Janus Configuration
Doppler log works with the principle of Doppler effect. When a sound beam is transmitted from a moving vessel, a difference (shift) in the frequencies is observed between transmitted sound and its reflected echo from the sea bed.
This frequency shift is known as the “Doppler shift”. The degree of shift in the frequency is proportional to the speed of the vessel.
Transducer is fitted on the ship’s keel which transmits a beam of acoustic wave at an angle, usually 60°, to the keel in the forward direction, this gives the component of the ship’s velocity towards the sea bed thus causing the Doppler shift of the received frequency.
We can, hence , calculate the speed of the ship, considering that there is no vertical motion.
In practice the ship has some vertical motion and the Doppler shift measurement will have a component of this vertical motion.
This problem is addressed by using two transducers one looking forward and the other looking aft. Such arrangement is called Janus Configuration.
Ref: Knowledge of Sea Website
