In this work the conditions and requirements that lead to a randomly phased angular spectrum of plane waves are investigated. An experimental technique is developed from which the complex spatial correlation of the electromagnetic fields at two spaced antennas may be obtained. It is shown that the complex correlation of these electromagnetic fields is strongly influenced by the amount of angular correlation within the spectrum of plane waves. From this theoretical development emerge criteria that may be used to test the hypothesis that a spectrum of plane waves was randomly phased. Experimental results from a multiple baseline field correlation experiment are presented. In this program the primary intent was to obtain data from D-region altitudes between 70 km and 90 km. The data from the E-region are primarily for comparison purposes. It is shown that the angular spectrum due to scattering in the ionospheric D-region is probably not randomly phased. In comparison, the results obtained from E-region scatter appear to support the validity of the randomly phased angular spectrum approximation. It is further shown that the angular power spectrum due to scattering in the D-region contains a plane undiffracted wave component which accounts for approximately 25% of the received power. Extensive effort was devoted to the identification of D-region echo amplitudes with a Rice probability density distribution. It is shown that the majority of the amplitude distributions analysed were Rayleigh distributed. The concept and credibility of the "off set” Rayleigh distribution are investigated and a theoretical interpretation of the observed facts is presented.