Aperture antenna effects after propagation through strongly disturbed random media

A strongly disturbed layer of ionization irregularities that is used as a propagation channel for radio waves can degrade the propagating wave and thereby affect the resulting measurements at the receiving antenna. The antenna aperture itself also affects measurements of the received signal by its inherent averaging process. Here an analytic solution for the two-position, two-frequency mutual coherence function, valid in the strong-scatter limit, is used to characterize the propagation channel. The channel itself consists of a thick slab of anisotropic electron density irregularities that are elongated in the direction parallel to the earth's magnetic field. Analytic expressions are obtained that give the effect of the aperture antenna on measurements of received power, decorrelation time (or distance), mean time delay, time delay jitter and coherence bandwidth. These quantities are determined as functions of the aperture diameter and of the angle between the magnetic field and the direction of propagation. It is shown that in strong turbulence aperture averaging can be a significant factor in reducing the received power by angular scattering loss, increasing the observed signal decorrelation time via aperture averaging, and reducing the time delay jitter by suppression of signals received at off-boresight angles. Results are presented for two cases. One-way propagation through an ionospheric communication channel is considered where both transmitter and receiver utilize aperture antennas. This result is easily extended to the case that one of the antennas is an omnidirectional point source, corresponding to the usual ease of transionospheric satellite communication from a small satellite antenna to a large ground based receiver. The second case involves transmission and reception of a radar signal that travels through a disturbed ionospheric channel to a target located in free space. This ease is applicable to the situation of a large antenna aboard a space based radar or to the case of a ground based defense radar.

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