What are Carriage requirement of doppler log as per SOLAS ?

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.