Kongsberg Simrad SN90 Reference Manual page 630

Simrad SN90 Reference Manual
The loss of energy is explained with two factors.
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 acoustic 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. TVG (Time Variable Gain) compensates for this loss of acoustic energy.
The TVG compensation is expressed as a logarithmic curve. Each curve is identified with
the equation X log TVG. The coefficient "X" is an integer. Each value for "X" will create
a different slope with a different gain compensation. Typical values for "X" are 10 to 40.
The drawing illustrates the TVG principle: The gain at time (B) is larger than at time
(A). The different settings determine how much gain the SN90 system will use as time
elapses along the X-axis. A higher value for "X" will make the angle (C) steeper.
The SN90 system allows you to choose your own compensation. You can choose any
value from 0 Log R to 40 Log R. If you choose 0 Log R the TVG compensation is
disabled. Some experimentation may be required to find the compensations that fit
your purposes for individual catch and environmental conditions. Typical time variable
gain compensation values may be:
0 log R
TVG compensation is not implemented. This option is hardly ever used.
20 log R
This setting offers weaker amplification close to the bottom. It is commonly used
to provide the best echoes for schools. The physical size of a school allows you to
detect it even with reduced gain.
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