Noise And Interference - Furuno CH-250 Operator's Manual

Sidelobe echo (false echo)
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 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.
Mainlobe
Figure 2-25 Sidelobe echoes
(A) Inteference from
other sonars (yours
or other's)
Figure 2-26 Noise and interference
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.
(B) Interference from
marine life
2-27