Doppler Log Errors
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 seawater temperature is accounted for. In modern transducers the angle at which the beam is transmitted is controlled with the help of phase difference and the factor c/Cosα is not used in calculating v.
Errors 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.
Errors 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 athwart-ship speed. Similarly rolling will cause an error in athwart ship, but not in forward speed.
Actual Speed = Indicated speed / Cos β Where β is the angle of pitching for forward speed and angle of roll in case of athwart-ship speed, e.g., if the ship is pitching at an angle of 10° and the indicated speed is 15 knots, then the
Actual speed = 15/ Cos 10°
i.e. Actual speed = 15.23 kts.
This error will increase with angle of pitch i.e. in rough seas. The digital readout will fluctuate between the actual speed (i.e. at the instant when the pitch angle is zero) and a value lower than the actual speed (i.e. when the angle of pitch is maximum).
The athwart-ship speed assumes importance only at the time of berthing where the rolling is negligible and hence the above formula is generally used in case of forward speed only.
Error due to inaccuracy in measurement of comparison frequency:
The difference in the frequencies received by the forward and aft transducers must be measured accurately as any error in this will be directly reflected in the speed of the vessel.
Error due to side lobes:
When the side lobe reception dominates over the main beam reception, there will be an error in the speed indicated. This error is more pronounced on a sloppy bottom, where the side lobe will be reflected at a more favorable angle and will have path length less than the main beam.
This error cannot be eliminated with the help of Janus configuration and to reduce this error the beam of the transmitted acoustic wave is reduced. However a minimum beam width of 4 to 6° is required.