Simrad FS70 -, Руководство оператора

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Theory of Operation
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2.3 Determining Target Position
The trawl scanning sonar processor measures the time from the start of the sound pulse
projected through water, to the reception of the sound pulse reflected back to the sonar
transducer. The measured time is then converted to distance by using the value of sound
speed through water.
Since the sound pulse is projected in a known direction, the bearing of the reflected object
is also known. This makes it possible to locate the object with respect to the sonar
transducer; the information will be used to plot the position of the reflected target on a
video graphic display monitor.
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.
2.5 Key Features Setting
The following paragraphs describe some of the key features of the FS 70 System.