Furuno GP-3500F - Manual (Page 217)

11. VIDEO SOUNDER OPERATION
11-45
Unstable bottom echo
The sea bottom echo may be displayed jagged like a saw. This results from that
the beam direction and distance to the bottom changes by the ship's rolling and
pitching in stormy weather.
Unfixed
bottom echo
False image
Every time the ultrasonic pulse is transmitted, some radiation escapes on each
side of the beam, called “side lobes.” Echoes from side lobes show on the
display as false images as below.
False Image
Main Lobe
Side Lobe
False image
Interference, noise
As shown in the figure below, unwanted signals are displayed cyclically or at
random. The source of these unwanted signals may be induction from electrical
machinery, ship's vibration, engine noise, interference from other sounders being
operated nearby, etc. As a general rule, echoes from interference usually appear
below the bottom echo. If TVG is ON, echoes and interference will become
stronger as the depth becomes deeper. Further, if “TRANSMISSION POWER”
on the ECHO SOUNDER SYSTEM SETUP menu is set to “2. LOW”, echoes will
be displayed in weaker colors, but the color of interference remains the same so
you can easily distinguish interference from echoes. If interference is present on
the screen, lower the TX frequency or turn on the interference rejector.
Interference
from other
sounder
Electrical
interface Interference
from other
ship's sounder
Interference, noise