Microwave backscattering and emission model for grass canopies

Microwave radar and radiometer measurements of grasslands indicate a substantial reduction in sensor sensitivity to soil moisture in the presence of a thatch layer. When this layer is wet it masks changes in the underlying soil, making the canopy appear warm in the case of passive sensors (radiometer) and decreasing backscatter in the active case (scatterometer). A model for a grass canopy with thatch is presented in order to explain this behavior and for comparison with observations. The canopy model consists of three layers: grass, thatch, and the underlying soil. The grass blades are modeled by elongated elliptical discs and the thatch is modeled as a collection of disk shaped water droplets (i.e., the dry matter is neglected). The ground is homogeneous and flat. The distorted Born approximation is used to compute the radar cross section of this three layer canopy and the emissivity is computed from the radar cross section using the Peake formulation for the passive problem. Results are computed at L-band (1.4 GHz) and C-band (4.75 GHz) using canopy parameters (i.e., plant geometry, soil moisture, plant moisture, etc.) representative of Konza Prairie grasslands. The results are compared to C-band scatterometer measurements and L-band radiometer measurements at these grasslands     

Microwave scattering by a plasma covered sphere

Laboratory measurements of the scattering of microwaves by a plasma covered sphere are presented for forward scattering angles. The radial electron density variation is deduced from these data and a simple model of the experiment.     

Microwave reflection tomographic array for damage detection of civil structures

Microwave tomographic imaging technology using a bifocusing operator has been developed in order to detect the internal voids/objects inside concrete structures. The imaging system consists of several cylindrical or planar array antennas for transmitting and receiving signals, and a numerical focusing operator is applied to the external signals both in transmitting and in receiving fields. An imaging algorithm using a numerical focusing operator was developed, which allows the recovery of a two-dimensional object from its scattered field. Numerical simulation demonstrated that a subsurface image can be successfully reconstructed by using the proposed tomographic imaging technology. For experimental verification, a prototype planar antenna array was fabricated and tested on a concrete specimen.     

Beam scattering by nonuniform leaky-wave structures

A method is presented for obtaining the scattered field due to a beam incident on a layered structure or on a dielectric grating whose properties vary slowly along a longitudinal direction. The scattered field is expressed in terms of a Green's function whose form is particularly simple for a large class of practical problems involving laser beams incident on structures capable of guiding leaky waves. The incident beam may then strongly couple to the leaky wave, thus producing a scattered field that contains a specularly reflected component and a diffracted component of the leaky-wave type. The presence of the latter component may cause the resulting scattered field to be considerably different from the field of the incident beam. A judicious choice of the leaky-wave structure can be used to produce a scattered field that exhibits any of a wide range of distributions. The method developed herein is illustrated by means of structures whose leakage-properties vary linearly. The results can be utilized either for efficiently coupling a beam into a surface wave, or for splitting the incident beam into two separate but closely similar outgoing beams, both applications being discussed in detail.     

Microwave imaging-Location and shape reconstruction frommultifrequency scattering data

The problem of determining the shape and location of an object embedded in a homogeneous dissipative medium from measurements of the field scattered by the object is considered in this paper. The object is assumed to be an infinite cylinder of known cross section illuminated by a TM plane wave and the scattered field is measured on a line segment perpendicular to the direction of incidence. Measurement data are carried out at three different frequencies for a homogeneous cylinder of known dielectric constant. The location and contour shape are determined using two different reconstruction algorithms, a Newton-Kantorovich (NK) method and the modified gradient (MG) method whose effectiveness and robustness are compared. Both methods are based on domain integral representations of the field in the body. They involve an iterative minimization of the defect between an integral representation of the field measured on the line and the actual measured data. The NK method involves a linearization of the nonlinear relation between the field and the contrast, as well as the solution of a direct scattering problem at each iteration. The MG method seeks the simultaneous reconstruction of the field and the characteristic function of the support of the scatterer without solving a direct problem at each step. Both methods employed the same initial guess and the a priori information that the characteristic function is nonnegative     

On the composite model for rough-surface scattering

The Kirchhoff method and the small perturbation method are usually used in combination in the composite model of rough-surface wave scattering. They are shown to be identical to the first order for scattering by a perfectly conducting body of a circularly polarized incident wave into a scattered wave polarized in the expected sense of circular polarization. Thus for this case, a single application of the Kirchhoff method is as accurate as the composite model and is simpler to apply. The identity remains valid for arbitrary directions of incidence and scattering, but not for other polarizations. Statements in the literature at variance with these results appear to be based on derivations which neglect some part of the complete solution.     

A GTD-based parametric model for radar scattering

This paper presents a new approach to scattering center extraction based on a scattering model derived from the geometrical theory of diffraction (GTD). For stepped frequency measurements at high frequencies, the model is better matched to the physical scattering process than the damped exponential model and conventional Fourier analysis. In addition to determining downrange distance, energy, and polarization, the GTD-based model extracts frequency dependent scattering information, allowing partial identification of scattering center geometry. We derive expressions for the Cramer-Rao bound of this model; using these expressions, we analyze the behavior of the new model as a function of scatterer separation, bandwidth, number of data points, and noise level. Additionally, a maximum likelihood algorithm is developed for estimation of the model parameters. We present estimation results using data measured on a compact range to validate the proposed modeling procedure     

A scattering model for snow-covered sea ice

The special properties of a robust radiative transfer model for scattering from layers of inhomogeneous rough-boundary slabs are presented. The model is applied to backscattering from saline and desalinated ice. Comparisons are made at single and multiple frequencies with some of the most complete sets of measurement data available, using measured physical and electrical characteristics of the ice as inputs to the model where possible. The results show close agreement. For example, for the saline ice backscatter data set, which consisted of measurements at two like and two cross polarizations at 5 and 13.9 GHz, the agreement with model predictions is within 2 dB except at 13.9-GHz cross polarization. Backscattering from >15-cm-thick saline ice is generally dominated by scattering from the top surface while backscattering from <8-cm-thick saline ice can be strongly influenced by returns from the ice/water interface, particularly at frequencies less than about 5 GHz     

A microwave scattering model for layered vegetation

A microwave scattering model was developed for layered vegetation based on an iterative solution of the radiative transfer equation up to the second order to account for multiple scattering within the canopy and between the ground and the canopy. The model is designed to operate over a wide frequency range for both deciduous and coniferous forest and to account for the branch size distribution, leaf orientation distribution, and branch orientation distribution for each size. The canopy is modeled as a two-layered medium above a rough interface. The upper layer is the crown, containing leaves, stems, and branches. The lower layer is the trunk region, modeled as randomly positioned cylinders with a preferred orientation distribution above an irregular soil surface. Comparisons of results obtained using this model with measurements from deciduous and coniferous forests show good agreement at several frequencies for both like and cross polarizations     

Microwave lowpass filters using hybrid uniplanar structures

Deals with the design of lowpass filters using hybrid uniplanar structure. To minimise the spurious response levels, a new compact quasi-lumped element concept is proposed. For a 2.5 GHz cutoff frequency filter experimental data show that spurious responses are lower than 20 dB up to 18 GHz     

Low-frequency scattering from cylindrical structures at obliqueincidence

Classical Rayleigh scattering theory is extended to the case of a homogeneous dielectric cylinder of arbitrary cross section whose transverse dimensions are much smaller than the wavelength. By assuming that the surface fields can be approximated by those of the infinite cylinder, the far zone scattered field is expressed in terms of polarizability tensors, whose properties are discussed. Numerical results are presented for circular, semicircular, triangular, and square cylinders. The results are applicable to the remote sensing of twigs, stalks, and vegetation needles at centimeter and millimeter wavelengths     

Analyses of electromagnetic scattering from wire-mesh structures

Various analytical and numerical methods have been developed recently to calculate the interaction of wire-mesh structures with electromagnetic waves. An intercomparison of these methods is reported for the two cases where the wire junctions are bonded and unbonded. In particular, it is found that the average boundary condition method of Kontorovich and Astrakhan appears to give valid results only if the interwire spacing of the mesh is small compared with a wavelength. However, we may safely conclude from both their work and ours that bonding the wire junctions may degrade the shielding performance of such meshes.     

Microwave scattering from mixed-species forests, Queensland, Australia

The potential of synthetic aperture radar (SAR) data for retrieving the above-ground and component (e.g., branch, trunk) biomass of mixed-species forests (including woodlands) typical to subtropical Queensland, Australia, was evaluated using a wave scattering model based on that of Durden et al. (1989). The model was parameterized using field data collected for nine forest types, which were selected through combined analysis of 1 : 4000 aerial photographs and light detection and ranging data. The simulated SAR backscatter data demonstrated a good correspondence at most frequencies and polarizations with Airborne SAR data. Analysis of scattering mechanisms revealed dominance of C-band horizontal-vertical (HV) volume scattering and increases with small-branch/foliage biomass, dominance of L- and P-band HH trunk-ground scattering and increases with trunk biomass, and dominance of L-band HV volume (branch) scattering and increases with large-branch biomass. The study concluded that above-ground biomass estimated using empirical relationships with selected SAR channels will be more reliable for forests of similar structural form due to dominance of microwave interaction with particular biomass components and the strength and consistency of relationships between these and the affiliated components that represent the total. In mixed-species forests, retrieval will be compromised by interaction with a greater diversity of structures and variability in relationships between structural components. Although empirical relationships with selected combinations of channels (e.g., L-band HH/HV) might allow retrieval of component and total biomass of forests containing trees of similar form (e.g., as mapped using Landsat sensor data), the use of SAR inversion models was considered a more appropriate route for retrieving the biomass of forests containing a mix of structural forms.     

Microwave breast imaging: 3-D forward scattering simulation

Active microwave imaging (MWI) is emerging as a promising technique for the detection of biomedical anomalies such as breast cancer because of the high electrical contrasts between malignant tumors and normal tissue. Previously, we have developed fast two-dimensional forward and inverse scattering algorithms for MWI systems. In this paper, we report the full three-dimensional (3-D) forward scattering simulation in order to account for 3-D effects and to provide a fast solver in future 3-D nonlinear inverse scattering methods. The 3-D fast forward method is based on the stabilized biconjugate-gradient fast Fourier transform (BCGS-FFT) algorithm. The method has been validated for various MWI measurement scenarios. Using this fast simulation method, we demonstrate the importance of accounting for 3-D effects in MWI, and we compare numerical results with the measurements from an experimental prototype.     

介质体电磁散射的偶极子模型法研究

提出用偶极子模型法来分析介质体的电磁散射.该方法以矩量法和Schaubert-Wilton-Glisson(SWG)基函数为基础,把介质体剖分成一定数量的四面体元.在介质体内,把含有公共面的体元对等效成电偶极子;在介质体表面,把边界面及其对应的体元等效成电偶极子.当等效偶板子单元离观察点大于临界距离时,用偶极子模型法计算阻抗矩阵元素.偶极子模型法简单易操作,不仅能大幅度降低阻抗矩阵的计算时间,还简化了边界条件的处理.数值结果表明了该方法的高效性及与原方法几乎相同的计算精度.     

A three-component scattering model for polarimetric SAR data

An approach has been developed that involves the fit of a combination of three simple scattering mechanisms to polarimetric SAR observations. The mechanisms are canopy scatter from a cloud of randomly oriented dipoles, evenor double-bounce scatter from a pair of orthogonal surfaces with different dielectric constants and Bragg scatter from a moderately rough surface. This composite scattering model is used to describe the polarimetric backscatter from naturally occurring scatterers. The model is shown to describe the behavior of polarimetric backscatter from tropical rain forests quite well by applying it to data from NASA/Jet Propulsion Laboratory's (JPLs) airborne polarimetric synthetic aperture radar (AIRSAR) system. The model fit allows clear discrimination between flooded and nonflooded forest and between forested and deforested areas, for example. The model is also shown to be usable as a predictive tool to estimate the effects of forest inundation and disturbance on the fully polarimetric radar signature. An advantage of this model fit approach is that the scattering contributions from the three basic scattering mechanisms can be estimated for clusters of pixels in polarimetric SAR images. Furthermore, it is shown that the contributions of the three scattering mechanisms to the HH, HV, and VV backscatter can be calculated from the model fit. Finally, this model fit approach is justified as a simplification of more complicated scattering models, which require many inputs to solve the forward scattering problem     

A model for edge scattering from dichroic surfaces

This paper discusses experimental and theoretical results for edge scattering from dichroic (frequency selective) surfaces. Measurements presented for half planes show the nature of the scattering from the edge for both reflection and transmission frequencies. An impedance boundary condition approach is used to calculate the diffraction from dichroic edges. In the analysis the dichroic surface is replaced with an equivalent surface impedance and the total scattered field calculated. Measurements support the theory demonstrating the usefulness of the approach. The method is also useful for calculating diffraction from dielectric edges.     

A diffuse scattering model for urban propagation prediction

A novel, simple diffuse scattering model based on a ray approach suitable for urban radio propagation is presented. The scattering model is arranged so that the main parameters, having a precise physical meaning, can be easily tuned using measurement results. The model is tested against 900-MHz measurements and the scattering contribution is shown to be important for both received power and channel dispersion in a typical microcellular case     

Four-component scattering model for polarimetric SAR image decomposition

A four-component scattering model is proposed to decompose polarimetric synthetic aperture radar (SAR) images. The covariance matrix approach is used to deal with the nonreflection symmetric scattering case. This scheme includes and extends the three-component decomposition method introduced by Freeman and Durden dealing with the reflection symmetry condition that the co-pol and the cross-pol correlations are close to zero. Helix scattering power is added as the fourth component to the three-component scattering model which describes surface, double bounce, and volume scattering. This helix scattering term is added to take account of the co-pol and the cross-pol correlations which generally appear in complex urban area scattering and disappear for a natural distributed scatterer. This term is relevant for describing man-made targets in urban area scattering. In addition, asymmetric volume scattering covariance matrices are introduced in dependence of the relative backscattering magnitude between HH and VV. A modification of probability density function for a cloud of dipole scatterers yields asymmetric covariance matrices. An appropriate choice among the symmetric or asymmetric volume scattering covariance matrices allows us to make a best fit to the measured data. A four-component decomposition algorithm is developed to deal with a general scattering case. The result of this decomposition is demonstrated with L-band Pi-SAR images taken over the city of Niigata, Japan.     

A nonrectangular patch model for scattering from surfaces

A model is derived for treatment of surfaces using nonrectangular patches. Tests show good agreement, where comparison is possible, with results published elsewhere. The results are shown to be reasonable even in cases where some of the quadrilaterals degenerate to triangles.