Onwa Marine Electronics KR-12X8 - Manual (Page 46)

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 4-3 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.
4.4 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 4-4.
Figure 4-4 Blind and shadow sectors
Vessel taller
than wharf
Mast, etc. In
path of radar
beam
Wharf
Wharf
Blind sector
(no echo)
Size of blind sector
depends on target
size and range.
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