Furuno FSV-25 Operator's Manual page 73

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 interval, with [TX Interval] on the [H-Scan Setting] 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 [Picture Setting] menu.
School of fish
Bottom
Ring of interference
(A) Interference from other
sonar (yours or others)
2. HORIZONTAL MODE
(B) Interference from marine life
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