Statistical properties of the ground diffraction pattern formed by ionospherically reflected radio waves are used to examine models of the angular spectra of reflections from the ionospheric D-region. In the daytime two distinct height regions characterized by differing reflection mechanisms are identified within the D-region. Below 85 km the angular spectrum of reflected waves frequently contains coherent components, whereas a spectrum of incoherent or randomly phased components is characteristic of reflections from the region above 85 km. There is evidence which suggests the presence on many occasions of isolated moving reflectors in the reflecting 'ionospheric screen' . . Both the mean angular spread and the fading speed of waves partially reflected from the D-region show an increase with increasing height of reflection.
A theoretical basis for studying the nature of ionospherically reflected radio waves is described. The expected statistical properties of the amplitude and phase variations of the radio wave ground pattern sampled at a pair of spaced receiving antennas are derived for general correlation conditions, assuming different models of the angular spectrum of reflected radio waves. The results provide a basis for experimental measurements of the angular spread and coherence ratio of the angular spectrum of reflected waves.
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