SONAR User Manual

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