Raymarine e7 User Reference page 139

Indirect Echoes
There are several types of indirect echoes or ghost images. These
sometimes have the appearance of true echoes, but in general they
are intermittent and poorly defined.
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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.
Using radar
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Description
False echo
True echo
Passing ship
Mast or funnel
True echo
False echo
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D1641-5
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.
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Description
True echo
Multiple echoes
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