Doppler log
Distance and speed measuring device.
Doppler log works on Doppler principle.
Since we know velocity of sound wave in water.
So, By measuring transmitting and receiving frequency vessel speed is found.
When vessel moves relative to a reference point on seabed.

At the forward part, under the ship’s keel, a transducer is fitted at an angle of 60 degree.
Let say, transmitting frequency f(o) and the receiving frequency is f(o) + f(d).
So, formula is
f(d) = 2 V f(o) / C
f (d) = Doppler frequency shift
f (o) = transmitting frequency
C = velocity of sound wave
V = relative speed

Doppler log error
Velocity of propagation
Multiple echo
Transducer error
Sensor error
Heavy weather error

Ground speed available in suitable depth
Accurate speed in shallow water
Better than other log
No moving parts

Errors
Astern propulsion can affect
Excessive trim can affect

Calibration of Doppler log
Transducer should be at an angle of 60 degrees.
Calibration by shore expert.
Calibration during dry dock.

SOLAS requirement
SOLAS Chapter V, Regulation 19.2
All ships of 300 GRT and above and passenger ships of all size shall be fitted.

Echo Sounder vs Doppler log
In the echo sounder, sound beams are sent vertically downwards, but in the Doppler log the transducers are angled about 60 degree to the keel.
A Doppler log uses a higher frequency than an echo sounder.