Simrad FM90, Reference Manual

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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 Sonar Head 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 range 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 compensation is designed to counteract the natural phenomena of geometric
spread and absorption loss. This is done in the FM90 using digital signal processing. The
TVG equalizes the echo presentation as a function of range to make the targets with the
same strength appear with the same intensity independent of their physical distance from
the Sonar Head.
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. The TVG curve can be selected
with several different slopes, each having different gain regulations. 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.
To the left of the TVG curve display is a slider with a range from -30 to 130. This represents
the system gain. Moving this slider down until the line intersects the green TVG curve will
Functions and dialog boxes