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2. HORIZONTAL MODE
2-29
Sidelobe echo (false echo)
An ultrasonic wave is emitted in the direction set by the [TILT] control (main beam),
however there are some emissions outside the main beam. These are called sidelobes.
The energy 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
represented on the display as a false echo as shown below.
Mainlobe
Sidelobe
Mainlobe echo
Sidelobe echo
The bottom echo
detected by sidelobe
appears at a certain
tilt angle when the
sidelobe points ver-
tically. Also, poor
soundome projection
may result in a similar
false echo.
Sidelobe echoes
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, interference from the sonar on other boats will show itself on the display as
in (A) in the figure below. This interference can be suppressed by changing the Tx rate on
the COM1 menu. Noise from marine life shows itself on the displays as in (B) in the figure
below. This type of noise can be suppressed with the interference rejector on the COM1
menu.
(A) Inteference from (B) Interference from
other sonars (yours marine life
or other's)
Noise and interference