Kongsberg Simrad ST90, Reference Manual

Page: 471 / 690

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469
User
This setting allows you to define your own TVG curve. To choose your own TVG
curve, use the dedicated function in the
Echogram
dialog box.
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 2 where
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
4 . In logarithmic
terms this is
10 log r
4 or
40 log r
. This compensation applies to single targets.
When a school of fish covers the whole acoustic beam cross section, the received echo will
increase with range simply because the acoustic beam cross section increases with range.
The received echo increases according to
r 2 , where r is now the radius of the acoustic beam.
This introduces an r 2 into the-two way spreading ( r 2 /r 4 ) so that the school compensation
becomes r 2 , or in logarithmic terms 20 log r .
In addition to the geometrical spreading, sound energy is absorbed in water. Absorption is
higher in salt than in fresh water and increases with the operating frequency. The received
echo must also be compensated according to the absorption.
The combined compensation for the geometrical spreading and absorption is the TVG
Related topics
Controlling the gain and range settings, page 94
Functions and dialog boxes, page 416
Active menu; Functions and dialog boxes, page 466
Ping-Ping Filter function
The
Ping-Ping Filter
analyses the historical information from previous consecutive pings in
order to remove unwanted noise and interference from the echo presentation.
How to open
You open this function from the
Active
menu.
Functions and dialog boxes