False Echo By Sidelobe; Noise And Interference - Furuno FSV-84 Operator's Manual

4.22.5

False echo by sidelobe

An ultrasonic wave is emitted only in the direction set by the TILT lever, 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.
4.22.6

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).
Bottom
4. SLANT MODE
4-47