False Echoes - Furuno FR1500 Mk3 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.
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.
Target ship
Own
ship
Mirror image
of target ship
Virtual image by mirror effect
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.
True
echo
False
echo
Shadow sector by stern block
-3.3-
Radar Radiator
Radar mast