ARPA is a radar processing system that automatically acquires and tracks targets (other vessels, obstacles) detected by the radar, calculating their true course, speed, and collision risk without requiring manual plotting.
What ARPA Calculates
- CPA (Closest Point of Approach) — the closest distance a tracked target will come to your vessel if both maintain current course and speed
- TCPA (Time to Closest Point of Approach) — how long until that closest point is reached
- True course and speed of the target, distinguishing actual vessel movement from relative motion seen on a basic radar display
- Trial maneuvers — simulating the effect of a proposed course or speed change before executing it
Why It Matters
Before ARPA, officers manually plotted target movement on a reflection plotter over several minutes to determine collision risk — slow and error-prone in busy traffic. ARPA automates this, giving continuous, real-time collision risk assessment for multiple targets simultaneously.
Alarms
ARPA systems generate automatic alarms when a tracked target’s CPA/TCPA falls within dangerous limits set by the officer of the watch, prompting timely action under COLREGs.
Exam Relevance
ARPA plotting, CPA/TCPA interpretation, and collision-avoidance decision-making using ARPA data are core practical and written competency exam topics for 2nd Mate FG and Master FG.