The propagation of radio energy over a smooth spherical earth of uniform dielectric constant and conductivity under conditions of standard refraction in the atmosphere, that is, an atmosphere in which the refractive index decreases uniformly with height at a rate of approximately 40 N-units per kilometer. Standard propagation leads to ray curvature due to refraction with a value approximately one- fourth that of the earth's curvature, giving a radio horizon that is about 15% farther than the distance to the geometric horizon. This is equivalent to straight-line propagation over a fictitious earth with radius of four-thirds the radius of the actual earth. See effective earth radius, superstandard propagation, substandard propagation, standard atmosphere.