Furuno FR-8122 EN - Manual (Page 79)

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.
2.2.1 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.
Multiple echoes
2.2.2 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 sidelobes 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.
Sidelobe echoes
Own ship
Target
True
echo
Multiple
echo
Sidelobe
True target