Sidelobe Echoes; Virtual Image - Furuno 1815 Operator's Manual

3. HOW TO INTERPRET THE RADAR DISPLAY
3.2.2

Sidelobe echoes

When the radar pulse is transmitted, some radiation escapes on each side of the
beam, called "sidelobes". If a target is where a target can be detected by the sidelobes
as well as the mainlobe, the side echoes can be shown on both sides of the true echo
at the same range. Sidelobes show normally only on short ranges and from strong tar-
gets. You can reduce the sidelobes with the sea clutter function.
Sidelobe
3.2.3

Virtual image

A large target close your ship can appear at two positions on the screen. One of them
is the true echo reflected by the target. The other is a false echo which is caused by
the mirror effect of a large object on or close your ship as shown in the following figure.
If your ship comes close to a large metal bridge, for example, a false echo can tempo-
rarily appear on the screen.
Mirror image
of target ship
3-4
Mainlobe (beam)
Sidelobe
Antenna
Target ship
Route for direct reflection
Your ship
Bridge
Route for
indirect reflection
True target
False echoes by sidelobes
True
echo
False
echo