1 DOPPLER LOG In this type of log
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DOPPLER LOG • In this type of log the Doppler shift of frequency due to movement of the transmitter ( ship ) is used to measure the speed of the ship. © • A very high degree of accuracy, reliability and usefulness is achieved in this log. © • The operation of this log is based on the measurement of Doppler effect. © 2
DOPPLER LOG • A transducer fitted on the bottom of the ship emits a continuous beam of sound vibrations in the water at an angle of about 60 degrees to the keel in the forward direction. © © The beam is bounced of the sea bed or a layer of water and received back at the transducer. 3
DOPPLER LOG • The difference in frequency between the transmitted and received signals is measured and is proportional to the speed of the ship. © • When the signal is bounced off the sea bed ( called bottom track ), the speed indicated will be in relation to the fixed sea bed and will therefore will be speed over the ground. © 4
DOPPLER LOG • If however, the bed cannot be tracked especially in deeper water ( above 200 mtrs ), then the signal is bounced off water layer ( between 10 to 30 mtrs ) ( water track ) and the speed indicated will be speed through the water. © Most Doppler logs have transducers to measure both fore and aft speeds ( ahead and astern ) as well as athwart ship transducers to measure speed in the athwart ship direction ( useful when berthing ). 5
DOPPLER LOG • Such a transducer which measures both along ship as well as athwart ship speed is called JANNUS CONFIGURATION. © • This log is the most accurate of all the logs described and is useful for both the ocean navigation as well as berthing and manouevering in close waters. © 6
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