“…The concept of an effective earth radius was introduced by Schelleng et al, [1933] and is also found in the works of Elmore [1975], Inoue [1975], Comit• Consultatif International des Radiocommunications (CCIR) [1978a], and Sasaki and Akiyarna [1979]. The use of Snell's law for rays refracted in a spherical atmosphere is applied by Schulkin [1952], Bean and Thayer [1959], Bean [1964], Armand and Kolosov [1965], Thayer [1967], Blake [1968], Bean and Dutton [1968], Plotnikov [1975], Gallop and Telford [1975], and Vickers and Lopez 1-1975].…”