Context
You can compare this gain setting with the volume control on your car radio. When the gain is increased, the echoes will appear stronger. Weak echoes will be easier to see. However, since you also increase the acoustic noise in the reception, the ST90 presentations will also show this noise. Too much gain may therefore "distort" the presentation.
Comparing the gain function with the volume control on your car radio is not very accurate. In fact, the gain in the ST90 is constant. The Gain function is used to adjust the sensitivity. This done by controlling the minimum level of detection. When you increase the gain level (more positive number) , you reduce the minimum level, and thus increase the sensitivity.
The echo strength (A) changes with time. The minimum level of detection (B) is adjusted up or down with the Gain function. Increasing the Gain with a more positive number reduces the minimum level, and this increases the sensitivity. Only echoes over the minimum level are shown in the echogram (C).
Do not confuse this Gain setting with the TVG (Time Varied Gain) setting.
TipDuring normal operating conditions we recommend that you keep the gain between 15 and 30. If you need to adjust the gain "from scratch" disable AGC (Automatic Gain Control) and RCG (Reverberation Controlled Gain)
to see the immediate effect of the gain adjustments. While you adjust the gain you may find it useful to have a known target (for example an island) as reference.