CR40
42
/50 MKII Radar operation
Chapter 7
83
Buoys are moored to the bottom with concrete via chain. The chain is
longer than the depth of the water to allow the buoys to ride in the
current and go up and down with the tide. Unfortunately this allows the
buoys to lean in the direction of
the current.
Radar reflectors built into
the buoy do not work well when the side of the buoy is pointed to the
sky. Therefore if you are going with the current, the images of the
channel markers may appear faint. If you are going against the current,
the buoys will be leaning in your direction and produce a stronger
image on the radar screen.
The iron mass and angles of the metal in the structure of a
bridge
can
cause unpredictable interference patterns on your radar. It is not unusu-
al for a reflected image to appear on the radar screen in front of you
just as you pass under the bridge. A similar effect is also common on
sailboats where the radar antenna is mounted close to an aluminum
mast.
Overhead cable crossings
can mimic a moving target on your radar
screen. The cable target usually appears to be on a collision course.
The entire length of cable does not appear on the screen, only a point
on the cable, and that point keeps changing giving the illusion of a
moving target.
7.1.3 Effects of ship’s movement
Radar images can be drawn in two ways on the radar display to show
the ship’s motion. The type of display modes are called “Relative
motion” and “True motion”. In Relative motion, the most common
radar display mode, own ship is permanently fixed at the center of the
display but radar echoes (targets) move in relation to your vessel. With
no movement of the ship, a steady display of fixed radar echoes is
shown. If the ship is moving ahead on a constant course, echoes
appearing at the top of the display will move downward across the
display. Your own ship’s position will always remain at the center of
the display.
If the vessel alters course to the right, the displayed echoes will be
displaced by an equal amount in bearing in a counterclockwise
direction, and vice versa. These changes in the display pattern with
ship movement is an extremely important characteristic to remember
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