ASDIC sent a directional underwater sound wave and measured the time taken for its echo to return from a submerged object. A steerable transmitter and compass reading supplied bearing, while a water-filled dome around the equipment helped reduce water-flow noise. The operator still had to sweep a limited arc and pause long enough at each bearing for the sound pulse to return. The system was sophisticated for its era, but it demanded careful searching and favourable operating conditions.
Chapter · 20:12-21:47
How ASDIC found a submerged submarine
A directional sound pulse, echo timing, and bearing indication gave an escort a submarine contact.