Furuno FCR-2107 series, Operator'S Manual

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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.
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 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.
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.