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2. SONAR MODE
2.9.5
Wake
A wake produced by own ship or another ship can be a strong reflecting object when
a narrow tilt angle is used. As the wake appears as a thick continuous line, it can be
easily distinguished from a school of fish. A wake contains many air bubbles which at-
tenuate ultrasonic energy, making it difficult to sound beyond the wake.
Own and other ship
Own ship
Tilt: 0° to 5°
2.9.6
False echo by sidelobe
An ultrasonic wave is emitted only in the direction set by the TILT control, however
there are some emissions outside the main beam. These are called sidelobes. The en-
ergy of the sidelobe is fairly weak but when the water is comparatively shallow and the
bottom is rocky and hard, strong signals are detected by the sidelobe. These are rep-
resented on the display as a false echo as shown below.
Sidelobe
The bottom echo detected by sidelobe appears at a certain tilt angle when the sidelo-
be points vertically. Also, poor soundome projection may result in a similar false echo.
2.9.7
Noise and interference
When the fishing ground is crowded with
many fishing boats, the sonar is subject to
interference from ultrasonic equipment
(echo sounder or sonar) on other boats as
well as those on own ship. For instance, in-
terference from the sonar operated on other
boats will show itself on the display as in (a).
This interference can be suppressed by
changing the Tx rate on the SONAR menu.
Noise from marine life shows itself on the
displays as in (b). This type of noise can be suppressed with "IR" in the SONAR menu.
2-12
s screw noise
'
Other ship
Own ship's
screw noise
Wake prodused by
other ship
Mainlobe
Own ship
s screw noise
'
Own ship's wake
Mainlobe echo
Sidelobe echo
Sidelobe echo
Sidelobe echo
b) Interference
a) Interference
from marine life
from other sonars
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