Effective Earth Radius

In telecommunication, effective Earth radius is 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 atmospheric refractive index.

For the standard atmosphere, the effective Earth radius is 4/3 that of the actual Earth radius. The ratio of the effective Earth radius to the actual Earth radius is called the k factor.