What does TCPA measure and how is it used in maritime operations?

• A. Time to Closest Point of Approach is calculated from relative motion data and used to assess collision risk and plan avoidance actions.
• B. Distance at closest point of approach is used to determine which vessel has right of way.
• C. Bearing rate alone determines collision risk.
• D. Time to collision is unrelated to avoidance decisions.

Answer: (A)

TCPA is about predicting when two vessels will be at their closest point to each other given their current speeds and courses. It’s calculated from relative motion data between your ship and the target (how far apart you are now and how fast and in what direction each vessel is moving). The smaller the TCPA, the sooner a potential collision could occur, so mariners use TCPA to gauge risk and decide on avoidance actions (such as altering course, changing speed, or increasing separation). This measure is most useful when used together with the distance at closest approach (CPA) to determine both how near the vessels will come and when that near moment will occur. Bearing rate alone doesn’t capture how soon a collision could happen, and time to collision is a related idea but TCPA specifically refers to the time to the closest point of approach under current motion, not an independent notion of “time to collision.”