Navigation General · Radar Plotting & CPA
On a radar plot, the Closest Point of Approach (CPA) is found by:
- ADrawing own ship's course line and measuring where it intersects the target's course line
- BExtending the relative motion line and measuring the perpendicular distance from own ship's position to that line✓ Correct
- CMeasuring the current range to the target and adding the target's speed times TCPA
- DDoubling the current range and dividing by the bearing rate of change
Explanation
After plotting the target's relative motion line (RML) — the line connecting successive plotted positions — the CPA is the point on that line closest to own ship's position (the origin of the plot). Geometrically, it is the foot of the perpendicular dropped from own ship's position to the RML. TCPA is the time to reach that point at the relative speed.
Authority: Radar Navigation Manual (Pub. 1310), Ch. 4
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