Thomas B. A. Senior

Relating Polaization Phase Difference of SAR Signals to Scene Properties

This paper examines the statistical behavior of the phase difference AO between the HH-polarized and VV-polarized backscattered signals recorded by an L-band SAR over an agricultural test site in Illinois. Polarization-phase difference (¿¿) distributions were generated for about 200 agricultural fields for which ground information had been acquired in conjunction with the SAR mission. For the over-whelming majority of cases, the AX distribution is symmetrical and has a single major lobe centered at the mean value of the disstribution ¿¿ Whereas the mean AX was found to be close to zero degrees for bare soil, cut vegetation, alfalfa, soybeans, and clover, a different pattern was observed for the corn fields; the mean ¿¿ increased with increasing incidence angle 0 from about zero at 0 = 150 (near-range of the image) to about 140° at 0 = 35°. The explanation proposed for this variation is that the corn canopy, most of whose mass is contained in its vertical stalks, acts like a uniaxial crystal characterized by different velocities of propogation for waves with horizontal and vertical polarization. Thus, it is hypothesized that the observed backscatter is contributed by a combination of propagation delay, forward scatter by the soil surface, and specular bistatic reflection by the stalks. Model calculations based on this assumption were found to be in general agreement with the phase observations.

Comparison of three high-frequency diffraction techniques

Three high-frequency methods of calculating the scattering from metallic edged bodies are compared. The first two are the physical and geometrical theories of diffraction, which have been well established since the late 1950s, and the third is the method of equivalent currents. It is shown that the three share remarkably similar features, although each has its particular virtues and limitations in practical applications.

Backscattering from resistive strips

Strips made of a resistive sheet material have lower backscattering cross sections than the corresponding perfectly conducting strips, and this is true in particular when the illumination is edge-on with the electric vector parallel to the edge. Attention is focused on this case. Using the moment method applied to an appropriate integral equation, data are obtained for the surface field and backscattered far field of a resistive strip for a variety of strip widths w and uniform resistances R . The front- and rear-edge contributions to the far field are then extracted. It is shown that for strips whose width is greater than about a half-wavelength the former is the same as for a half-plane having the same resistance, whereas the latter is proportional to the square of the current at that point on the half-plane corresponding to the rear edge of the strip. The implications of these results on the selection of a strip resistance for low backscattering are discussed.

Combined resistive and conductive sheets

To simulate a thin layer of material whose permittivity and permeability both differ from the values for the surrounding medium, a combination resistive and conductive sheet is defined and its properties described.

Scattering by a narrow gap

For a plane wave incident on a cavity-backed gap in a perfectly conducting plane, the coupled integral equations for the induced currents have been solved numerically and the far-field scattering computed. The results are compared with a quasi-analytic solution previously derived. For a narrow gap the agreement is excellent for all cavity geometries and for all material fillings that have been tested. >

Scattering by gaps and cracks

The contribution from gaps and cracks where two component structures come together is a matter of increasing concern in radar scattering. For the two-dimensional problem of a narrow gap filled with a resistive or impedance material in an otherwise perfectly conducting plane, the integral equations for the currents induced in the gap are approximated under the assumption that w >

Derivation and application of a class of generalized boundary conditions (electromagnetic scattering)

Boundary conditions involving higher order derivatives are presented for simulating surfaces whose reflection coefficients are known analytically, numerically, or experimentally. Procedures for determining the coefficients of the derivatives are discussed, along with the effect of displacing the surface where the boundary conditions are applied. Provided the coefficients satisfy a duality relation, equivalent forms of the boundary conditions involving tangential field components are deduced, and these provide the natural extension to nonplanar surfaces. As an illustration, a metal-backed uniform dielectric layer is simulated. It is shown that fourth-order conditions are capable of providing an accurate simulation for layers at least a quarter of a wavelength in thickness. >

Generalized impedance boundary conditions in scattering

The authors present generalized surface impedance conditions that improve medium characterization. One of the most common conditions that is often used to simulate a dielectric coating is the standard impedance boundary condition, and the improved boundary condition discussed can be viewed as a generalization of this, distinguished by the presence of higher-order derivatives of the field. By virtue of the derivatives, the conditions are less local in character, and the additional degrees of freedom can be used to better simulate the material properties of a surface. Following a review of the standard impedance condition, the new conditions are introduced. Several forms are presented, each with coefficients which are functions of the geometry and material properties of the surface. The applications of these to a variety of scattering problems are then discussed. >

Simple expressions for a function occurring in diffraction theory

The Maliuzhinets (integral) function arises in connection with diffraction by a half-plane. Two simple expressions are derived which, when used in conjunction with known identities, serve to approximate the function to a high degree of accuracy throughout the entire complex plane.

Electromagnetic Field Penetration into a Cylindrical Cavity

For an E-polarized plane wave incident on a perfectly conducting cylindrical shell having a longitudinal slit aperture, the fields inside the cavity are determined by a numerical solution of the E-field integral equation. Selected data are presented and the first few complex frequency (SEM) singularities are determined for a variety of aperture sizes.