5-4
5.1 Preparation
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5
Basic Concept of Collision Avoidance
There are two aspects in collision avoidance: collision prediction and avoidance. Collision prediction is to
predict that two or more vessels will happen to occupy the same point at the same time, while collision
avoidance is to maneuver vessels not to occupy the same point at the same time.
In practical operation of vessels, a spot of collision has to be deemed to be a single point but a closed zone.
This closed zone is conceptually defined as a CPA (Closest Point of Approach). In collision prediction, the
time to be taken until a ship reaches the CPA is defined as a TCPA (Time to CPA).
Fig. 5-2 shows a diagram caked “Collision Triangle”.
CPA
Target Vessel True Vector
Own Ship True Vector
Own Ship
TCPA（Time to CPA）
Target vessel
CPA ring
Relative Vector
Collision Triangle
Fig. 5-2 Collision Triangle
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