fathometer

syvyysmittari puhekieltä A depth finder that uses sound waves to determine the depth of water.
1977, w:Richard O'Kane|Richard O'Kane, Clear the Bridge: The War Patrols of the U.S.S. Tang (Ballantine Books, 2003), page 347:

A Fathometer is just a vertical echo-ranger; its pings would sound the same and could drive a submarine out of position just as effectively as horizontal echo-ranging and might even make a contact.