Jose A. Martinez-Lorenzo

SAR IMAGING OF SUICIDE BOMBERS WEARING CONCEALED EXPLOSIVE THREATS

This paper deals with the problem of detecting potential suicide bombers wearing concealed metallic and dielectric objects. The data produced by Millimeter-Wave-Radar system, working on a Mulptiple Frequency-Multiple Transmitters and Multiple Receivers configuration (MF-MTMR), is synthetically generated by an electromagnetic code based on Finite Differences Frequency Domain (FDFD) method. The numerical code provides the scattered field produced by the subject under test, which is later processed by using a multiple bistatic Synthetic Aperture Radar (SAR) algorithm. The blurring effect produced by the Point Spread Function (PSF) in the SAR image is removed by applying a regularized deconvolution algorithm that uses only magnitude information (no phase). Finally, the SAR algorithm and the deconvolution procedure are tested on a person wearing metallic and dielectric objects. The SAR response of dielectric rods is quite different from the metallic pipes. Our algorithm not only distinguishes between cases but also is capable of estimating the dielectric constant of the rods. Each constitutive parameter directly maps to the dielectric constant of explosive compounds, such as TNT or RDX, making feasible the detection of potential suicide bombers.

Physical Limitations on Detecting Tunnels Using Underground-Focusing Spotlight Synthetic Aperture Radar

This paper examines the feasibility of underground-focusing spotlight synthetic aperture radar (UF-SL-SAR) systems for tunnel detection applications. A general formulation is reviewed for generating UF-SL-SAR imaging by using multiple frequencies across a wide band and by focusing in space to subsurface points using well-known ray refraction at the nominal ground surface. A full-wave finite-difference frequency-domain model is used to consider wave propagation in realistic soil with loss- and frequency-dependent dielectric constant and a randomly rough ground surface, both of which serve to obscure and distort the returned tunnel target signal. Imaging results are presented for two representative soil scenarios: dry sand and moist clay loam. Considering the ground surface ray refraction for focusing greatly improves the SAR image relative to conventional SAR focusing at the ground surface. Using UF-SL-SAR, a small shallow tunnel is reasonably imaged for the sand case, despite the roughness of the ground interface. However, for higher conductivity moist clay loam, the clutter from the rough surface overwhelms the significantly attenuated target signal, which must propagate through the lossy intervening soil. It is demonstrated that, despite ideal focusing, the tunnel is successfully imaged only for the sand case.

Zooming and Scanning Gregorian Confocal Dual Reflector Antennas

The zooming and scanning capabilities of a Gregorian confocal dual reflector antenna are described. The basic antenna configuration consists of two oppositely facing paraboloidal reflectors sharing a common focal point. A planar feed array is used to illuminate the subreflector allowing the antenna to scan its beam. The resulting quadratic aberrations can be compensated by active mechanical deformation of the subreflector surface, which is based on translation, rotation and focal length adjustment. In order to reduce the complexity of the mechanical deformation, least squares fit paraboloids are defined to approximate the optimal correction surface. These best fit paraboloids considerably reduce scanning losses and pattern degradation. This work also introduces two different zooming techniques for the Gregorian confocal dual reflector antenna: the first consists of introducing a controlled quadratic path error to the main reflector aperture; and the second is based on reducing the size of the radiating aperture of the feeding array.

Sparse Array Optimization Using Simulated Annealing and Compressed Sensing for Near-Field Millimeter Wave Imaging

The optimization and use of a sparse array configuration for an active three dimensional (3D) millimeter wave imaging system for personnel security screening is presented in this work. The combination of the optimization procedure with the use of Compressed Sensing techniques allows drastic reduction in the number of sensors, thereby simplifying the system design and fabrication and reducing its cost. Representative simulation results showing good performance of the proposed system are provided and supported by sample measurements.

Fourier-Based Imaging for Multistatic Radar Systems

Fourier-based methods for monostatic and bistatic setups have been widely used for high-accuracy radar imaging. However, the multistatic configuration has several characteristics that make Fourier processing more challenging: 1) a nonuniform grid in k-space, which requires multidimensional interpolation methods, and 2) image distortion when the incident spherical wave is approximated by a plane wave. This contribution presents a Fourier-based imaging method for multistatic systems, solving the aforementioned limitations: the first, by using k-space partitioning and applying interpolation in each domain; the second, by approximating the spherical wave with multiple plane waves. Both solutions are fully parallelizable, thus allowing calculation time savings. Validation and benchmarking with a synthetic aperture radar backpropagation algorithm have been performed through 2-D and 3-D simulation-based examples. Imaging results from radar measurements have been assessed.

Reconstructing Distortions on Reflector Antennas With the Iterative-Field-Matrix Method Using Near-Field Observation Data

This work extends the mathematical formulation of the iterative-field-matrix method for observed data from the near-field region of a Perfect Electric Conductor. The method is used as a diagnosis tool for reflector antennas, to determine the positions and extent of distortions from their idealized shapes. The new formulation is tested on a reflector antenna with several significant bumps, and excellent results are achieved. This work also presents an example where the Method of Moments is used to generate the synthetic data and the inversion is performed using Physical Optics. Such a configuration ensures that the forward model is unbiased with respect to the inversion model, demonstrating that the new formulation is also robust for these realistic scenarios.

An Inverse Fast Multipole Method for Geometry Reconstruction Using Scattered Field Information

A novel inverse fast multipole method (FMM) application for accelerating inverse problem solution is presented. The idea is based on the multipole expansion properties of the scattered fields and reconstructed equivalent currents, which allow an easy inversion of the FMM operators, resulting in a forward solution of the inverse problem, i.e., without matrix inversion or cost function minimization. In addition, this technique allows the use of reconstruction domain discretization larger than half a wavelength and overcomes the restriction of having the entire target enclosed by a reconstruction domain, features that also contribute to the reduction of calculation time. Two 3D application examples are presented, highlighting the achieved inverse FMM speed-up with respect to previous inverse scattering methods for geometry reconstruction.

Wave Scattering by Dielectric and Lossy Materials Using the Modified Equivalent Current Approximation (MECA)

An approximation is presented for the calculation of the equivalent currents based on the oblique incidence of a plane wave on the interface between free space and lossy dielectric media. The Snell reflection coefficients establish the relation between incident and reflected waves, then, the boundary conditions establish the equivalent electric/magnetic currents in the free space region using the total magnetic/electric fields. The interface is discretized into planar triangular facets, on which currents are assumed to have constant amplitude and linear phase variation in order to analytically calculate the scattered fields. Our modified equivalent current approximation (MECA) reduces to the well-studied physical optics (PO) formulation in case of PEC surfaces. Simulations to validate the formulation are performed over electrically large canonical geometries - a lossy dielectric sphere, a rough plate and a right dihedral - to compare the calculated Radar cross section (RCS). Good agreement between MECA and analytical/method of moments (MoM) results is demonstrated.

Experimental results for standoff detection of concealed body-worn explosives using millimeter-wave radar and limited view ISAR processing

In the increasingly important problem of identifying suicide bombers wearing explosives concealed under clothing, it is essential to detect suspicious individuals at a distance. Current systems employ multiple sensors to determine the presence of explosives on people, including observing and following individuals, identifying explosive residues or heat signatures on the outer surface of their clothing, or by characterizing explosives using penetrating X-rays or terahertz wave radar. At present, radar is the only modality that can penetrate and sense beneath clothing at a distance of 10 to 50 meters without causing physical harm. Unfortunately, current radar systems require impractically large synthetic apertures to effectively distinguish people wearing concealed body-worn improvised explosive devices from innocent individuals. We explore these limitations and propose strategies to maximize detection with limited apertures.

On the Use of Improved Imaging Techniques for the Development of a Multistatic Three-Dimensional Millimeter-Wave Portal for Personnel Screening

The design and evaluation of an active three dimensional (3D) millimeter wave imaging system for personnel security screening is presented in this work. The system is able to produce a high- resolution 3D reconstruction of the whole human body surface and reveal concealed objects under clothing. Innovative multistatic millimeter wave radar designs and algorithms, which have been previously validated, are combined to signiflcantly improve the previous reconstruction results. In addition, the system makes use of a reduced amount of information, thus simplifying portal design. Representative simulation results showing good performance of the proposed system are provided and supported by sample measurements.