False Echoes - Furuno FR-2105 Operator's Manual

3.2

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.
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.
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 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.
Target ship
Own
ship
True
echo
Mirror image
of target ship
Virtual image by mirror effect
False
echo
-3.3-
Radar
antenna
Radar
mast
Blind zone
Blind zone by stern block