Navigation General · Radar Plotting & CPA
Own ship is on course 000°T, speed 10 knots. Target A has been plotted and the relative motion line shows the target approaching from 045° relative bearing, range decreasing from 10 to 7 miles in 9 minutes. Target A will pass ahead if own ship:
- AMaintains course and speed
- BAlters course to starboard to open the CPA
- CCannot be determined without completing the full vector triangle✓ Correct
- DReduces speed only, without course change
Explanation
Whether the target will pass ahead or astern, and by how much, requires knowing the complete relative motion line — specifically where that line passes relative to own ship's position. Observing that range is decreasing on a steady bearing only establishes collision risk. Determining the pass-ahead or pass-astern geometry and the effect of any maneuver requires completing the OAB vector triangle.
Authority: Radar Navigation Manual (Pub. 1310), Ch. 4
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