What is "echosounder"

Usage examples of "echosounder".

An echosounder only told you what was directly below, and no submarine would venture under trawlers.

According to the terms of the joint fishery, Soviet ships weren't allowed to use their echosounders in American waters, but no captain would sail blindly while Ameri­cans were off the bridge.

It was a pity, thought Lawrence, that he had no echosounder that would operate here, but he doubted if either acoustic or radio waves could penetrate through more than a few meters of the dust.