Furuno GD-1700 Operator's Manual page 141

Virtual image
A relatively large target close to your ship may show 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.
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 or blind spot may be produced. Within this sector, targets
can not be detected.
Target ship
Own
ship
True
echo
Mirror image
of target ship
Virtual image
Radar
mast
Shadow sector
Shadow sectors
3. RADAR OPERATION
False
echo
Radar
antenna
3-47