Furuno FSV-84 manual Sidelobe echoes false echoes, Noise and interference

2. HORIZONTAL MODE

2.23.5Sidelobe echoes (false echoes)

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 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.

Mainlobe echo

Sidelobe	Sidelobe echo
	Mainlobe
	The bottom echo
	detected by sidelobe
	appears at a certain
	tilt angle when the
	sidelobe points ver-
	tically. Also, poor
	soundome projection
	may result in a similar
	false echo.
	Sidelobe echoes

2.23.6Noise 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 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 H-SCAN SETTING menu.

(A) Inteference from	(B) Interference from
other sonars (yours	marine life
or other's)

Noise and interference

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