convert the relative bearing to a true bearing. vessel’s course and speed through the water
Care must be taken to reconcile features on the You can use this difference in movement on
screen with those on the chart. And the ap- your screen to distinguish between station-
parent directions to objects will be displaced ary objects useful to navigation, and moving
by half the radar’s beam width. The best radar objects that can pose risks of collision. In
LOPs are arcs based on radar ranges. order to decide upon a safe action, it is es-
To aid radar navigation, important objects sential to know the other vessel’s heading
such as major lights or fairway buoys are fit- and speed. To do this we plot a combination
ted with Racons. As discussed, these produce of our own movement and the other ship’s
an identifying flash on the radar screen along relative movement.
with an enhanced blip. To allow time to plot and assess which action
Some radar presentations are now oriented to take, the plot should start when the echo is
to compass directions rather than to vessel some distance from the center (your vessel).
heading, usually with north at the top of the The triangle of motions is often named by
screen. As the vessel yaws, the heading mark- the points ‘W’, ‘O’, and ‘A’. A series of ranges
er moves while the picture is stable. Another and bearings are taken in a time interval of
new feature is true motion, where the center six minutes (1/10th hour).
spot moves across the screen at your vessel’s The first position is named O and the time
same course and speed. Chart-plotters can noted. After the six minutes, the range and
also superimpose a radar image onto the bearing is taken and marked A. This O-A line
chart display, allowing you to match up radar is projected past the center to show the closest
returns to charted buoys and landmarks. point of approach (CPA). A CPA of less than
The Radar Plot
one mile should be considered potentially
dangerous due to the small scale of the radar
Most small-craft radars default to a “ship’s
picture and the inaccuracy of radar bearings.
head-up” display. The heading marker is al-
In the plotting interval of six minutes, your
ways at the top of the screen and any change
own vessel will have moved over a known
in the craft’s heading shows as rotating move-
course and distance (1/10th of the speed). This
ment of the radar picture. When plotting in
direction and distance is laid back from O and
this mode you need to convert the relative
named W. W-O is “Way of Own Ship.” Join W
adar
bearings into compass bearings. The relation-
to A. This represents the course and speed of R
ship between the two changes, of course, with
the vessel. W-A represents “Way of Another.”
each course change. Failure to make this ad-
Use arrows on each line to indicate direction.
justment is a frequent cause of mistakes.
With this information, one can consider
You should take bearings of objects with
evasive maneuvers. Different options may be
ion
the cursor through the center of the echo
applied to the plot to see the possible effect
and, when taking ranges, use the inner edge
before the alteration is made. Alternatively, a
of the range marker through the inner edge
safe distance can be decided upon and taken
of the echo. At the moment of bearing, note
a
v
i
gat
back to the plot (a new O/A line) to deter-
N
the heading along with the time. The accura-
mine the required alteration.
cy of the plot is only as good as the accuracy
To apprise the situation visibly, sketch in
with which the ship’s head was noted.
a boat shape at the center of the plot in the
When your vessel is stopped, all ranges and
direction of your own heading and another
bearings produce a “true motion picture.”
shape at A in the direction W-A (the other
All moving objects show the real direction
vessel’s heading).
of their movement; stationary objects do
With practice and reasonable accuracy,
not move. When your vessel is moving, all
the radar plot gives a good general idea of
stationary objects appear to move in a direc-
the situation, assuming the other vessel has
tion opposite to your movement and at your
maintained a steady course and speed. It is
speed. A harbor entrance, for example, will
important to continue plotting an approach-
appear to come toward you on the screen.
ing echo until it is past and clear, especially
All echoes from moving vessels move across
after you have made an alteration, to see
the screen in a direction and rate determined
what effect has been made to the CPA.
by your own course and speed and the other
851
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