User guide

901-10037001/1.1 FS 20/25 Operators Manual
Page 2.4 Theory of Operation
Kongsberg Simrad Mesotech Ltd.
Port Coquitlam, BC - Canada
After the TVG correction, the absolute levels of the received signals will be
determined by the acoustic response of the reflecting target.
The sonar processor system repeatedly measures the TVG corrected target levels
by digitizing a sequence of samples after each sound pulse transmission. Each
sample is then plotted on the video display at the appropriate position according to
its range and bearing. The level of the target strength sample determines the color
used to plot each sample.
The process can be repeated with the transducer pointed in different directions,
forming an image of a large area of the bottom, (or the trawl net geometry) and
displaying it on the video screen.
In simple word, the TVG function controls the gain of the receiver so that a school
with a given size and density is presented with approximately the same strength on
the display, inside the regulated TVG range. This can also be seen as a filter,
because it reduces he noise close to the vessel.
2.1.4 Key Feature Setting
The following paragraph will explain some of the key feature of the FS 20/25 System
performance.
2.1.4.1 AGC
Automatic Gain Control (AGC)
The AGC “Automatic Gain Control” algorithm increases the gain during the low
acoustic return 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”
as explained in paragraph 1.1.4.3
2.1.4.2 RCG
Reverberation Control Gain (RCG)
The RCG filter senses the noise level (reverberation, propeller noise, etc.), and
adjusts the gain of each 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.