Kongsberg Simrad ST90, Reference Manual

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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.
Details
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 ST90 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.
40 log R
This setting offers larger amplification closer to the bottom. It is commonly used to
provide the best echoes from single fish.
Other options are provided when this function us used in an Inspection view. The echo
presentation in the Inspection view resembles the echogram you see on an echo sounder. The
Inspection view can be regarded as an "echo sounder" with an adjustable direction. In the
ST90 system we use the phrase echogram to describe the information provided by the view.
No TVG
TVG compensation is not implemented. This option is hardly ever used.
School
This setting offers weaker amplification close to the bottom. It has been designed
to provide the best echoes for schools. The physical size of a school allows you to
detect it even with reduced gain.
Fish
This setting offers larger amplification closer to the bottom. It has been designed to
provide the best echoes from single fish.
Simrad ST90 Reference Manual