Whenever man is shipwrecked and takes to a lifeboat or a raft, crashes in an aircraft, or gets into trouble mountaineering or on an expedition, he requires help and rescue. Before any action can be taken by potential rescuers it must be known that a disaster has taken place. At present such knowledge is often acquired in a negative manner, by non-arrival at an expected destination or loss of communication contact. Even if positive information is received by a distress message, the exact location of the incident is often in doubt. Extensive air searches are then required before a rescuer can be guided to the spot.