Echosounder

An echosounder is an acoustic instrument that measures water depth by timing a sound pulse from a hull transducer to the seabed and back. Depth follows from the two-way travel time and the sound speed in water, about 1,500 m/s, as depth equals half the travel time times the sound speed. Single-beam units give one depth under the keel; multibeam units fan many beams across track for full swath coverage. Accuracy depends on a measured sound-velocity profile, heave compensation, and transducer draft. Hydrographic echosounding is the primary depth source behind nautical charts and is graded against IHO S-44 vertical-uncertainty limits.