24
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
scanner 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.
Figure 3-7 Blind and shadow sectors
3.6 Nautical Chart and RadarPicture
Under normal conditions, a picture which
is similar to a nautical chart can be obtained
on the radar display. However, a radar can-
not:
• show targets which are below the hori-
zon.
• show a target which is hidden by a larger
one.
• see around corners (for example, sea-
wall).
• distinguish between two targets which are
very close together, either in range or
bearing. For example, a vessel towing
another will probably appear as one ves-
sel, as both will be covered by the beam
at the same time.
The nautical chart and radar picture shown
in Figure 3-8 are from the Kada Inland Sea
in southwestern Japan.
Figure 3-8 Nautical chart and associated
radar picture
Vessel taller
than wharf
Wharf
Wharf
Blind sector
(no echo)
Mast, etc. in path
of radar beam
Size of blind sector
depends on target
size and range.
Actual topography Radar display
Black areas are
strong reflection
targets. Coastline
difficult to
display.