Signalling system

Claim 1

A SIGNALLING SYSTEM, COMPRISING IN COMBINATION: TRANSMITTER MEANS FOR PROVIDING A FIRST CARRIER SIGNAL HAVING A PLURALITY OF SUB-CARRIER SIGNALS MODULATED THEREON, SAID TRANSMITTER MEANS HAVING FIRST MEANS FOR GENERATING SAID FIRST CARRIER SIGNAL, A PLURALITY OF SUB-CARRIER GENERATING MEANS FOR GENERATING SAID PLURALITY OF SUB-CARRIER SIGNALS, AND MODULATION CIRCUITRY FOR MODULATING SAID SUBCARRIER SIGNALS ON SAID FIRST CARRIER SIGNAL; A PASSIVE RESPONDER DEVICE RESPONSIVE TO SAID FIRST CARRIER AND RELATIVELY MOVABLE WITH RESPECT TO SAID TRANSMITTER MEANS, SAID PASSIVE RESPONDER DEVICE BEING OPERATIVE TO PROVIDE A CODED SECOND CARRIER HAVING A SELECTED GROUP OF SAID SUB-CARRIER SIGNALS MODULATED THEREON, THE FREQUENCY OF SAID SECOND CARRIER BEING DIFFERENT FROM THE FREQUENCY OF SAID FIRST CARRIER; RECEIVER MEANS FOR RECEIVING AND DEMODULATING SAID SECOND CARRIER TO PROVIDE A COMPOSITE SUB-CARRIER SIGNAL CONTAINING THE FREQUENCIES OF SAID SELECTED GROUP OF SUB-CARRIER SIGNALS; DETECTOR MEANS FOR RECEIVING AND PHASE-MODULATING SAID COMPOSITE SUB-CARRIER SIGNAL TO PROVIDE A DIGITAL SIGNAL CODED IN ACCORDANCE WITH THE SELECTION OF SUB-CARRIER SIGNALS PROVIDED IN SAID RESPONDER, SAID DETECTOR MEANS INCLUDING A SYNCHRONOUS DETECTOR RESPONSIVE TO AT LEAST ONE COMPONENT OF SAID COMPOSITE SUB-CARRIER SIGNAL RECEIVED FROM SAID RESPONDER AND TO A FURTHER SIGNAL RECEIVED DIRECTLY FROM SAID TRANSMITTER, SAID SYNCHRONOUS DETECTOR BEING OPERABLE TO COMPARE THE PHASE OF SAID COMPONENT OF SAID COMPOSITE SUB-CARRIER SIGNAL AND SAID FURTHER SIGNAL, AND OPERATIVE TO REJECT AS NOISE COMPONENTS OF SAID AT LEAST ONE COMPONENT OF SAID COMPOSITE SUB-CARRIER SIGNAL WHICH DO NOT OCCUR IN PROPER PHASE RELATIONSHIP WITH SAID FURTHER SIGNAL.