False Echo By Sidelobe; Noise And Interference - Furuno CSH-5 MARK-2 Operator's Manual

False Echo by Sidelobe

In the preceding chapters, it was explained that an ultrasonic wave is
emitted only in the direction set by the TILT lever, but, in practice,
there are some emissions outside the main beam that are called
"sidelobes". Energy of the sidelobe is fairly weak but when the sonar
is used in comparatively shallow water with a hard and rocky bot-
tom, strong target signals are detected by the sidelobe. These are rep-
resented on the screen as a false echo as shown below. To weaken the
sidelobe echoes, set the VER BEAM ANGLE to WIDE on the SCAN
MENU.

Noise and Interference

In case the fishing ground is crowded with many fishing boats, the
sonar is subject to interference from ultrasonic equipment such as an
echo sounder, sonar, etc. on board other boats as well as those on
board own ship.
For instance, interference from the sonar operated on board other
boats will appear as a ring as shown in (A). This interference can be
suppressed by properly changing TX cycle. Electrical equipment on
own ship can also cause interference to the sonar as shown in (B).
The noise from some marine life appears on the screen as in (C). This
noise can be suppressed by the IR control.
(A)
Noise and Interference
Mainlobe echo
Sidelobe echo
The seabed echo
detected by sidelobe
appears at a certain tilt
angle when the sidelobe
points vertically.
(B)
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(C)