Furuno FR-1500 - Manual (Page 81)

-3.3-
Side lobe echoes
Every time the radar pulse is transmitted, some radiation escapes on each side of the beam, called
“side lobes.” 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.
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 by mirror effect Shadow sector by stern block
Shadow sectors
(see above right)
Funnels, stacks, masts, or derricks in the path of the scanner block the radar beam. If the angle
subtended at the scanner is more than a few degrees, a non-detecting sector may be produced.
Within this sector targets can not be detected.
Radar Radiator
Radar mast