Furuno 851 MARK-2 - Manual (Page 34)

3-4
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. Figure 3-6
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
obstruction but at the range of the
legitimate contact
• when plotted, their movements are
usually abnormal, and
• their shapes may indicate they are not
direct echoes.
Heading
marker
Indirect
echo
True
echo
Heading
marker
Indirect
echo
True
echo
Direct
path
Target
Indirect
path
Obstruction
(mast, funnel,
etc.)
Antenna
Bridge
Indirect
echo
Target
Indirect
path
Direct
path
Own
ship
Figure 3-6 Indirect echoes
Blind and shadow sectors
Funnels, stacks, masts, or derricks in the
path of antenna may reduce the intensity of
the radar beam. If the angle subtended at
the antenna is more than a few degrees a
blind sector may be produced. Within the
blind sector small targets at close range
may not be detected while larger targets at
much greater ranges may be detected. See
Figure 3-7.
Vessel taller
than whart
Wharf
Wharf
Blind sector
(no echo)
Mast, etc. in path
of radar beam
Size of blind sector
depends on target
size and range.
Figure 3-7 Blind and shadow sectors