7. FISH FINDER (SOUNDER)
7-28
Unstable bottom echo
The bottom echoes can have a saw-tooth ap-
pearance. This occurs 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.
MainlobeMainlobe
SidelobeSidelobe
False image
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