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An acoustic system for detecting submarines; the letters ASD stand for "Anti-Submarine Division", the part of the Admiralty responsible for the work.
WWI Philip Vassar Hunter collaborated with Admiral W. W. Fisher and William Bragg formed a three-man committee to guide the work done by the Admiralty on the development of methods of detecting, locating and destroying submarines. The best known and by far the most fruitful product of this association was ASDIC.
The American name SONAR was later more widely used for this technique