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2. HORIZONTAL MODE
2-14
2.12.2 Suppressing bottom tail
As described earlier, fish schools near the bottom are sometimes difficult to detect because
you have to discriminate fish echoes from the bottom reflections. To discriminate fish
echoes near the bottom, choose the short Tx pulselength on the COM1 menu to decrease
the tail of bottom reflections.
1. Press the MENU key to open the menu.
2. Press ▲ to choose MENU and then press ◄ to choose the COM1 menu.
3. Press ▲ or ▼ to choose PULSELENGTH and then press ►. The following dialog box
appears.
PULSELENGTH
LONG
SHORT
4. Press ► to choose SHORT.
5. Press the MENU key to register your selection and close the menu.
2.12.3 Displaying weak echoes clearly
Echoes from targets (such as the bottom or a fish) return to the transducer in order of the
distance to them, and when their intensities are compared at the transducer face, those
from nearer targets are generally stronger when their reflecting properties are nearly equal.
The sonar operator will be quite inconvenienced if these echoes are directly displayed on
the screen, since he won’t be able to judge the actual size of the target from the size of
echoes displayed on the screen. To overcome this inconvenience, use the TVG function. It
compensates for propagation loss of sound in water: amplification of echoes on short range
is suppressed and gradually increased as range increases, so that similar targets are
displayed in similar intensities irrespective of the ranges to them.
The TVG also functions to suppress unwanted echoes and noise which appears in a certain
range area on the screen.
Without TVG
Proper TVG setting
Noise caused by
strong reflection
of surface
turbulence.
Nearer target
echoes appear
in larger size.
Reduce
gain for
nearby
areas,
using
TVG.
Echoes of equal
size targets are
presented in the
same color and size.
Noise is rejected.
How TVG works
Note: Excessive TVG may eliminate short-range echoes.