Raymarine E90W User Reference Handbook page 142

• 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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Vessel vectors (CPA graphics) overview
CPA graphics show vectors for your vessel and a selected target.
A vector is a line on-screen showing the predicted courses of your
vessel and the selected target if you both remain on your present
course. These vectors vary in length due to vessel speed and vector
length set in the MARPA Setup menu.
True mode
With the display set in True mode, the vectors of your vessel and
the target are shown extended to their intersection point. The
CPA is shown as a line that is placed on your vessel's vector at
the point of the CPA. The length and direction of the line indicates
the distance and bearing of the target at CPA. The text indicates
CPA and TCPA. The text next to the target symbol indicates its true
course and speed.
E-Series Widescreen User reference