Furuno FAR-2107(-BB) - Manual (Page 134)

2. RADAR OBSERVATION
2-4
Virtual image
A relatively large target close to your ship may be represented at two positions
on the screen. One of them is the true echo directly reflected by the target and
the other is a false echo which is caused by the mirror effect of a large object on
or close to your ship as shown in the figure below. If your ship comes close to a
large metal bridge, for example, such a false echo may temporarily be seen on
the screen.
True
echo
False
echo
Own ship
Target ship
Mirror image
of target ship
Virtual image
Shadow sectors
Funnels, stacks, masts, or derricks in the path of the antenna block the radar
beam. If the angle subtended at the antenna is more than a few degrees, a
non-detecting sector may be produced. Within this sector targets can not be
detected.
Radar
antenna
Radar
mast
Shadow sector
Shadow sectors