False Echo By Sidelobe; Noise And Interference - Furuno CSH-5L/CSH-8L Operator's Manual

6. INTERPRETING THE DISPLAY
6.5

False Echo by Sidelobe

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 bottom, strong target signals
are detected by the sidelobe. These are represented on the screen as a false
echo as shown below. To weaken the sidelobe echoes, set VER BEAMWIDTH to
WIDE, on the SONAR menu.
Sidelobe
6.6

Noise and Interference

If 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 the 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).
Those types of noise can be suppressed with the interference rejector.
(A) Interference from
other sonar
6-4
Mainlobe echo
Sidelobe echo
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 echo
(B) Electrical
interference
Noise and interference
(C) Marine life noise