system of claim 1 , wherein the bit code which comprises the first component of each symbol is arranged to be transmitted using phase shift keying (PSK) and the frequency of the carrier signal of each symbol is arranged to be changed from symbol to symbol by hopping the carrier frequency on which the symbol is transmitted through a predetermined continuously repeating sequence of frequencies.

4. The underwater communication system of claim 3 , wherein the transmitter comprises PSK modulator arranged to effectively hop the frequency of the carrier signal carrying the signal thus modulated, the receiver comprising frequency/phase detector means for detecting both of the two components of the received signals which detector is arranged to feed a correlator to which the amplitude detector is responsive for providing output signals corresponding to the data transmitted.

5. The underwater communication system of claim 4 , wherein the frequency/phase detector comprises a phase quadrature detector.

6. The underwater communication system of claim 1 , wherein the correlator unit of the receiver is arranged to employs optimal correlation processing of the signals and thus is arranged to continuously generates replicas of all the possible signals at each epoch in a bank of correlators, thereby to facilitate symbol recognition based on a maximum likelihood detection principle.

7. The underwater communication system of claim 1 , wherein synchronisation is arranged to be achieved by transmitting a wideband header signal at the start of each signal sequence, the receiver being arranged to detect the wideband header signal.

8. The underwater communication system of claim 1 , wherein a tail signal is arranged to be appended at the end of each signal sequence.

9. The underwater communication system of claim 8 , wherein compensation for relative motion between the transmitter and receiver is arranged to be effected in dependence on the relative times of arrival of the head and the tail signals.

10. The underwater communication system of claim 8 , wherein relative motion compensation is arranged to be implemented using time of arrival estimates of each symbol.

11. An underwater navigation system comprising the communication system of claim 1 .

12. An underwater positioning system comprising the communication system of claim 1 .

13. An underwater communication system comprising:

an acoustic signal transmitter; and

a remotely positioned acoustic signal receiver, wherein

data is encoded prior to transmission as a plurality of symbols each having two components one of which comprises a distinctive bit code and the other of which appertains to the character of the symbol as a whole, the character of successive symbols being stepped through a predetermined continuously repeating sequence of distinctive steps each of which occurs once in the sequence by hopping the carrier frequency on which the symbols are to he transmitted through a predetermined continuously repeating sequence of frequencies;

the distinctive bit code comprises the first component of each symbol is transmitted by the transmitter using phase shift keying (PSK); and

the signal receiver is operated synchronously with the signal transmitter and comprises a correlator unit responsive both to the bit code and to the character 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 responsive to the outputs from the correlator means for providing an output signal corresponding to the data transmitted; wherein

the bit code of each symbol and the character of successive symbols are changed in accordance with the predetermined sequence and synchronising operation of the transmitter and receiver.