Definition
CPA is the minimum separation that will occur if both vessels hold course and speed, and TCPA is the time remaining until that moment; a negative TCPA means the point has passed. Alarm limits are set to suit the waters, and a small CPA with a short TCPA demands early and substantial action. Both figures assume no manoeuvre, and are invalidated the instant either vessel alters, so they must be re-evaluated.
Want to apply this on the water?
Ask Saily out loud for an example, a scenario or a simpler explanation.
Talk to Saily