A Doppler log measures a vessel’s speed by transmitting sound pulses and analyzing the frequency shift (Doppler effect) of the signal reflected back — a method independent of satellite positioning.
How It Works
The Doppler log transmits sound waves toward the seabed (or, in deep water where seabed return isn’t practical, toward the water mass itself). As the ship moves, the frequency of the returning echo shifts slightly compared to the transmitted frequency — this shift is directly related to the ship’s speed, similar in principle to how a passing ambulance siren changes pitch as it approaches and moves away.
Speed Through Water vs Speed Over Ground
- Speed through water — the vessel’s speed relative to the surrounding water mass, unaffected by current
- Speed over ground — the vessel’s actual speed relative to a fixed point on Earth, which includes the effect of current
A Doppler log operating in “bottom track” mode (using seabed return, available in shallower water) gives speed over ground; in “water track” mode it gives speed through water. GPS separately provides speed over ground as well, and comparing the two data sources helps identify current effects.
Why It Matters for Ship Handling
Doppler log data is particularly valuable during close-quarters maneuvering — berthing, unberthing, and pilotage — where precise, real-time speed information (including lateral movement in some dual-axis systems) is more immediately useful than GPS-derived speed, which can have slight lag or lower precision at very low speeds.
Exam Relevance
Doppler log principles and its practical application during ship handling are covered within Navigation Equipment topics in deck officer competency exams.