Kongsberg Simrad ST90, Reference Manual

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Reducing noise and reverberation with the AGC (Automatic
Gain Control) function
Identifying echoes from fish and schools can often be challenging when they are hidden
by noise and reverberation. When sonar conditions are poor, effective tuning of the filters
in the ST90 system may be necessary to achieve a satisfactory result. Automatic Gain
Control (AGC) is one of the tools provided to remove noise and reverberation from the
echo presentation.
Context
The
AGC
(Automatic Gain Control) function runs an automatic analysis of the echo
strength. Based on this analysis the ST90 system maintains a proper dynamic range based
on all the current echo values. This is done by automatically reducing the gain if you have
reverberation and noise, or increasing the gain if the acoustical conditions permit it.
Tip
The most practical way to use the AGC (Automatic Gain Control) is to first switch it off.
Adjust the echo presentation using the normal gain functionality until you have the best
possible result, then activate the AGC.
The AGC works best with even background (homogenous noise conditions) and pronounced
targets. With uniform environmental conditions you may benefit from strong compensation.
However, if you are looking at scattered fish, reduce the automatic gain control, and try to
add normal gain instead.
If you work close to other vessels, the noise from their propellers may be misinterpreted as
echoes. This will also reduce the visibility of other echoes. The same applies if the bottom
conditions place your vessel with decreasing depth on one side of the vessel and your targets
on the other side. Reduce the automatic gain control.
If you have a large school located close to your vessel (typically closer than 150 meters)
the school gives you a strong echo. The AGC may misinterpret the school echo for noise.
Reduce or disable the automatic gain control.
The filters in the ST90 system are implemented in “series”. The gain compensations are
made before the AGC. For this reason, your current gain setting will always have an effect
on the automatic gain control.
Procedure
1 Click in any view to make it active.
The active view is identified with a thicker border.
2 Open the
Active
menu.
Operating procedures