CPA (Closest Point of Approach)

The closest point of approach is the minimum range a tracked target will reach if both vessels hold present course and speed, derived from the relative-motion triangle of own ship and target. With the time to CPA it is the primary numeric measure of collision risk on radar and ARPA. A CPA of zero means a collision course; a small but non-zero CPA at high closing speed still demands early Rule 8 action. CPA is computed in the relative frame, so a change by either vessel invalidates the figure and a fresh solution is required.