Suppressing Bottom Tail - Furuno CH-250 Operator's Manual

2.11.2 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, select the short Tx pulselength on the COMN menu to decrease the tail of bottom
reflections.
1. Press the [MENU] key to open the menu.
2. Press to select MENU and then press
3. Press or to select PULSELENGTH and press
PULSELENGTH
LONG SHORT
4. Press to select SHORT.
5. Press the [MENU] key to register your selection and close the menu.
2.11.3 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.
Noise caused by
strong reflection
of surface
turbulence.
Note: Excessive TVG may eliminate short-range echoes.
Nearer target
Without TVG
echoes appear
in larger size.
Figure 2-14 How TVG works
to select the COMN menu.
. The following dialog box appears.
Proper TVG setting
Reduce
gain for
nearby
areas,
using
TVG.
Echoes of equal
size targets are
presented in the
same color and size.
Noise is rejected.
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