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2-1
2. FINE TUNING THE SONAR PIC-
TURE
2.1 Eliminating Unwanted Echoes
Echoes from targets such as bottom and fish return to the transducer in order of dis-
tance to them, and when we compare their intensities 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 dis-
played on the screen, since the actual size of the target cannot be judged from the size
of echoes displayed on the screen. To overcome this inconvenience, the TVG function
is incorporated. It compensates for propagation loss of sound in water; amplification
of echoes on short rang is suppressed and gradually increased as range increases so
that similar targets are displayed in the similar intensities irrespective of the ranges to
them.
The CSH-5L/CSH-8L incorporates two TVG functions, NEAR and FAR, and they
mainly compensate for propagation loss on short and long ranges respectively, cen-
tered at the ranges shown below.
How TVG works
The TVG is also used to suppress unwanted echoes and noise which appear in a cer-
tain range area on the screen such as sea surface reflections and cruising noise. To
obtain the proper TVG setting, follow the procedure below.
1. Press the MENU key.
2. Use the RANGE control to choose [MENU MODE] at the top of the screen.
3. Use the GAIN control to choose [SONAR] to display the [SONAR] menu.
4. Use the RANGE control to choose [TVG NEAR].
5. Use the GAIN control to adjust gain.
6. Use the RANGE control to choose [TVG FAR].
7. Use the GAIN control to adjust [TVG FAR] (setting range: 0-10).
8. When sea surface reflections or plankton layers disturb the picture, adjust [TVG
NEAR] to eliminate them. They will be eliminated by decreasing the setting by "1"
or "2."
9. On a long range, locate a fish school which is approaching own ship. Note that the
tilt should be kept adjusted so that the fish school is always placed in the center
of the sonar beam, i.e., so that the fish school is displayed in strongest colors pos-
sible. Check that the fish echo is displayed in the same color while it approaches.
NEAR
FAR
300 m