How does manual plotting complement ARPA when target data is unreliable?

• A. Manual plotting replaces ARPA entirely.
• B. Manual plotting can provide reliable CPA/TCPA calculations when ARPA data is questionable.
• C. The operator can independently measure range and bearing and compute CPA/TCPA, ensuring continued safety even if ARPA data is questionable.
• D. Manual plotting is not useful.

Answer: (C)

Redundancy in target tracking is the key idea here. When ARPA data is unreliable, manual plotting gives you an independent way to assess what’s happening and keep safe.

By taking measurements of range and bearing to the target at known times and plotting both vessels on a chart, you create your own track of relative motion. Watching how these positions change over time lets you project the ships’ paths and directly compute the closest point of approach (the smallest distance you would come to the other vessel) and the time at which that closest approach would occur. This provides CPA and TCPA estimates that you can trust even if the ARPA track is questionable, so you can maintain safe separation.

It’s not about replacing ARPA, but about having a reliable, independent check when ARPA data can’t be trusted. Manual plotting is useful because it preserves situational awareness and decision-making capability under data doubt.