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3. Radar Observation
3-3
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.
Own ship
Target
Tr ue
echo
Multiple
echo
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.
Spurious
target
True echo
Main-lobe
Side-lobe
Antenna
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
that 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.
Tr ue
echo
False
echo
Own ship
Target ship
Mirror image
of target ship
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 may be
produced. Within this sector targets can not be detected.
Radar
antenna
Radar
mast
Shadow sector