Furuno DS-85 Operator's Manual page 6

REMARKS ON USAGE
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-85 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
tions. 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 water mass
(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/sinθ
c: Underwater velocity
Note that the sound velocity in water changes with water temperature and water pressure but the
DS-85 readout is automatically compensated for change by using a temperature sensor.
Remarks on usage
The DS-85 measures ship's speed by detecting the Doppler shift frequency of the echo reflected
by a water mass (water layer containing plankton and other micro-organisms) located within the
measuring area, which is usually about 2 m. In some instances, however, no signal is returned
because of too few plankton in the sensing depths. This phenomenon can occur in particular areas
in particular seasons. The probable cause is the plankton are lying in deep water because an ice-
melted cold water mass covers the sea surface. Similar cases may also occur in a freshwater lake.
Under these circumstances the DS-85 will not show the correct ship's speed.
to the waterline towards ship's fore and aft direc-
θ
V
Sea
surface
θ f o
f r = f o + f d
Watermass
iv