Abstract: An electronic two way signalling apparatus for permitting two or more persons not in line-of-sight with each other to activate transceivers for broadcasting and receiving radio frequency signals alerting those persons of the presence of others within the range of the broadcast signals. The apparatus carried by such persons provides a fixed outgoing radio frequency signal on a random broadcast time of short duration and reception of incoming radio frequency signals at time when a broadcast is silent, and each such apparatus is identical in operation.