Furuno FSV-85 Operator's Manual page 124

4. SLANT MODE
False echo by sidelobe
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 en-
ergy 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 rep-
resented on the display as a false echo as shown below.
Side lobe
Noise and interference
Interference from a sonar on another ship appears on the screen as shown in (A). (B)
shows Interference from electrical equipment on board own ship. Noise from marine
life appears on the display as shown in (C).
(A) Interference
from other sonar
4-36
Bottom
Bottom Bottom
False echo
Main lobe
Sea surface
reflections
(B) Electrical
interference
(C) Noise from
marine life