Furuno BB FAR28x7 - Manual (Page 127)

2. RADAR OBSERVATION
2-3
2.2 False Echoes
Occasionally echo signals appear on the screen at positions where there is no
target or disappear even if there are targets. They are, however, recognized if
you understand the reason why they are displayed. Typical false echoes are
shown below.
Multiple echoes
Multiple echoes occur when a transmitted pulse returns from a solid object like a
large ship, bridge, or breakwater. A second, a third or more echoes may be
observed on the display at double, triple or other multiples of the actual range of
the target as shown below. Multiple reflection echoes can be reduced and often
removed by decreasing the gain (sensitivity) or properly adjusting the A/C SEA
control.
Own ship
Target
True
echo
Multiple
echo
Multiple echoes
Sidelobe echoes
Every time the radar pulse is transmitted, some radiation escapes on each side
of the beam, called “sidelobes.” If a target exists where it can be detected by the
side lobes as well as the main lobe, the side echoes may be represented on
both sides of the true echo at the same range. Side lobes show usually only on
short ranges and from strong targets. They can be reduced through careful
reduction of the gain or proper adjustment of the A/C SEA control.
Target B
(True)
Target B
(Spurious)
Target A
Sidelobe echoes