As per SOLAS CH V / Regulation 19, 2.3.4
All ships of 300 gross tonnage and upwards and passenger ships irrespective of size shall be fitted with, speed and distance measuring device, or other means, to indicate speed and distance through the water.
Doppler log is based on the principle of Doppler shift in frequency measurement i.e. apparent change in frequency received when the distance between source and observer is changing due to the motion of either source or observer or both.
In Doppler log an observer is moving with a source of sound towards a reflecting plane, then the received frequency.
By measuring the received frequency & knowing the value of transmitted frequency and velocity of sound in water, the speed of the vessel can be determined.
You may also know –
ERRORS OF DOPLLER LOG: –
Error in transducer orientation-
The transducers should make a perfect angle of 60° with respect to the keel or else the speed indicated will be inaccurate.
Error in Oscillator frequency-
The frequency generated by the oscillator must be accurate and constant, any deviation in the frequency will result in the speed indicated being in error.
Error in propagation velocity of acoustic wave-
The velocity of the acoustic wave at the temp of 16°C and salinity of 3.4% is 1505 m/sec, but generally it is taken as 1500 m/sec for calculation.
This velocity changes with temperature, salinity or pressure. To compensate the error due to temperature variation, a thermistor i.e. a resistance whose value changes with temperature, is mounted near the transducer and change in velocity of the acoustic wave through water from the standard value due to the change in sea water temperature is accounted for.
In modern transducers the angle at which the beam is transmitted is controlled with the help of phase difference.
Error due to ship’s motion-
During the interval between transmission and reception, the ship may marginally roll or pitch and thereby the angle of transmission and reception can change and for a two degree difference between the angle of transmission and reception, the net effect will be an error of 0.10% of the indicated speed which is marginal and can be neglected.
Error due to the effect of rolling and pitching
The effect of pitching will cause an error in the forward speed but it has no effect on the athwartships speed. Similarly rolling will cause an error in athwartships, but not in forward speed.
Actual speed = Indicated speed / Cos β