Collision Avoidance And Relative Motion.3-11 - Furuno 1751 Operator's Manual

Colllslon Avoklance artd Relative Motion
3. APPUCATION
Collision Avoidance and Relative Motion
CAUTION
The 1972 Convention on the International Regulations for Preventing ColUsions at Sea
(1972 COLREGS) pwvides statutory requirements for the operation of any vessel, including
the posting of lookouts. The operator of the vessel is responsible for comptiance with those
requirements whether or not a radar is in use.
Collisions at sea sometimes occur because the radar picture doesn't match the information
provided by the eye in clear weather and because of the misunderstanding of relative motion.
In a relative motion display, your ship is represented by the spot of light fixed at the center
of the screen, regardless of the speed of your own ship. With both your own ship and the
target in motion, the successive pips of the target do not indicate the actual or true movement
of the target. If your own ship is in motion, the pips of fixed objects, such as land masses,
move on the display at a rate equal to and in a direction opposite to the motion of your own
ship. Only when your ship is stopped or motionless do target pips move on the display in
accordance with their true motion. Figure 3-12 illustrates the relative and true motion of a
target contacted by radar.
In Figure 3-12, ship A, at geographic position Al on true course of 001" at 14 knots initially
observes ship B on the PPI at bearing 179" at 4.1 nautical miles. 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 are 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 initi^ly observes ship A on bearing 001" at 4.1 nautical miles.
The radar operator aboard ship A will determine that relative movement of ship B is
approximately 66.5 degrees, whereas the operator aboard ship B will determine that the
relative movement of ship A is approximately 238 degrees. These figures were obtained
using a maneuvering board.
Figure 3-12 Radar Echo on Each Ship
(North-up Relative Motion Mode)
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