ARPA stands for Automatic Radar Plotting Aid, a radar system that helps ship and submarine operators track nearby vessels and judge collision risk. It takes repeated radar measurements of targets and estimates their motion across the sea. This matters because large vessels turn and stop slowly, so crews need warning long before two paths meet.
ARPA turns a radar screen into a prediction tool, not just a map of objects.
Key Facts
- Relative speed toward collision can be estimated from closing speed: closing speed = distance change / time.
- Time to closest point of approach: TCPA = time until the predicted minimum separation occurs.
- Closest point of approach: CPA = smallest predicted distance between two vessels if both keep course and speed.
- Speed, time, and distance are related by d = vt.
- ARPA usually needs several radar scans before it can compute a reliable target track.
- A collision warning is triggered when predicted CPA is too small and TCPA is within a chosen time limit.
Vocabulary
- ARPA
- Automatic Radar Plotting Aid is a radar-based system that tracks targets and predicts collision risk.
- Radar
- Radar is a detection system that sends radio waves and measures their echoes to locate objects.
- CPA
- Closest point of approach is the smallest predicted distance between two moving vessels.
- TCPA
- Time to closest point of approach is the predicted time remaining before the closest separation occurs.
- Relative motion
- Relative motion describes how another vessel appears to move compared with your own vessel.
Common Mistakes to Avoid
- Treating a radar contact as a fixed object is wrong because most contacts are moving vessels whose future position depends on course and speed.
- Ignoring TCPA is wrong because a small CPA may not be urgent if it occurs much later, while a moderate CPA can be dangerous if it occurs soon.
- Assuming ARPA is instantly accurate is wrong because the system needs multiple radar scans to build a dependable track.
- Using ARPA without visual or radio confirmation is wrong because radar errors, sea clutter, and target changes can make predictions incomplete.
Practice Questions
- 1 A target is 6.0 nautical miles away and the range decreases to 4.5 nautical miles after 10 minutes. What is the closing speed in nautical miles per hour?
- 2 An ARPA display predicts a TCPA of 12 minutes and the closest point of approach will be 0.8 nautical miles. If the ship's safety limit is CPA greater than 1.0 nautical mile, should the operator treat this as a collision risk?
- 3 Explain why ARPA predictions can become unreliable if a target ship changes course after the system has plotted its track.