Doppler spectra obtained from simulations of ocean-like surface bistatic scattering are shown under the first order small slope approximation of rough surface scattering. Spectral properties are examined as a function of wind speed, polarization, and observing geometry in order to assess the potential for the bistatic remote sensing of various sea surface properties.

Radar images of a 1-D perfectly conducting rough surface are compared for monostatic and for the equivalent bistatic imaging configurations. The results show that, while first-order “single” scattering images are largely similar; higher-order scattering effects can show differing signatures for these two imaging modalities.

We present a method to estimate and quantify geometric parameters of a buried object in near-field microwave images. These parameters are shape, location, size, and orientation. Unlike the parameter estimation based on a conventional ground-penetrating radar image, a 3-D well-focused coherent image is generated. Instead of choosing a$z$-slice of the 3-D image, both 2-D projection and shadow projection images are used to perform the parameter estimation. The shape of the buried object is determined based on both$\boldsymbol x$- and$y$-polarized images. Other geometric parameters, such as location, size, and orientation, are estimated by the method of spatial moments. Both numerical and experimental data of an object buried in a dielectric slab are used to demonstrate the efficacy of the proposed geometric parameter estimation algorithm.

The Global Navigation Satellite System reflectometry (GNSS-R) community has traditionally used the Kirchhoff approximation to model how the coherent reflected power reduces as a function of the RMS height of the rough surface observed. While this approximation has been valid for most GNSS-R applications to date, the community has recently raised the question of the model's validity when the normalized coherent reflected power is very low ( ≤ -20dB) due to increased interest in measuring the coherent reflected power in situations where the incoherent reflected power is dominant. To investigate the trend of the normalized coherent reflected power below -20dB, Monte Carlo simulations were performed by randomly generating Gaussian rough PEC surfaces with various RMS heights and averaging over multiple realizations at each RMS height to obtain a reflected power whose incoherent component has been averaged out, leaving only the coherent reflected power behind. In this process, the statistics of the Rician random variable obtained when coherent specular scattering from a rough surface is measured must also be considered, and it can be shown that the accuracy of the coherent power estimate is dependent on the number of realizations used and the ratio of the coherent and incoherent powers. This analysis also shows that a large number of averages can be required to obtain reasonable accuracy in estimating the coherent power in the limit of low coherent to incoherent power ratio.

A shadow projection approach for near-field microwave imaging is developed to distinguish the size and shape of objects embedded in layered media. An ordinary microwave image may only present a small portion of the front surface of an object due to the lensing effect of refraction at the boundary between two adjacent layers with a high dielectric contrast. Therefore, instead of looking at the primary image, the projection onto the interface behind the object is investigated to better discern the object’s true size and shape. A diffraction tomographic algorithm is modified to accentuate the shadow image. The efficacy of this shadow projection method is demonstrated by 3-D numerical simulations of air voids in a dense dielectric slab.

The second-order small perturbation method is applied to investigate brightness temperature corrections caused by the rough interfaces of a two-layer medium. The spectral weighting functions of the two rough interfaces are extracted from the solution, and their properties examined. It is found that the functions are identical for the two interfaces as the spectral variable approaches zero, indicating an identical weighting of the surface height variance on each interface and an additive effect on the brightness temperature at nadir. Sample results for some realistic scenarios show that surface roughness in a two-layer medium can increase or decrease the observed brightness temperature at shallower angles, and in the case of a wideband measurement, can shift the interference pattern in frequency.

In this paper, we propose a novel greedy algorithm referred to as look-ahead hybrid matching pursuit (LAHMP) for multipolarization through-wall radar imaging (TWRI). From the viewpoint of compressive sensing, the task of multipolarization TWRI can be formulated as a problem of sparsity pattern recovery under the joint sparsity model. A newly developed greedy algorithm for joint sparsity model, hybrid matching pursuit (HMP), combines the strengths of orthogonal matching pursuit and subspace pursuit and improves the accuracy of the sparsity pattern recovery. Besides, the look-ahead strategy can select an optimal atom by evaluating its effectiveness on the overall reconstruction quality. Through integrating the virtues of HMP with the look-ahead strategy, the proposed LAHMP aims to more accurately select atoms corresponding to the true targets behind walls. Experiments based on measured radar data show that, compared to existing greedy algorithms, LAHMP provides better image quality at affordable expense of computational complexity.

In this paper, we consider the problem of fusion of multi-polarization radar images and develop a new greedy algorithm referred to as hybrid matching pursuit (HMP). By combining the strengths of the orthogonal matching pursuit algorithm and the subspace pursuit algorithm, HMP can enhance target reflections and attenuate background clutter. Experimental results based on measured radar data demonstrate that HMP offers better image quality with higher target-clutter-ratio compared to some popular greedy algorithms.

This paper investigates the algorithms that are used to predict the full polarimetric bistatic normalized radar cross-section of rough surfaces. These include small perturbation method (SPM), the physical optics (PO) approach, the small slope approximation (SSA) and the integration equation method (IEM) and its derivatives improved IEM and advanced IEM. The methods are then compared to ground truth values obtained from multiple Monte Carlo runs a numerical Method of Moments (MOM) code using the same surface statistics. Effects of using band-limited exponential instead of a true exponential correlation function for surface statistics are also explored. Mean L1-norm error values integrated over the hemisphere are given between AIEM and MOM and SSA and MOM.

Through-wall radar images have limited resolution due to the finite bandwidth and aperture size of man-portable systems. These images may be enhanced by optimizing image sparseness. The inverse imaging function is posed as a constrained nonlinear optimization problem that is solved using the Lagrange multiplier (LM) method. This robust approach has the advantage over related image optimization algorithms in that the LMs are solved within the algorithm rather than being chosen a priori. This yields a system of equations that are solved iteratively rather than by gradient search. Through-wall and free-space measurement examples are presented using a small portable wideband UHF radar system to collect scattered energy from several trihedral (pointlike) targets to demonstrate the enhancement of the radar images.

A high-frequency asymptotic technique based on the Uniform Geometric Theory of Diffraction (UTD) is employed for building interior imaging. The analysis is implemented using a ray-tracing technique to account for multiple scattering interactions in a building, along with a set of heuristic diffraction coefficients for dielectric wedges and corners. Imaging of the synthetic aperture radar data is carried out by the conventional fast Fourier transform method to transform to the downrange domain, and combined with a coherent near-zone imaging function for cross-range. Comparisons with experimental data for a scaled-down building model are given to demonstrate the suitability and efficacy of our analysis for through-wall building imaging. The UTD ray mechanisms account for the dominant scattering features observed in the image.

A numerical study of the retrieval of sea surface height profiles from low grazing angle radar observations is described. The study is based on a numerical method for electromagnetic scattering from 1-D rough sea profiles, combined with the ldquoimproved linear representationrdquo of Creamer for simulating weakly nonlinear sea surface hydrodynamics. Numerical computations are performed for frequencies from 2975 to 3025 MHz so that simulated radar pulse returns are achieved. The geometry utilized models a radar with an antenna height of 14 m, observing the sea surface at ranges from 520 to 1720 m. The low grazing angles of this configuration produce significant shadowing of the sea surface, and standard analytical theories of sea scattering are not directly applicable. Three approaches for retrieving sea height profile information are compared. The first method uses a statistical relationship between the surface height and the computed radar cross sections versus range (an incoherent measurement). A second method uses the phase difference between scattering measurements in two vertically separated antennas (ldquovertical interferometry) in the retrieval. The final technique retrieves height profiles from variations in the apparent Doppler frequency (coherent measurements) versus range and requires that time-stepped simulations be performed. The relative advantages and disadvantages of each of the three approaches are examined and discussed.

Retrieval of one dimensional ocean surface profile information is studied through the use of S and X band radar backscattering data at low grazing incidence. The required backscatter data is obtained through the use of numerical electromagnetic scattering codes. The simulations compute backscatter at multiple frequencies in each time step, so that range-resolved radar cross sections are obtained after an FFT operation. Profile retrievals are obtained through a simple "tilted Bragg" assumption, which allows local incidence angles on the surface to be directly determined from RCS data. These local incidence angles are then translated into surface slopes, and further integrated to construct the estimated profile. Initial results from this process are reported in this paper; further studies are in progress to quantify the overall accuracy achieved as a function of various surface and sensor parameters.

The variations in the radar cross section (RCS) of two-dimensional floating targets are studied by means of a numerical model. Of particular interest is the rolling of a target on the waves. It is found that the rolling motion has the most significant effect on the RCS for low-grazing angles of incidence. The effect is very pronounced for a high-RCS shiplike target due the corner-reflector effect between the sea surface and the ship hull, and less noticeable for a low-RCS shape. A simplified means of estimating the variation is also presented

A numerical model is applied in a Monte Carlo study of scattering from a three dimensional penetrable object below a lossy dielectric rough interface. Both time and frequency domain results are investigated to illustrate the relative importance of coherent and incoherent scattering effects in the sample problem considered. Results show that introducing a reduced transmission coefficient is reasonable for object coherent scattering predictions in this example, and that incoherent object/surface interaction effects approximately follow a simple scaling behavior as surface roughness is increased.

Channeling phenomenon is a type of forward scattering and multipath that occasionally occurs when a well-formed sea wave briefly reflects or focuses the out-of-plane energy toward the receiver, yielding a higher than expected, broadband power gain. A series of X-band, forward-scattering measurements in a large wave tank observed multiple channeling events at low grazing angles. The wave tank simulated a one-dimensional, 1/10th-scale Pierson-Moskowitz surface at sea states 0, 3, and 5. During the measurements, channeling events were observed under crosswind wave conditions when a properly constructed wave trough, coinciding with the radar's line of sight, channeled or focused the out-of-plane, forward-scattered energy toward the receiver. Presented is a summary of the channeling events observed and a physical model of the channeling phenomenon based on experimental observations and numerical simulations.

The results of a numerical investigation of the electromagnetic scattering from two-dimensional (2-D) targets on a time-evolving sea surface are presented. The 2-D radar cross section (RCS), or "echo width," is computed as a function of time as the sea surface evolves linearly using the spectrally accelerated generalized forward-backward method. It is shown that the RCS varies with time on the order of seconds, which is much longer than the typical pulse width or pulse repetition rate of search radars. It is also observed that for low-grazing angles of incidence the primary sea surface influence on the RCS comes from the long waves in the ocean spectrum.

A numerical model for computing scattering from a three-dimensional (3D) dielectric object above or below a rough interface is described. The model is based on an iterative method of moments solution for equivalent electric and magnetic surface current densities on the rough interface and equivalent volumetric electric currents in the penetrable object. To improve computational efficiency, the canonical grid method and the discrete dipole approach (DDA) are used to compute surface to surface and object to object point couplings, respectively, in O(N log N), where N is the number of surface or object sampling points. Two distinct iterative approaches and a preconditioning method for the resulting matrix equation are discussed, and the solution is verified through comparison with a Sommerfeld integral-based solution in the flat surface limit. Results are illustrated for a sample landmine detection problem and show that a slight surface roughness can modify object backscattering returns.

Gaussian beam (GB) representation methods are used to analyze the electromagnetic coupling into and the scattering by a large nonuniform cavity. The aperture field in the cavity is decomposed into beams using the Gabor expansion, and shooting techniques are then employed. The method is illustrated only for the two-dimensional (2-D) case. The GBs are tracked axially using the rules of beam optics which ignore any beam distortion upon reflection at the walls. The effects of beam distortion are not significant for relatively slowly varying waveguide cavities. The field scattered into the exterior by the termination within the cavity is found using a reciprocity integral formulation which requires a knowledge of the beam fields near the termination. Numerical results based on this GB approach are presented and compared with results based on an independent reference solution.>