Indirect echoes, Side-lobe echoes – Furuno FMD-811 User Manual
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Indirect echoes
Indirect echoes may be returned from either
a passing ship or returned from a reflecting
surface on your own ship, for example, a
stack. In both cases, the echo will return from
a legitimate contact to the antenna by the
same indirect path. The echo will appear on
the same bearing of the reflected surface,
but at the same range as the direct echo. Fig-
ure 2-6 illustrates the effect of an indirect
echo. Indirect echoes may be recognized as
follows:
• they usually occur in a shadow sector
• they appear on the bearing of the obstruc-
tion but at the range of the legitimate con-
tact
• when plotted, their movements are usu-
ally abnormal, and
• their shapes may indicate they are not di-
rect echoes.
Figure 2-6 Indirect echoes
oes may be observed on the display at
double, triple or other multiples of the ac-
tual range of the target as shown in Figure
2-4. Multiple reflection echoes can be re-
duced and often removed by decreasing the
sensitivity or properly adjusting the STC.
Figure 2-4 Multiple echoes
Side-lobe echoes
Every time the scanner rotates, some radia-
tion 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-lobe echoes may
be represented on both sides of the true echo
at the same range, as shown in Figure 2-5.
Side-lobes show usually only at short ranges
and from strong targets. They can be reduced
through careful reduction of the sensitivity
or proper adjustment of the STC.
Figure 2-5 Side-lobe echoes
Own ship
Target
True
echo
Multiple
echo
Spurious
target
True echo
Main-lobe
Side-lobe
Scanner
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mark
True
echo
Indirect
echo
Heading
mark
True
echo
Indirect
echo
Indirect
path
Direct
path
Scanner
Obstruction
(mast, funnel,
etc.)
Own
ship
Target
Bridge
Direct
path
Indirect
echo
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Indirect
path