Kongsberg Simrad SN90 Reference Manual page 482

Simrad SN90 Reference Manual
You can choose from four different AGC settings. The settings define the difference
between the maximum and minimum echo level for the automatic gain control to take
effect.
The chosen AGC (Automatic Gain Control) setting is only be applied to the currently
active view. Activate the relevant view before you select the AGC setting.
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 SN90 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.
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