How does Doppler Speed Log work

How does Doppler Speed Log work?

Working Principle

The Doppler Speed log uses the Doppler Effect to measure the vessel's speed. The Doppler Effect is the frequency shift observed due to the relative motion between the transmitter and receiver. A common example of the Doppler Effect is a train: when the train approaches, the pitch of the whistle is higher than normal. You can hear the change in pitch as the train passes by.

DL9000 has a pair-beam, one pointing towards the bow and the other towards the stern. It emits ultrasonic waves at an angle θ to the waterline in the directions of the vessel's bow and stern. Then, the frequency of the received signal is compared with that of the transmitted signal to measure the Doppler frequency shift, thereby calculating the vessel's speed. Doppler frequency shift caused by relative motion results in the ultrasound reflected from the water body (plankton or any underwater object) being received at the frequency fr = fo + fd, where fd is the frequency shift count calculated in the receiving circuit.

V = fd / fo * C / sin θ

Where C is the propagation speed of sound in water. Please note that the speed of sound in water varies with water temperature and water pressure. The DL 9000 automatically compensates for these changes by using a temperature sensor.

Sea surface

Sea bed

Remarks on usage
The DL9000 measures vessel speed by detecting the Doppler frequency shift of echoes reflected within the measurement area (usually about 3 meters) from the water body (a water layer containing plankton and other microscopic organisms). However, in some cases, no signal is returned due to an insufficient number of plankton at the sensing depth.

                             Sea bed

This phenomenon may occur in specific areas and during certain seasons. A possible reason is that plankton reside in deep water because the sea surface is covered by melting cold water masses. Similar situations may also occur in freshwater lakes. In such cases, the device will be unable to display the correct vessel speed.