Definition
Radar energy is refracted slightly more than light in the standard atmosphere, so the radar horizon extends about six per cent further than the optical one. A useful approximation with heights in metres is 2.2 times the sum of the square roots of scanner height and target height, giving nautical miles. A scanner at 9 m detecting a 25 m cliff top gives roughly 2.2 times 8, about 17.6 nautical miles.
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