Furuno GP-1670F Operator's Manual page 106

7. FISH FINDER OPERATIONS
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.
7-26
Mainlobe Mainlobe
Sidelobe Sidelobe
False image