Abstract: A signalling device employing radio frequencies in which the transmitter produces a weak, or relatively weak, encoded transmission within a crowded frequency band and an associated receiver which includes a processor capable of detecting and analyzing the weak signal by applying significance to each signal parameter including pulse width, word length, pulse characteristic, number of bits; and, imposing the further requirement that the same parameters must be perceived a second time within a prescribed 0.2-0.8 sec. interval to insure that the signal has been produced by a correctly encoded transmitter and not by a mere random reception.