Brm 2 - Doppler Log-1.docx

Doppler Log

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, the received frequency f(r) = f(t) (c+v)/ (c-v), where f(r) = received frequency f(t)= transmitted frequency c= velocity of sound in seawater and v= velocity of the vessel By measuring the received frequency ‘f(r)’ and knowing the value of transmitted frequency ‘f(t)’ and velocity of sound in seawater (c), the speed of the vessel ‘v’ can be determined. PRINCIPLE A transmitting transducer below the ship continuously emits a beam of sound vibrations in the water at a specific angle (usually 60 degrees to the keel) in the forward direction. A second transducer aboard receives the echo caused by diffuse reflection from the seabed. A Doppler log uses a higher frequency than an echo sounder. As a higher frequency means a shorter wavelength, it makes possible a smaller beam-angle and so avoids the dimensions of the radiating face

of the transducer becoming too large. Also, the emitted power of the sound vibrations spreads less and thus the echo is stronger. Also every point of the seabed is hit by the beam and causes a stronger or weaker echo in the direction of the receiving transducer. All these points are situated at a different angle of about 60 degrees (as stated above) to the horizontal plane. The frequencies received aboard must differ for all these points. However, the average received frequency is approx. that from exactly 60 degrees to the horizontal. Hence, though the distance between the ship and seabed does not change, the received frequency will differ (owing to the Doppler Effect) from the transmitted frequency. From the Doppler frequency shift, which can be measured, the speed of the vessel can be found. A second transmitting transducer directs a beam in a backward direction and a second receiving transducer receives its echoes. The speed of sound waves in the water depends however on the temperature and, to a smaller degree, on the salinity and the water pressure. LIMITATIONS 1) Errors in transducer orientation: the transducers should make a perfect angle of 60 deg. With respect to the keel or else the speed indicated will be inaccurate. 2) 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.

3) Error in propagation velocity of acoustic wave : The velocity of the acoustic wave at the temp. of 16 deg Celsius and salinity of 3.4% is 1505m/sec, but generally it is taken as 1500 m/sec for calculation. This velocity changes with temperature, salinity or pressure. Various methods have been devised to overcome this error. 4) 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. 5) 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 athwartship speed. Similarly rolling will cause an error in athwartship, but not in forward speed. Janus Configuration Placement of two transmitting transducers in the longitudinal direction measures the forward and backward speed and this is called the Janus Configuration. As for the alongships transducers, athwartship transducers are mounted along the beam of the vessel at a 90 degree angle to the F&A transducers. All above are called a “Janus Configuration”. Calibration of Doppler speed log This is done with an accurate measuring method with the GPS Speed measurement method which is nowadays, considered highly accurate.