Furuno DS-80 manual Principle of Operation

PRINCIPLE OF OPERATION

The Doppler speed log measures ship's speed by using the Doppler Effect, which is observed as a frequency shift resulting from relative motion between a transmitter and receiver or reflector of acoustic or electro- magnetic energy. A common example of the Doppler Effect is a train. When a train is approaching, the whistle has a higher pitch than normal. You can hear the change in pitch as the train passes.

The DS-80 has a pair-beam, one directed in the fore direction and the other in the aft direction, which emits ultrasonic waves at an angle of θ to the waterline towards ship’s fore and aft directions. The frequency of the received signal is then compared with that of the transmitted frequency to measure doppler shift to calculate ship’s speed.

The relative motion causes the Doppler shift and the ultrasonic waves reflected at the watermass (plankton or any underwater objects) are received at the frequency of fr = fo + fd where fd is the number of frequency shifts counted at the receiver circuit. To

calculate ship’s speed, the following formula is used.

V = fd/fo • c/2sinθ

c: Underwater velocity

Note that the sound velocity in water changes with water temperature and water pressure but the DS-80 readout is automatically compensated for change by using a temperature sensor.

V

SEA

SURFACE

θ fo

fr = fo + fd

WATERMASS

3