FS70 Trawl Sonar System
8 974-24007001/5.0
2.5.1 Automatic Gain Control (AGC)
The AGC “Automatic Gain Control” algorithm increases the gain during low acoustic
returns and reduces the gain during strong acoustic returns. The speed with which the gain
is adjusted is determined by the setting of the “RX Gain Response”
2.5.2 Reverberation Controlled Gain (RCG)
The RCG filter senses the noise level (reverberation, propeller noise, etc.), and adjusts the
gain of each of the received beams in order to eliminate noise on the display. The strength
of the filter can be selected in the menu. With maximum strength selected, the RCG will
effectively reduce the bottom in shallow water, while variation on the bottom will be
displayed.
Note that scattered fish can be perceived as reverberation. The RCG filter must be used
with care if scattered schools are to be detected.
2.5.3 RX Gain Response and Type
The setting of the “RX Gain Response” determines the response of the filter algorithm as
selected by the “RX Gain Type”. For the AGC, this setting determines the speed by which
the gain is adjusted. For the RCG, this setting determines the influence of the previous ping
average over the current ping. If set to “Slow” the overall average is given by the sum of
20% current and 80% of previous average. In “Medium” this ratio is 50% to 50%, and in
“Fast” the ratio is 80% to 20%.
2.5.4 Pulse Length Control
The FS 70 Trawl Sonar Heads are capable of changing the acoustic pulse length that is
transmitted. The surface processing unit sets the pulse length based on the operating range
of the sonar head. It is generally better to use a longer pulse as the operating range
increases. However, the actual value can be optimized to enhance the details in the sonar
image (Optimize Resolution) or to increase the target visibility at long distance away from
the sonar head (Maximum Range).
Optimize Resolution - The pulse length selection can be optimized to enhance the detail of
the targets. It has been experimentally determined that the pulse length corresponding to
40% of the sample period can increase the image resolution and details for targets that are
generally larger than a few samples.
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