Indirect Echoes; Blind And Shadow Sectors - Furuno FR-7062 Operator's Manual

4.3 Indirect Echoes

Indirect echoes may be returned from either a
passing ship or returned from a reflecting sur-
face on your own ship, for example, a stack. In
both cases, the echo will return from a legiti-
mate contact to the antenna by the same indi-
rect 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 obstruc-
tion but at the range of the legitimate con-
tact
• when plotted, their movements are usually
abnormal, and
• their shapes may indicate they are not direct
echoes.
Direct
path
Obstruction
(mast, funnel.
Antenna
etc.)
Target
Indirect
path
Direct
path
Own
ship
Bridge
Indirect
echo
Figure 4-3 Indirect echoes
4-2
Heading
line
Target
Indirect
path
True
echo
Indirect
echo
Heading
line
True
echo
Indirect
echo
4.4

Blind and Shadow Sectors

Funnels, stacks, masts, or derricks in the path
of antenna may reduce the intensity of the ra-
dar 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 tar-
gets at close range may not be detected while
larger targets at much greater ranges may be
detected. See Figure 4-4.
Vessel taller
than wharf
Wharf
Wharf
Blind sector
(no echo)
Figure 4-4 Blind and shadow sectors
Mast, etc. in
path of radar
beam
Size of blind sector
depends on target
size and range.