Raymarine a Series Installation And Operation Instructions Manual page 214

Blind Sectors
Obstructions such as funnels and masts near the radar antenna
may obstruct the radar beam and cause radar shadows or
'blind sectors'. If the obstruction is relatively narrow, there will
be a reduction of the beam intensity, though not necessarily a
complete cut-off. However, for wider obstructions there may
be a total loss of signal in the shadow area. There may also
be multiple echoes which extend behind the obstruction. Blind
sector effects can normally be minimized by careful selection of
the scanner site prior to installation.
Rain or Snow Clutter
The radar can see echoes from rain or snow. Returns from
storm areas and rain squalls consist of countless small echoes
that continuously change size, intensity and position. These
returns sometimes appear as large hazy areas, depending on
the intensity of the rainfall or snow in the storm cell. The images
in the table below show how the Rain control can clear up this
clutter:
Rain
clutter off
Rain
clutter on
Sea Clutter
Radar returns from waves around the vessel can clutter the
centre of the radar picture, making it difficult to detect real
targets. Such 'sea clutter' usually appears as multiple echoes
on the display at short range scales, and the echoes are
not repetitive or consistent in position. With high winds and
extreme conditions, echoes from sea clutter may cause dense
background clutter in the shape of an almost solid disc. Sea
clutter can be suppressed using the sea clutter settings. The
images in the table below show how the sea clutter settings can
clear up some of this clutter:
212
Sea
clutter off
Sea
clutter in
auto
Interference
When two or more radar-equipped vessels are operating within
range of each other mutual radar interference can occur. This
usually appears as a spiral of small dots from the display centre
This type of interference is most noticeable at long ranges.
This interference can be suppressed using the interference
rejection settings. The images in the table below show how
the Interference rejection settings can clear up some of this
interference:
Interfer-
ence re-
jection off
Interfer-
ence re-
jection on
a Series / c Series / e Series