Furuno GP-3700F, Operator'S Manual

Page: 182 / 224

11. ECHO SOUNDER
11-34
Unstable bottom echo
The bottom echoes can have a saw-tooth appearance. This oc-
curs in heavy weather because pitching and rolling changes the
direction of the ultrasonic pulses and the vertical motion of the
ship causes the distance to the bottom to change.
False echo
When an ultrasonic pulse is transmitted, some energy leaves
from each side of the beam. This energy has the name “sidelobe”.
Echoes from sidelobes show on the screen as false images like in the illustration
shown below.
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 near-
by, 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 be-
comes deeper. If interference is present on the screen, lower the TX frequency or turn
on the interference reduction (see section 11.7).
Unstable
bottom echo
Mainlobe Mainlobe
Sidelobe Sidelobe
False image
Interference
from other
echo sounder
Interference
from other
ship’s echo
sounder
Electrical
interference