Simrad GO XSE Series Operator's Manual page 128

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Red (C1/C2 and D1/D2), indicating the target is on an
•
approaching path to own vessel. Its relative speed at that point is
greater than the VelocityTrack speed threshold.
Not colored (C3 and D3/D4), indicating it is temporarily neutral
•
because its relative speed at that point is less than the
VelocityTrack speed threshold.
Blue (C4/C5 and D5), indicating the target is diverging away
•
from own vessel and its relative speed at that point is greater
than the VelocityTrack speed threshold.
The radar palette
Different colors (palettes) can be used to represent detail on your
radar panel.
Orientation
Radar orientation is indicated on the upper left corner of the radar
panel as either HU (Heading UP), NU (North Up) or CU (Course up).
Head-up
In head-up mode, the heading line on the PPI is oriented on the 0°
on the bearing scale and towards the top of the screen. The radar
image is displayed relative to own ship, and when the ship turns the
radar image rotates.
Ú Note:
Head-up is only available in relative motion mode, and it
is the only orientation mode available if the radar is not
connected to a heading source.
North up
In north up mode, the 0° indication on the PPI represents north. The
heading line on the PPI is oriented according to own ship heading
obtained from the compass. When the ship turns the heading line
changes its direction according to the ship's heading, while the
radar image remains stabilized.
The north up orientation is not available if no heading source is
connected to the radar. If heading data is lost, the system will
automatically switch to head-up orientation.
Course up
In course up mode, the top of the bearing scale indicates the ship's
true course measured from north at the time course up was
Radar | GO XSE/XSR Operator Manual