ARPA (Automatic Radar Plotting Aid)

ARPA automatically acquires and tracks radar targets and computes each target’s closest point of approach and time to CPA, displaying course, speed, and a vector for collision assessment. The functional standard is IMO Resolution A.823(19), now folded into the radar performance standard MSC.192(79). ARPA must track at least 20 targets, present a trial-maneuver facility, and trigger CPA and TCPA alarms against operator-set limits. It does not relieve the watchkeeper of the Rule 7 duty to plot systematically, since acquisition lag and target swap can corrupt the computed solution.