Sofar

To its public list of gadgets for locating castaways at sea (Very pistols, dye markers, a hand-cranked automatic radio, etc.) the U.S. Navy added another that has vastly more range and accuracy than al] the rest. The latest, perfected too late for use in World War II but due for plenty of use, nevertheless, is a long-range underwater sound system called Sofar (Sound Fixing and Ranging).

Sofar, a by-product of wartime submarine detection research, is as curious as it is practical. A five-pound bomb, tossed into the water by the survivor, explodes under hydrostatic pressure 3,000 to 4,000 feet under sea. Sound waves, carried by the water, are picked up by three or more shore stations. By careful comparison of the arrival times of the signals, the stations can chart the position of the explosion, through triangulation.

Sofar's range is one of its most startling features: a test bomb dropped at Dakar was heard in the Bahamas, 3,100 miles away.

The most notable finding of the Navy's experimenters--and the basis of Sofar--is the sound-carrying property of water 2,000-6,000 feet below the surface. This zone conserves, focuses and carries sound waves over great distances. Sped by the sound-carrying properties of deep water (a function of temperature and density), the explosion of Sofar's underwater bomb comes to the receiving stations as a noise something like a rolling kettledrum crescendo, ending in a sharp report which can be timed to a tenth of a second.

Thus, by comparing the time of readings, three stations can make sure they all heard the same bomb. Each also records the compass direction from which the sound came. In a matter of 40 minutes the readings can be compared, plotted on a chart. The spot where the three bearing lines intersect is the spot where the bomb was dropped. The Navy says it can plot the bomb explosion accurately within one mile at distances up to 2,000 miles from the shore stations.

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