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4. INTERPRETING THE DISPLAY
4-5
4.5.3 Surface noise
When the sea is rough or the ship passes over a wake, surface noise may
appear at the top of the screen. It can be suppressed with the CLUTTER
function.
Surface Noise
Surface noise
4.5.4 Aerated water
When the sea is rough or the ship makes a quick turn, gaps in the bottom echo
on the screen may appear. This is caused by air bubbles which block
propagation of the sound wave. Generally low frequency ultrasonic waves are
interrupted more easily than high ones.
Ultrasonic wave
blocked by
aerated water
Aerated water
4.5.5 False image
Every time the ultrasonic pulse is transmitted, some radiation escapes on each
side of the beam, called “side lobes.” Echoes from side lobes show on the
display as false images as below.
False Image
Main
Lobe
Side
Lobe
False image