Basic Concept Of Collision Avoidance - JRC JMR-5406-X Instruction Manual

6.2.3 Basic concept of collision avoidance

There are two aspects in collision avoidance: collision prediction and avoidance. Putting it in extreme
terms, 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 manoeuvre 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). Further, the time
taken to arrive at CPA is required for collision prediction, and the danger of collision is judged
considering the TCPA (Time to CPA). The following figure illustrates a diagram caked "Collision
Triangle".
CPA ring
Section 6 Target Tracking and AIS
TCPA
(Time to CPA)
Collision
triangle
Own ship
6-4
True vector of
target vessel
Target vessel
Relative vector
True vector of own
ship