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Electromagnetic Scattering Concepts Applied to the Detection of Targets Near the Ground

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Electromagnetic Scattering Concepts Applied to the Detection of Targets Near the Ground 70 - 19,318 HILL, David Allen, 1942- ELECTROMAGNETIC SCATTERING CONCEPTS APPLIED TO THE DETECTION OF TARGETS NEAR THE GROUND. The Ohio State University, Ph.D., 1970 Engineering, electrical University Microfilms, A XEROX Company , Ann Arbor, Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED ELECTROMAGNETIC SCATTERING CONCEPTS APPLIED TO THE DETECTION OF TARGETS NEAR THE GROUND DISSERTATION Presented in P a rtia l Fu lfillm en t of the Requirements for the Degree Doctor of Philosophy in the Graduate School of the Ohio State University by David Allen H i l l , B .E .E ., M.Sc. ******** The Ohio State University 1970 Approved by ' \ ^ Q W V ^ ---- Adviser Department of Electrical Engineering FOREWORD The remote sensing of d is tin c t objects on the surface of the earth by electromagnetic waves is a very broad goal to which this dissertation is addressed. Needless to say, the optimum method has not yet been found and the study described in these pages can at most contribute new ideas and analytical results on several phases of the overall problem. S p e c ific a lly , the major con­ tributions presented w ill be briefly summarized below. New results for dipole radiation above a plane dielectric earth are obtained, for the case of aperiodic, or impulsive excitation in a number of special cases. These are simple in mathematical form and corraborate, apply and extend the work of others. Applications of resulting formulas are described, so as to determine the form of the total illuminating wavefront at the dielectric interface and the multiple interactions between a dipole scatterer (radiator) and the plane dielectric earth. These results are contained in Chapter I. The model of a scattering body as several induced e le c tric and magnetic dipoles, valid at s u ffic ie n tly long illum inating wavelengths, is applied to express the chanaes in induced dipole strengths, as functions of frequency, when such a body is near a plane dielectric or highly conducting earth. The complex resonances of a scat­ tering object, described by poles in the complex frequency plane may be influenced by the plane earth in several ways. The pole locations and pole-shifts produced by d ie le c tric and conducting h a lf­ spaces are estimated for conducting spherical and spheroidal scat- terers in several cases. Including the changes in illumination as well as the complex pole locations, formulas which express the dependence of backscattered signal upon signal frequency for such objects above a plane dielectric earth are derived. These results are presented in Chapter I I . The use of a small number of harmonically related source fr e ­ quencies to illuminate a scatterer permits one, upon coherent detection and recombination of the backscattered signal at all fre­ quencies, to produce a characteristic waveform related to the target size and shape. This technique and in particular the synthesis of a ramp response waveform by this means has been proposed by others as a method of target identification. Applying a difference equation which depends only upon the complex poles of the scattering function, a method is now suggested fo r target id en tific a tio n and discrim i­ nation. This difference equation, described by Corrington for a lumped constant network, is used to predict current and future values of the transient response from N prior values of the re­ sponse, where N is the number of complex poles. In the detection of a target at the earth's surface, knowledge of the complex poles of the target in the presence of earth would thus prescribe the coefficients of the difference equation, which in principle would be satisfied by the characteristic waveform produced a t any location of source and receiver. This independence of the target recogniti or detection scheme on direction of source and receiver is ex­ tremely important in reducing the number of constants which must be known for each object. In Chapter II I, a method for applying the difference equation is proposed and illustrated for spherical and spheroidal targets. The use of this technique in a linear predictor-correlator device is also applied to the detection of spherical targets in the presence of simulated vegetative clutter on a conducting ground, using experimental data from a m ultiple frequency anechoic chamber f a c ilit y , with encouraging resu lts. ACKNOWLEDGMENTS The author would lik e to acknowledge the many helpful ideas and the assistance of his graduate adviser, Professor E.M. Kennaugh. Dr. D.L. Moffatt was responsible for the experimental data as well as some useful suggestions. A debt of gratitude is also owed to the members of the reading corrmittee, Professors C.E. Warren and W.H. Peake, for their critique of the original dissertation draft. The research reported in this dissertation was sponsored in part by the A ir Force Cambridge Research Laboratories, O ffice of Aerospace Research, under Contract F44620-67-C-0095, and by The Ohio State University Research Foundation. v VITA April 21, 1942 Born - Cleveland, Ohio June 1964........ B.S.E.E., Ohio University, Athens, Ohio 1964-1966 ........ Acting Instructor, Electrical Engineering Department, Ohio University, Athens, Ohio June 1966 ........ M.So., Ohio University, Athens, Ohio 1966-present.. Research Associate, ElectroScience Laboratory (formerly Antenna Laboratory), The Ohio State U niversity, Columbus, Ohio Publications "The Effects of Irre g u la r Contour on Image Glide-Path Systems," M.Sc. Thesis, Ohio U niversity, 1966. "Transient Dipole over a Dielectric Half-Space," Proceedings of the Conference on Environmental Effects on Antennas, Boulder, Colorado, July 1969. vi Fields of Study Major Field: Electrical Engineering Studies in Electromagnetic Field Theory. Professors E.M. Kennaugh and R.G. Kouyoumjian Studies in Radar Systems Professor A.A. Ksienski Studies in Communications Professor C.E. Warren Studies in Applied Mathematics Professor S. Drobot Studies in Classical Physics Professor W.H. Shaffer vii TABLE OF CONTENTS Page FOREWORD............................................................................................................... i i ACKNOWLEDGMENTS................................................................................................. iv VITA....................................................................................................................... v ii LIST OF FIGURES................................................................................................. x Chapter I THE TRANSIENT FIELDS OF DIPOLES IN THE PRESENCE OF A DIELECTRIC HALF-SPACE............................... 1 A. Introduction 1 B. Classical Sommerfeld-Weyl-Van der Pol Solution 2 1. Exact frequency-domain forms 2 2. Applications 7 3. Extension to aperiodic excitation 3 C. Vertical Electric Dipole 10 1. On-axis solution in upper medium fo r anisotropic lower medium 10 2. Surface fields for a dipole at the interface of an anisotropic dielectric 17 3. Magnetic field for general observation point in upper medium 21 4. On-axis solution in the lower medium 26 5. Dipole and field point in the lower medium 28 D. Horizontal Electric Dipole in Upper Medium 30 E. Magnetic Dipole 36 1. On-axis solution for vertical orientation 36 2. On-axis solution for horizontal orientation 37 F. Approximate Expressions fo r Return Fields 40 I I EFFECT OF GROUND ON THE DIPOLE MODES OF SCATTERERS............................................................................ 51 A. Introduction 51 B. Dipole Modes of Scatterers in Free Space 56 viii Chapter Page C. Change in Dipole Modes Due to Ground When Dipole Modes do not Interact 56 1. New polarizability 56 2. New poles 61 D. Coupling of Dipole Modes 69 E. Backscattered Field 74 I I I THE DIFFERENCE EQUATION IN TARGET DETECTION AND DISCRIMINATION.............................................. 77 A. Introduction 77 B. The Difference Equation for Transient Responses of Scatterers 78 C. Discrimination of Targets by a Difference Equation Receiver 86 D. Detection of Targets in the Presence of Clutter Using a Difference Equation Receiver 93 IV SUMMARY AND CONCLUSIONS............................................................. 102 REFERENCES............................................................................................................. 105 LIST OF FIGURES Figure Page 1 Vertical dipole above a conducting half-space ................ 3 2 Vertical electric dipole above a uniaxial anisotropic dielectric half-space ................................................................ 11 3 The scattered electric field on the z-axis as a function of tim e............................................................................................ 16 4 The magnetic field at the surface of a dielectric half-space ...................................................................................... 20 5 Isometric view of ct = ..12........................................... 24 O9 6 Isometric view of ct = 15 ........................................... 25 0$ 7 Isometric view of pH . ct = 18 ............................................. 26 O9 8 The Hertz vector in the lower medium on the z-axis... 28 9 Vertical electric dipole within a dielectric half-space ................ 31 10 Scattered electric field in a dielectric medium 31 11 Horizontal electric dipole above an isotropic dielectric half-space ...................................................................................... 31 12 The scattered horizontal electric field on the z-axis as a function of time ................................................
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