FM Sonar

This American sonar was a high-frequency, short-range sonar designed to detect moored mines. It used frequency modulation, sweeping through frequency as it swept in bearing, so that a nondirectional hydrophone could determine the bearing of a return signal from its frequency alone. It was one of the first true search sonars, though its range (not more than 800 yards or 730 meters under ideal conditions)was inadequate for any purpose but plotting minefields. 

The sonar had a Plan Position Indicator, similar to those used on the best radar, which showed mines as bright green "pears"; the sonar also made an audible sound when its beam swept across a mine, a clear ringing note that  the crews promptly dubbed "hell's bells." The audible component was important because helped distinguish actual mines from shoals of fish; the shoals gave a less distinct return that produced a muddy tone that a trained operator could easily distinguish from an actual mine.

The technology was originally developed for minesweepers, but the minesweeper force was unimpressed with its capability, and its development and deployment was taken over by the submarine service under Charles Lockwood. Lockwood saw the potential for the sonar to allow his submarines to penetrate Japanese minefields, and the first units reached the fleet in time for Operation BARNEY in July 1945, a raid by nine submarines on the Sea of Japan. The nine boats successfully penetrated the minefields in Tsushima Strait, although one boat was later lost to a conventional depth charge attack before the remaining eight boats escaped through La Perouse Strait.


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