Kongsberg Simrad FM90i, Справочное руководство

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Description
When an acoustic pulse is sent through the water, it will gradually lose its energy. The
greater the distance between the transducer and the target(s), the greater the loss of energy.
•
Geometric spread
Once transmitted, the acoustic energy will spread out to form a circular beam. The width
of this beam increases with the physical distance to the target(s).
•
Absorption loss
Depending on the salinity and temperature, the water will absorb some of the energy
from the transmission. The absorption loss increases as the physical distance to the
target(s) increases.
Both the geometric spread and the absorption will also have an effect on the returned echo
signal. That is why we normally refer to these factors as the
two-way transmission loss .
The TVG (Time Variable Gain) compensation is designed to counteract the natural
phenomena of geometric spread and absorption loss. In the FM90i, the TVG compensation
is made using digital signal processing software. By means of algorithms, the time variable
gain compensation converts the echo presentation as a function of range. This makes the
targets with the same strength appear with the same intensity independent of their physical
distance from the transducer.
Tip
Click the
TVG
button on the top bar to enable the TVG Correction filter. This filter reduces
TVG-amplified noise at further ranges where no targets appear. You can adjust the
percentage of this filter (applied to the range) in the
Display Widget
.
Details
The TVG compensation is expressed as a logarithmic curve. You can choose from a
selection of curves. Each curve has a different slope creating a different gain compensation.
Four adjustable factors are used to set the gain curve.
A Factor
Represents spreading loss.
B Factor
Represents one-way absorption loss.
C Factor
Represents a base or starting gain level.
L Factor
Limits the maximum gain to reasonable levels.
Simrad FM90i Reference Manual