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COLLISION AVOIDANCE
ColHsions
at
sjoa
somotime'^
oceur
becouse
the
radar*
piclui'G
doean't
molch
Ihe
information
providcd
by
Ihe eye in clear wealher and
because of the
misunderslanding of relative motion.
In a
relative motion
display your ship is represented by the spot of light
l'ixed at
the
center of the screen,
whatever the
speed of your own ship.
With
both your own
ship and the target in motion,
the successive pips of
the
target do not indicatc
the
actual or true
movement
of tlie target.
If own
ship is in motion,
the pips of fixed objects,
such as landmasses,
move on
the display at a rate equal to and in a direction opposite to the motion of
own
ship.
Only
when
your
ship is
stopoed
or motionless do target pips
move on the display in accordance with their true motion.
Fig.48 Ulustrates
the relative and true motion of a target contacted by radar.
In Fig.48,
ship
A,
at
geugraphic
position
Al
on
true course
of 001® at
14
knots initially observes ship B on the PPI at bearing 179° at 4.1 nm.
The
bearing and distance to the ship changes as ship A proceeds from position
Al
to A3.
The
changes
in
the
position
of
ship
B
relative
ship
A
nre
illustrated
in
the
successive
PPI
presentations
corresponding
to
the
geographic
positions
of
ships
A
and
B.
Likewise,
ship
B at geographic
position
Bl,
on
true
course
25°
at 21
knots initially observes
ship
A on
bearing ooi® at 4,i nm.
The radar operator aboard ship A will determine that relative movement of
ship
B
is
approximately
66.5°,
whereas
the
operator
aboard
ship
B
will
determine that the relative movement of ship A is approximately 238°.
These
figures were obtained using a maneuvering board.
Fig.48
Relative Motion vs. True Motion
Assessing the Risk
The moment
an echo appears on the screen its range and relative bearing
should be measured and its true or magnetic bearing noted.
This is best
done on a chart or ptot.
Coilision nsk can be assessed by carefully watch-
ing the true or magnetic bearing of an approaching vessel.
If the bearing
of the target does not appreciably change a possibility of coilision may exist.
You
should
take
proper
action
in
accordance
with
the
Regulations
for
Preventing Collisions at Sea.
47
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