Effective radius of the earth
The radius of a hypothetical earth for which the distance to the radio horizon, assuming rectilinear propagation, is the same as that for the actual earth with an assumed uniform vertical gradient of a refractive index. (For the standard atmosphere, the effective radius is 4/3 that of the actual earth.)
Reference: C.C.I.R.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
