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24.12.2013

Doppler speed logs

Doppler speed measuring log vessel  the Doppler effect, which consists  in that the signal emitted from a moving  object taken fixed observer offset frequency wherein  the offset frequency is proportional to the degree of  speed of a moving object.

 
For the sake of clarity, we consider the  measurement front to rear speed vessel.
 
The figure illustrates a system  with a pair of beams. 
Ultrasonic waves are radiated at an angle ? to the  sea surface in the direction of movement of the ship  in the opposite direction. If the speed of the vessel  V, then the source of the ultrasonic wave  (transmitter) approaches or retreats  from the reflection point on the sea bottom at a speed  Vcos?.
 
This relative movement causes d oplerovsky shift and ultrasonic waves  reflected from the seabed and received by  the transducer at frequencies F + F Dand F -F .  lag processor calculates the difference fo + F and F -F to determine values only d oplerovskogo shift F .
(F o + F D ) - (F o -F D ) = 2F D
 
Since, theoretically, F D can be defined as
D = 2Vcos? XF o / C (C: Speed of sound in water),  and F , C and ? are known, then, with the known value of F  can calculate the speed V.

Note that the speed of sound in the water depends on the  water temperature and pressure, but the lag indications  automatically compensated through  use of the temperature sensor (which it is).


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