Furuno CH-250 Manuel d'utilisateur télécharger pdf (Page 45)

2-27

Sidelobe echo (false echo)

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

Sidelobe

Mainlobe echo

Sidelobe echo

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.

Figure 2-25 Sidelobe echoes

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 rate on

the COM1 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 COM1 menu.

(A) Inteference from (B) Interference from

other sonars (yours marine life

or other's)

Figure 2-26 Noise and interference

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Furuno CH-250 Manuel d'utilisateur Page 45