| Definition: | the radius of an ideal spherical Earth without atmosphere for which propagation paths are along straight lines, the heights and ground distances being the same as for the actual Earth in an atmosphere with a constant vertical gradient of refractivity
NOTE 1 – The concept of effective radius of the Earth implies that the angles with the horizontal planes made at all points by the transmission paths are not too large.
NOTE 2 – For an atmosphere having a standard refractivity gradient, the effective radius of the Earth is taken as equal to 8500 km, which corresponds approximately to 4/3 of the actual radius of the Earth.
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