Furuno DS-80 - Manual (Page 11)

3
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 f r =
f o + f d where f d is the number of frequency
shifts counted at the receiver circuit. To
calculate ship’s speed, the following formula
is used.
V = f d /f o • c/2sin
θ
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
θ fo
fr = fo + fd
WATERMASS
SEA
SURFACE