Furuno FSV-30 Operator's Manual page 79

Sidelobe echoes (false echoes)
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 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.
Sidelobe
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 operated 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 INTERVAL on the H-SCAN SETTING 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 H-SCAN
SETTING menu.
(A) Inteference from
Mainlobe
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
other sonars (yours
or other's)
Noise and interference
2. HORIZONTAL MODE
Mainlobe echo
Sidelobe echo
(B) Interference from
marine life
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