Kongsberg Simrad SN90 Reference Manual page 467

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.
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 SN90 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.
User
This setting allows you to define your own TVG curve. To choose your own TVG
curve, use the dedicated function in the
Additional information for the technically minded
When a sound pulse is travelling from the transducer through the water it will undergo
geometrical spreading. The sound intensity falls off with the range squared (1/r
r=range). The same applies to the echo from a target in the water column; sound intensity
falls off with 1/r 2 . The total two-way spreading is therefore:
1/r 2 x 1/r 2 = 1/r 4
To remove this effect, the received echo must be compensated according to r
logarithmic terms this is 10 log r
381294/E
Echogram
4 or 40 log r. This compensation applies to single targets.
Functions and dialog boxes
dialog box.
2 where
4 . In
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