Raymarine C140w User Reference Handbook page 103

Multiple Echoes
Multiple echoes are not very common but can occur if there is a large
target with a wide vertical surface at a comparatively short range.
The transmitted signal will be reflected back and forth between the
target and your own ship, resulting in multiple echoes, displayed
beyond the range of the true target echo, but on the same bearing.
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.
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.
Using radar
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.
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