In radar and radio propagation studies, a value for the radius of the earth that may be used in place of the actual radius to correct for refraction by the atmosphere.

The effective earth radius is a convenient fiction that makes straight the actual curved path of a radio ray in the atmosphere by presenting it relative to an imaginary earth with a radius larger than the radius of the real earth, thus maintaining the relative curvature between earth and radio ray. The effective earth radius R_e is approximately