Interrogator-responder signalling system

Claim 1

AN INTERROGATOR-RESPONDER SIGNALLING SYSTEM COMPRISING AN INTERROGATOR STATION, A RESPONDER DEVICE MOVABLE INTO SPACED RELATION WITH THE INTERROGATOR STATION, AND A COMMUNICATION CHANNEL COUPLED BETWEEN THE INTERROGATOR STATION AND A REMOTE LOCATION, SAID INTERROGATOR STATION INCLUDING A TRANSMITTER, AT LEAST TWO RECEIVERS, AND A SWITCHING MEANS ASSOCIATED WITH EACH OF THE RECEIVERS, SAID TRANSMITTER BEING OPERABLE TO GENERATE A CARRIER SIGNAL OF A FIRST FREQUENCY WHICH IS MODULATED BY A PLURALITY OF SIDEBAND SIGNALS, SAID RESPONDER DEVICE INCLUDING MEANS FOR DETECTING THE CARRIER SIGNAL AND FOR GENERATING A RESPONSE SIGNAL OF A SECOND FREQUENCY MODULATED IN ACCORDANCE WITH SELECTED ONES OF THE SIDEBAND SIGNALS, EACH OF SAID RECEIVERS INCLUDING A MEANS FOR DETECTING THE RESPONSE SIGNAL AND FOR PASSING AUDIO SIGNALS CORRESPONDING TO THE SIDEBAND SIGNALS SELECTED BY AND IDENTIFIABLE WITH THE RESPONDER DEVICE, EACH RECEIVER INCLUDING A FURTHER MEANS FOR PASSING ANOTHER AUDIO SIGNAL PECULIAR TO AND IDENTIFIABLE WITH THE RECEIVER, EACH OF THE SWITCHING MEANS BEING COUPLED TO PASS SIGNALS FROM