Navigation and Seamanship

3.2.7. Doppler log

Doppler Log works with the principle of Doppler Effect. When a sound beam is transmitted from a moving vessel, difference (shift) in the frequencies is observed between transmitted sound and its reflected echo from a target. This frequency shift is known as the “Doppler Shift”. The degree of shift in the frequency is proportional to the speed of the vessel . The advantage of Doppler log is that it can indicate speed relative to seabed rather than speed through the water. Doppler shift of frequency of sound wave is due to movement of the transmitter (ship) is used to measure the speed of the ship. In the case of echo sounder sound beams are sent vertically downwards, but in the Doppler log the transducers are angled horizontally about 60^o to the keel . The propagation time of the pulse and its echo plays no role.

Depending on the size of the vessel up to four transducers are fitted. The frequencies usually vary between 100 kHz and 1 MHz. Accuracy is affected by temperature and salinity of the water (which alter the speed of propagation ) and Doppler logs may have sensors to monitor and correct for variations in these parameters.

ARPV	Advanced Remotely Piloted Vehicle
SVDR	Simplified Voyage Data Recorder

GMDSS Global Maritime Distress & Safety System

The Doppler shift could be explained with simple example. As the train approaches a stationary person, the pitch (frequency) of the sound of the train is higher than when the train passes. As the train recedes from the listener, the pitch decreases. Both radio and sound waves obey the Doppler principle, although radio waves travel at the speed of light .The Doppler Effect is a frequency shift that results from relative motion between a frequency source and a listener. If both source and listener are not moving with respect to each other (although both may be moving at the same speed in the same direction), no Doppler shift will take place. The Doppler log can measure the speed to the nearest 0.01 knot or 5 mm/s, provided there is no air bubbles (which reflect sound waves) to pass through the beams (aeration).

Working Principle

A transmitting transducer below the ship continuously emits a beam of sound vibrations in the water at an angle (usually 60˚ to the keel) in the forward direction. A second transducer aboard receives the echo caused by diffuse reflection from the seabed. A Doppler log uses a higher frequency than an echo sounder.