Doppler log

 Principle:

Whenever there is a relative movement between a transmitting source and a receiver, there will be a ‘Doppler shift’ in the frequency received. This shift is termed as Doppler effect.

            fr= ft (c+v)/(c-v)

Where

fr = Received Frequency

ft = Transmitted frequency

c =Velocity of sound in Sea Water (1500m/sec)

v = Velocity of the vessel

How Doppler log works?

• A transducer emits continuous a high frequency sound pulse in the forward direction at an angle of 60° to the keel.
• Higher the sound frequency, smaller the transducer, narrower the beam and higher the accuracy.
• The beam bounces back from the sea bottom.
• The frequency of the bottom echo will be higher when the ship is moving ahead or lower if she is moving astern.
• The Doppler equation is solved to obtain ship’s speed.
• When signal is bounced off the sea bed, (called Bottom Track), the speed displayed the “Speed over the ground (SOG)”    

Advantages of Doppler Log:

• Most accurate

• Can measure ahead, astern & athwartship movements

• Can be used for ocean navigation as well as berthing and maneuvering in close waters.

• Can measure very low speeds

• This log is most prevalent in today’s marine world.

What is Janus Configuration?

Most Doppler logs have Janus Configuration where

• Transducers pointing ahead measures speed.

• Transducer pointing Astern is used for accuracy check.

• Transducers point abeam to measure athwart ship speed (while berthing).

Errors of Doppler 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 showing in error.

Error in propagation:- The velocity of the acoustic wave at a temperature of 16°C and salinity of 3.2% is 1505 m/sec but taken as 1500 m/sec for calculation. This velocity changes with temperature, salinity and pressure. To compensate the error due to temperature change, a thermister is mounted near the transducer and change in velocity of the acoustic wave through the water from the standard value due to the change in sea water temperature is accounted for.

Error in ships‟ motion:- During the period of transmission and reception, the ship may have a marginal roll or pitch and thereby the angle of transmission and reception can change and a two degree difference in the angle of transmission and reception can have a 0.10% error in the indicated speed, which is marginal and can be neglected.

Error due to rolling/pitching:- The effect of pitching will cause an error in the forward speed and not the athwartship speed. Similarly, rolling will have an effect on the athwartship speed, not the forward speed.

Actual speed = Indicated speed/Cosß

Error due to inaccuracy in measurement of frequency:- The difference in the frequencies received by the forward and aft transducers must be measured accurately. Any error in this will be directly reflected in the speed of the vessel.

Error due to side lobe:- When the side lobe reception dominates over the main beam reception, there will be an error in the speed indicated. The error is more pronounced on a sloping bottom as the side lobe is reflected at a more favourable angle and will have path length less than the main beam. This error can be eliminated with the help of the Janus configuration and to reduce this error, the

beam of the transmitted acoustic wave is reduced.