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 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. |
|
|
|
|
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) |
|