Propagation of RF Signals through Structured Ionization. Theory and Antenna Aperture Effect Applications

摘要

This report presents a review of the theory of propagation of RF signals through random ionospheric disturbances that would result from high altitude chemical releases or nuclear detonations. Starting with Maxwell's equations, an analytic expression for the two-position, two-time, two-frequency mutual coherence function is derived for strong, anisotropic scattering conditions. The physics that are contained in this important function and the approximations that are used. The first is the usual frozen-in approximation which models the ionosphere as a rigid structure that drifts across the line-of-sight. The second, called the turbulent model, is developed in this report which decouples the spatial and temporal fluctuations in the ionosphere. This model may be more accurate at times before striations have formed in the ionosphere or when multiple layers of striations with different velocities are drifting across the line-of-sight. The second part of this report presents several new results and applications of the theory. The generalized power spectral density (GPSD), which is the Fourier transform of the mutual coherence function, is used to compute the second order statistics of the signal at the output of an anisotropic aperture antenna. Analytic results are presented for antennas with Gaussian beam profiles and numerical results are presented for uniformly weighted apertures. These results are then generalized to transponder communication link geometries with two independent propagation paths and four antennas.