Principle Of Doppler Speed Log - Furuno DS-50 Operator's Manual

PRINCIPLE OF DOPPLER SPEED
LOG
A doppler speed log measures ship's speed by utilizing
the principle of the Doppler effect, which defines that a
signal emitted from a moving object is heard with its
frequency shifted at stationary locations and the degree
of the frequency shift is proportional to the speed of the
moving object.
For ease of understanding measurement of ship's
fore-aft speed is explained in this paragraph.
Although the DS-50 employs three directional beams,
let's suppose that only two beams are used as shown at
right.
Ultrasonic pulses are emitted at an angle of θ relative to
water line toward ship's fore and aft direction. If the
ship's speed is "V", the source of the ultrasonic pulse
approaches or goes away the reflecting points on the
seabed at a speed of Vcosθ.
This relative motion causes the Doppler shift and the
ultrasonic signals reflected at the seabed are received at
frequencies of "fo + fd" and "fo - fd" by the transducer.
In the processor unit of the DS-50, difference of "fo +
fd" and "fo - fd" are computed to extract only the
Doppler shift factor "fd".
(fo+fd) -(fo-fd) = 2fd
Since the 'fd' is theoretically given by
fd = 2Vcosθ x fo/C (C: Sound velocity in water)
and fo, C and θ are known, V can be calculated if the "fd" is given.
Note that the sound velocity in water changes with water temperature and water pressure but
the change by water temperature is automatically compensated for by using temperature sensor.
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