A Doppler log, also known as a Doppler speed log, is a type of marine navigation instrument that measures the speed and distance of a ship through the water using the Doppler effect.
The Doppler effect refers to the change in frequency of a wave as the source of the wave and the observer move relative to each other.
In the case of a Doppler log, the instrument sends out ultrasonic waves into the water, which bounce off the seabed or particles in the water and return to the instrument.
The frequency of the returning waves is affected by the speed and direction of the water relative to the instrument, allowing the Doppler log to calculate the speed and distance of the ship through the water.
Doppler logs are commonly used on ships as a primary means of measuring speed through the water, and can also be used in conjunction with other navigation instruments to calculate position and course.
There is a transducer placed at the bottom of the ship that measures the speed. It can measure both speeds through water (STW) and Speed Over Ground (SOG).
The transducer is of a configuration known as Jannus Configuration that measures speed not only in the fwd-aft direction but also athwartships.