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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 COM1 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
to select the COM1 menu.
3.Press or
to select PULSELENGTH and press
. The following dialog box appears.
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.
Without TVG Nearer target | Reduce | |
echoes appear | ||
in larger size. | gain for | |
Noise caused by | nearby | |
areas, | ||
strong reflection | ||
using | ||
of surface | ||
TVG. | ||
turbulence. | ||
|
Proper TVG setting
Echoes of equal size targets are presented in the same color and size. Noise is rejected.
Figure 2-14 How TVG works
Note: Excessive TVG may eliminate