VALUABLE DEVICE

Otago Daily Times, Issue 26848, 12 August 1948, Page 6

VALUABLE DEVICE

FIXING OF POSITIONS AIRCRAFT OR SHIPS IN DISTRESS DATA FROM UNITED STATES P.A. WELLINGTON, Aug. 11. The United States Government had agreed to make available to member States of the International Civil Aviation Organisation in the South Pacific region technical data on a positionfixing devise known as “ Sofar,” which is now under development, said the Minister of Defence, Mr Jones, tonight. By means of this device it will De possible to establish almost immediately the exafct position of aircraft or a ship in distress at sea. Aircraft and ships now depend on the reception of distress messages sent by radio, but this method is not always reliable. “ Sofar ” is a device based on the theory that there exists in the temperate latitudes of the Pacific Ocean a sound velocity structure which is such that velocity decreases with depth to approximately 350 fathoms and beyond that it increases with depth. In accordance with the laws of wave refraction, this plane of minimum velocity forms a natural sound “channel’' over the entire area of the ocean. Due to this channelling effect, sound originating at or near the depth of minimum sound velocity is detectable at very great distances. A “Sofar” netwoxk consists of at least three widely-separated hydrophone installations bordering the ocean area to be covered. Hydrophones are anchored at the sound channel deptn ana connected by cable to monitoring equipment on shore. To determme the position of a vessel in distress, a special type of explosive acoustic signal v.-.sisneu. to expioue ai appj.uiamai.eiy 300 fathoms is dropped overboard. When it reaches the approximate aepch of the sound channel a pressure aetonator sets off the main charge and the resulting sound proceeds from that in aii uirectiuns. <-■ .

The channelling effect concentrates che sound in one plane through the ocean, enabling distant but sensitive uydrophones to detect it. When the <,ouna reaches each listening hydrophone, on which a continuous watch is Kept, it is recorded and timed very accurately. The time of arrival for each station is transmitted by radio to a piotung centre, wnere an accurate iix is made on the source of the distress signal.