The prediction of maximum usable frequencies for radiocommunication over a transequatorial path

Times of reception of 15 Mc/s radio waves over a transequatorial path of 7 500 km have been recorded throughout the recent period of declining solar activity (1950-54). The analysis of these times has shown that predictions of maximum usable frequency made by the usual control-point method were, in general, too high by about 4 Mc/s, and at times by as much as 7 Mc/s or more. This is contrary to the normal experience for long transmission paths lying within a single hemisphere. When a transmission mechanism involving multiple geometrical reflections is assumed instead of the forward-scattering mechanism implied by the control-point method, it is found that the path can be considered, for the purpose of predicting maximum usable frequencies, to consist of three reflections. The discrepancies between prediction and observation, which still remain after a 3-reflection mechanism has been invoked, are attributed mainly to reflections from the sporadic-E region at the southernmost reflection point, although it is possible that lateral deviation of the radio waves is also a contributing factor.