Abstract: A security system suitable for monitoring the presence of the occupants of a vessel such as a sailing yacht has two receivers at fixed locations on the yacht for receiving signals from one of a number of transmitters each worn by a respective crew member. The receivers are connected to a detector having a comparator triggered when a predetermined relative signal strength is generated by the receivers indicating the presence of a transmitter in an "unsafe" region acting to cause triggering of an alarm and automatic ejection of a life buoy or other life saving equipment.