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Part No. 96000001
NB. In very shallow water (2m or less) it may be more advantageous to use Fixed Gain. To put the
system into Fixed Gain, enter zero (0) for both Spreading Loss and Absorption.
For more detailed information on absorption and spreading loss, please refer to Appendix VI Basic
Acoustic Theory.
5.8.3 Time Variable Gain
Absorption and spreading loss are the main components of the Time Variable Gain (TVG)
computation.
TVG Equation
TVG = 2*R* α/1000 + Sp*log(R) + G
α = Absorption Loss db/km
Sp = Spreading loss coefficient
G = Gain from Sonar Control setting
TVG is employed in underwater acoustics to compensate for the nature of the reflected acoustic
energy. When an acoustic pulse is transmitted in a wide pattern, the first returns will generally be
from the nadir region and very strong. As the receive window time lengthens, the weaker returns
are received. Using a fixed gain would apply either too much gain for the early returns or
insufficient gain for the later returns. The solution is to use TVG. The function of TVG is to increase
gain continuously throughout the receive cycle. Therefore, smaller gain corresponds with the first
returns (normally the strongest) and higher gain corresponds to the later returns (normally the
weakest). This function is represented in, what is called, the TVG curve.
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