Radar Observer · Plotting — Speed of Relative Motion
On a relative plot, the target's echo moves 2.0 NM along the relative-motion line in 6 minutes. The speed of relative motion (SRM) is:
- A12 knots
- B20 knots✓ Correct
- C2 knots
- D120 knots
Explanation
SRM = distance along the relative track ÷ time = 2.0 NM ÷ 6 min. Using the six-minute rule, distance in 6 min × 10 = speed: 2.0 × 10 = 20 knots. The SRM and the direction of relative motion (DRM) are read directly off the plot.
Authority: Bowditch (Pub. No. 9), Radar Plotting
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