A collision avoidance system is devised which is composed of a ship-borne radar and a conventional VHF communication system with some additional devices. Ship-borne radars and VHF communication systems are used quite widely today. The functions of the system are listed below ; (1) collision warning by a buzzer which is equivalent to a horn of a car. (2) identification of a target with which a collision may occur by giving a mark on the radar scope as well as identifying lamp display. (3) exchange of information on manoeuvrings and others through VHF telecommunication. An evaluation test at sea was executed in the vicinity of Port Yokohama where "Noji-ma", a patrol ship, was employed as the target and "Sumida", also a patrol ship, was calling station. The functions of the system were almost satisfactory and the covered range was up to 3.5 miles for the range of "Low Sensitivity" of the radar receiver. However, interferences from other radars as well as other VHF transmitters were obser-ved occasionally, since the congestion of the ship traffic as well as electromagnetic wave are heavy in the port. The tested system has this shortcoming, however, the authors feel that if the system is produced very economically, its merits are beyond its shortcoming.
