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 o + F Dand F o -F D . lag processor calculates the difference fo + F D and F o -F D to determine values only d oplerovskogo shift F D .
(F o + F D ) - (F o -F D ) = 2F D
Since, theoretically, F D can be defined as
F D = 2Vcos? XF o / C (C: Speed of sound in water), and F o , C and ? are known, then, with the known value of F D 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).