Furuno 1830 - Collision Avoioance

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