Date of Award

Spring 1989

Document Type

Dissertation

Degree Name

Doctor of Engineering Science in Electrical Engineering

Department

Electrical and Computer Engineering

First Advisor

Gerald Martin Whitman

Second Advisor

Yunjin Kim

Third Advisor

Felix K. Schwering

Fourth Advisor

Edip Niver

Abstract

A forest is a highly scattering medium at millimeter wave frequencies. The propagation of cw millimeter wave signals in forests has been studied previously, both theoretically and experimentally. Published experimental data verified that continuous wave transmission is possible over lengths of the order of a few hundred meters and that a forest acts to strongly scatter energy in the forward direction. The cw studies yielded the determination of the range dependence, beam broadening effects and depolarization effects of millimeter wave signals in a forest. However, pulse broadening effects, which are of importance particularly in the case of digital signal transmission, remained to be studied. The main purpose of this dissertation is to provide a theory of these effects applicable to the millimeter wave region. A second purpose of the dissertation is to refine the previously developed cw theory by linking it to the experimental cw data by an optimization scheme. In Part I of this study a periodic sequence of gaussian plane wave pulses is assumed to impinge upon a forest half-space. The forest is taken to be statistically homogeneous and to consist of a random distribution of particles which scatter and absorb radiation. A theory of millimeter wave pulse propagation in a forest is developed using the scalar time-dependent equation of radiative transfer. The forest is assumed to be described by a scatter function which consists of a strong narrow forward lobe superimposed over an isotropic background. The power intercepted by a receiving antenna in the forest is computed as a function of path length and travel time. It is demonstrated through numerical computations that the detection of a transmitted signal is indeed feasible and that pulse broadening occurs at large penetration depths. In Part II a parametric inversion scheme is developed which permits the determination of forest parameters at millimeter wave frequencies. Using the available experimental data for the cw case, the inversion scheme is applied to the time-independent equation of radiative transfer. By initially choosing values for the unknown parameters, and then judicially varying parameters using an optimization technique similar in concept to "simulated annealing" and requiring that the difference between the experimentally and theoretically determined values of received power be minimal, the desired unknown forest parameters are found. The method appears to provide meaningful parameter characterization of the forest despite the limited available experimental data.

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