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Publication of a patent filed 12 years ago by The Plessey Company of Ilford
(GB 2 339 617)—but kept a military secret until now—improves the
accuracy of underwater submarine trackers. Inherited by Marconi after Plessey’s
break-up, the patent exploits the fact that sound travels faster in the
direction of flow of a liquid than against it. A frame has opposing pairs of
ultrasound transmitters and receivers, about 10 centimetres apart. The time it
takes the ultrasound to cross the frame is affected by any sound which arrives
from outside the frame because this creates a fluid flow. Comparing the
ultrasound delays gives an accurate fix on the direction of the external
sound—and the location of the object (such as a dolphin or submarine) that
made it.