JRC JMA-5204 Instruction Manual page 253

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 TCPA Time to CPA
Collision triangle
Own
ship
Fig. 5-2 Collision Triangle
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5.1 PREPARATION
Target
vessel
Relative vector
CPA ring
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