Speed log (Doppler)

A Doppler speed log measures a ship’s speed by transmitting acoustic pulses from a hull transducer and reading the frequency shift of the returns. In shallow water the beams reflect off the seabed for speed over ground; beyond bottom-tracking depth they backscatter from water layers for speed through water. Dual-axis Doppler logs give both fore-and-aft and athwartship speed, so they read transverse and bow-and-stern motion for berthing. The through-water output feeds ARPA for sea-stabilized relative-motion tracking, which IMO Resolution A.823(19) calls for, while the over-ground output supports ground-stabilized navigation.