The principle of the Doppler log is based on the Doppler effect, which is a change in the frequency or wavelength of a wave about an observer who is moving relative to the source of the wave.
In the case of a Doppler log, the wave is sound, and the observer is a transducer mounted on the hull of a ship. The transducer emits sound waves, which reflect off the seabed and return to the transducer.
As the ship moves, the transducer experiences a change in the frequency of the reflected sound waves, due to the Doppler effect.
If the ship is moving towards the seabed, the frequency of the reflected sound waves will be higher than the transmitted frequency. If the ship is moving away from the seabed, the frequency of the reflected sound waves will be lower than the transmitted frequency.
By comparing the transmitted frequency and the received frequency, and taking into account the velocity of sound in seawater, the Doppler log can calculate the speed of the ship relative to the seabed.
This provides an accurate measurement of the ship’s speed through the water, which is important for navigation, pilotage, and safety.
The Doppler log can also provide additional information such as distance traveled, heading, and even water depth. It is a valuable tool for ships of all sizes, from small recreational vessels to large commercial vessels.