DIRECT ANSWER
CPA is the predicted closest distance; TCPA is the time until that point. A small CPA with a short positive TCPA deserves early attention.
The four signals
Use the values together.
| Signal | Meaning | Use |
|---|---|---|
| Radar | Detected targets and range | See traffic, including non-AIS returns |
| AIS | Broadcast identity and movement data | Identify equipped vessels |
| CPA | Closest predicted passing distance | Judge separation |
| TCPA | Time until CPA | Judge urgency |
Decision loop
Check the visual scene, confirm the target trend, make a clear course or speed change early, and monitor whether CPA increases. Never rely on one display alone.