Unit installation

Theory of Operation
974-24007001/4.1 9
2.4 Forming an Image
The sound pulse projected will be attenuated as it travels through
the water from the transducer to the target and back. Much of this
attenuation is a predictable function of the total time or the distance
the sound pulse travelled through water. Increasing the receiving
gain with time can compensate for this decrease in the signal level.
This is done automatically in the sonar with a Time Varying Gain
(TVG) circuit.
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 colour 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 words, 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.