Showing papers by "Constantine A. Balanis published in 1987"

Backscatter analysis of dihedral corner reflectors using physical optics and the physical theory of diffraction

Abstract: Physical optics (PO) and the physical theory of diffraction (PTD) are used to determine the backscatter cross sections of dihedral corner reflectors in the azimuthal plane for the vertical and horizontal polarizations. The analysis incorporates single, double, and triple reflections; single diffractions; and reflection-diffractions. Two techniques for analyzing these backscatter mechanisms are contrasted. In the first method, geometrical optics (GO) is used in place of physical optics at initial reflections to maintain the planar nature of the reflected wave and subsequently reduce the complexity of the analysis. The objective is to avoid any surface integrations which cannot be performed in closed form. This technique is popular because it is inherently simple and is readily amenable to computer solutions. In the second method, physical optics is used at nearly every reflection to maximize the accuracy of the PTD solution at the expense of a rapid increase in complexity. In this technique, many of the integrations cannot be easily performed, and numerical techniques must be utilized. However, this technique can yield significant improvements in accuracy. In this paper, the induced surface current densities and the resulting cross section patterns are illustrated for these two methods. Experimental measurements confirm the accuracy of the analytical calculations for dihedral corner reflectors with right, acute, and obtuse interior angles.

Dihedral corner reflector backscatter using higher order reflections and diffractions

Abstract: The uniform theory of diffraction (UTD) plus an imposed edge diffraction extension is used to predict the backscatter cross sections of dihedral corner reflectors which have right, obtuse, and acute included angles. UTD allows individual backscattering mechanisms of the dihedral corner reflectors to be identified and provides good agreement with experimental cross section measurements in the azimuthal plane. Multiply reflected and diffracted fields of up to third order are included in the analysis for both horizontal and vertical polarizations. The coefficients of the uniform theory of diffraction revert to Keller's original geometrical theory of diffraction (GTD) in far-field cross section analyses, but finite cross sections can be obtained everywhere by considering mutual cancellation of diffractions from parallel edges. Analytic calculations are performed using UTD coefficients; hence accuracy required in angular measurements is more critical as the distance increases. In particular, the common "far-field" approximation that all rays to the observation point are parallel is too gross of an approximation for the angular parameters in the UTD coefficients in the far field.

Oceanic low-angle monopulse radar tracking errors

Abstract: Radar systems often experience difficulties when tracking low-altitude targets over the ocean because of multipath effects. Whenever the radar cannot resolve the target from its image, it will track a false target position which can move far above or below the actual position. In this paper, mathematical models are utilized to quantitatively determine the degradation in tracking ability of a monopulse radar due to multipath. The model incorporates provisions for the antenna sum and difference patterns, including sidelobes, and for the antenna polarization. Divergence factors are utilized to account for the curvature of the earth's lossy surface. More accurate calculations of the phase length of the direct and reflected rays using the spherical earth model are included. Smooth and rough surface models are used to model the prevailing sea state. The smooth surface model determines both stable and unstable equilibrium directions toward which the target position is indicated. The rough surface model defines a band of maximum error in the indicated position, as a function of the surface waveheight, and it includes both the rough specular and the rough diffuse reflection term.

A Conjugate Direction Method for Geophysical Inversion Problems

Abstract: In geophysical tomography, algebraic methods are often used to linearize the nonlinear problem of determining the characteristics of an underground region given measurements of the earth's attenuation to electromagnetic or seismic waves. In this way, a set of linear equations is developed such that the unknowns are the picture elements (pixels) of the region being scanned.Classically, these linear equations have been solved using the algebraic reconstruction technique (ART) algorithm. In this paper, a new algorithm that is a member of the set of conjugate direction (CD) methods is developed and comparisons are made between this algorithm and the ART algorithm for data arising from simulated electromagnetic probing. This new method, which we call the constrained conjugate gradient (CCG) algorithm, is shown to have a much faster convergence to a final solution than the ART algorithm. In addition, for applications involving high-contrast anomalies (for example, tunnel detection) the CCG is shown to have superior performance in locating the anomalous region for almost all test cases considered.

Conjugate direction method for geophysical inversion problems.

Abstract: In geophysical tomography, algebraic methods are often used to linearize the nonlinear problem of determining the characteristics of an underground region given measurements of the earth's attenuation to electromagnetic or seismic waves. In this way, a set of linear equations is developed such that the unknowns are the picture elements (pixels) of the region being scanned.Classically, these linear equations have been solved using the algebraic reconstruction technique (ART) algorithm. In this paper, a new algorithm that is a member of the set of conjugate direction (CD) methods is developed and comparisons are made between this algorithm and the ART algorithm for data arising from simulated electromagnetic probing. This new method, which we call the constrained conjugate gradient (CCG) algorithm, is shown to have a much faster convergence to a final solution than the ART algorithm. In addition, for applications involving high-contrast anomalies (for example, tunnel detection) the CCG is shown to have superior performance in locating the anomalous region for almost all test cases considered.

Electromagnetic scattering by impedance structures

Abstract: The scattering of electromagnetic waves from impedance structures is investigated, and current work on antenna pattern calculation is presented A general algorithm for determining radiation patterns from antennas mounted near or on polygonal plates is presented These plates are assumed to be of a material which satisfies the Leontovich (or surface impedance) boundary condition Calculated patterns including reflection and diffraction terms are presented for numerious geometries, and refinements are included for antennas mounted directly on impedance surfaces For the case of a monopole mounted on a surface impedance ground plane, computed patterns are compared with experimental measurements This work in antenna pattern prediction forms the basis of understanding of the complex scattering mechanisms from impedance surfaces It provides the foundation for the analysis of backscattering patterns which, in general, are more problematic than calculation of antenna patterns Further proposed study of related topics, including surface waves, corner diffractions, and multiple diffractions, is outlined

Diffractions from a lossy polygonal plate in the presence of an antenna

Abstract: The coefficients are written in terms of the Maliuzhinets function of order n which has been approximated in terms of elementary functions [ 3 ] . The Maliuzhinets function arises from the solution of the boundary value problem of the canonical wedge with an impedance surface boundary condition. The validity of this boundary condition has been discussed in [ 4 ] . The edge is described by the relative surface impedance q and is considered to be opaque. The coefficients of the imperfect conductor UTD revert to the perfectly conducting UTD coefficients of [l] as q approaches zero for both normal and oblique incidences.