Raymarine E Series User Reference Handbook page 143

Safety notices
MARPA can improve collision avoidance when used wisely. It is
your responsibility to exercise common prudence and navigational
judgement.
There are conditions where acquiring a target may become difficult.
These same conditions may be a factor in successfully tracking a
target. Some of the conditions are:
• The target echo is weak. The target is very close to land, buoys
or other large targets.
• The target or your own vessel is making rapid manoeuvres.
• Choppy sea state conditions exist and the target is buried in
excessive sea clutter or in deep swells.
• Choppy sea state conditions exist yielding poor stability; own
vessel's heading data is very unstable.
• Inadequate heading data.
Symptoms of such conditions include:
• target acquisition is difficult and the MARPA vectors are unstable;
Using radar
• the symbol wanders away from the target, locks-on to another
target, or changes to a lost symbol target.
In these circumstances, target acquisition and tracking may need to
be re-initiated and in some cases might be impossible to maintain.
Better quality heading data might improve performance in these
circumstances.
How a MARPA risk is assessed
Each target is monitored to ascertain whether it will be within a
certain distance from your vessel within a certain time. If so, the
target is designated as dangerous, and an audible warning is
sounded and a warning displayed. The target symbol changes to
the dangerous target symbol and flashes to indicate that it is a
dangerous target. Selecting the appropriate softkey will silence the
alarm and remove the warning.
If a target is lost, either because the MARPA software has lost
contact with it, or because it has moved out of range, an audible
alarm is sounded and an on-screen warning appears. The
on-screen symbol will change to the target lost symbol. Selecting
the appropriate softkey will silence the alarm and remove the
on-screen warning and the target lost symbol.
Effective range for MARPA targets
MARPA target acquisition is only available at radar range scales of
up to 12 nm, although tracking continues at all ranges.
If you change to a smaller range scale, targets may be beyond the
range of your radar scanner and will be lost. In such cases, an
on-screen warning indicates that the target is off-screen.
Status symbols for MARPA targets
Each target is shown in the radar application as a symbol, indicating
its status:
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