Mojtaba Dehmollaian

Refocusing Through Building Walls Using Synthetic Aperture Radar

Through-wall imaging/sensing using a synthetic aperture array technique is studied by employing ultrawideband antennas and for wide incidence angles. The propagation through building walls, such as brick and poured concrete in response to point sources near the walls, is simulated by using high-frequency methods. Reciprocity is used to find the responses of point targets behind walls, which are then used to simulate the synthetic aperture radar (SAR) imaging through the walls. The effect of building walls on the target-image distortions is investigated by simulations and measurements. It is shown that by using the idea of match filtering, the effect of the wall can be compensated for, and the point target response can be reconstructed, provided that the wall parameters are known. An optimization method based on minimization of squared error in the SAR image domain within an area confined within the expected point-spread function is used to estimate the wall parameters and sharpen the image simultaneously. A controlled experiment within the laboratory environment is performed to verify the methods presented. It is shown that for an ultrawideband system operating over a frequency band of 1-3 GHz, highly distorted images of two point targets in close proximity of each other behind a wall can be resolved after refocusing. A dual-frequency synthetic method is also presented that can improve the cross-range resolution of the refocused image.

Through-the-Wall Imaging Using Differential SAR

An algorithm for imaging of targets behind walls is proposed to reduce the wall reflection and enhance the signal-to-clutter ratio. The image formation is based on differential synthetic-aperture-radar image formation employing a continuous-wave radar system. In this approach, instead of using individual backscattered signals, the image is formed by employing the difference signals obtained by subtracting two successive signals along the track. This way, specular reflections are totally eliminated without the need for the knowledge of the wall parameters. This also affects the point target response which is corrected by an integration process. By using backscattered fields from small trihedral corner reflectors behind a poured concrete wall, measured over a frequency band of 1-2.5 GHz, the proposed method is demonstrated.

Refocusing through building walls using synthetic aperture radar

In standard synthetic aperture radar (SAR) imaging the effect of the wall is not usually taken into account. For walls with high permittivity (and/or relatively large thickness) such as brick and poured concrete walls, this effect becomes important. In this paper, numerical and experimental study is performed to address the effect of the wall on the target images. The shift in the target position in the range direction is observed. An important influence of the wall, which is widening of the point spread functions along the cross range direction, is indicated. This effect significantly disturbs (defocuses) the target image. In order to capture this effect, transmissivity through building walls are measured and methods for refocusing are provided. The significance of refocusing methods is illustrated when multiple targets close to each other, are placed behind walls.

Electromagnetic Scattering From Foliage Camouflaged Complex Targets

In this paper, a hybrid target-foliage model based on existing electromagnetic techniques is developed to investigate the scattering behavior of hard targets embedded inside a forest canopy at high frequencies. The proposed model is composed of two basic scattering models, one for foliage and the other for the hard targets. The connection between these two models, which accounts for the interaction between the foliage scatterers and the target and vice versa, is accomplished through the application of the reciprocity theorem. Wave penetration through the forest canopy and near-field and far-field scattering from the canopys constituents is calculated using a coherent discrete scattering model that makes use of realistic tree structures. Calculation of scattering from a hard target illuminated by the reduced incident field and the scattered field of nearby vegetation is carried out using an iterative physical optics (PO) method formulated for fast computation of foliage-target interaction. To reduce the number of iterations, geometrical optics (GO) approximation is initially used for determining the shadowed areas over the hard target when illuminated by individual foliage scatterers. Furthermore, using a scaled measurement system at millimeter-wave frequency, the accuracy of the iterative PO model is demonstrated, employing a complex target that occupies a volume as big as 86lambdatimes33lambdatimes20lambda

Analytical, numerical, and experimental methods for through-the-wall radar imaging

In this paper a physics-based approach for image formation of targets behind complex wall structures is presented. Analytical and numerical techniques are used for the development of forward scattering models which are then exploited in construction of matched filters for ultra-wideband synthetic aperture radars operating over a wide rang of incidence angles. Special scattering models for different wall types including cinder block and reinforced concrete walls are presented using efficient numerical and approximate analytical techniques. These allow for construction of SAR images as well as development of a refocusing algorithm. An experimental ultra-wideband radar is set up in the laboratory environment for the evaluation of the models presented. Also, a radar measurement configuration is proposed that allows for elimination of direct reflection from the walls.

An Approximate Solution of Scattering From Reinforced Concrete Walls

Scattering from reinforced concrete walls, such as vertical rebar and crossed rebar walls, is analyzed by using an approximate analytical approach. The solution is obtained for a plane-wave incidence with an arbitrary angle of incidence and polarization under the thin-wire approximation. The formulation is based on derivation of the Greens function of a 1-D periodic sources inside a dielectric slab. Since the metal thickness is assumed to be small compared to the wavelength, the induced currents are predominantly axial and the transverse components of the induced currents are ignored. This drastically simplifies the solution for this problem which is obtained using a straightforward point-matching technique. The solution for two-dimensional periodic crossed rebar concrete walls is also cast in terms of the periodic Greens function obtained for the 1-D periodic structure of the vertical rebar walls. Using the finite-difference time-domain (FDTD) simulation results, it is shown that the induced currents on thin crossed re- bars (diameter <lambda0/5) have primarily a progressive phase equal to that of the incident field in both directions. This observation also points to a simple approximate solution where the problem of scattering from a 2-D periodic crossed rebar concrete wall is decomposed into two coupled 1-D periodic structures of vertical and horizontal rebar walls. The Bragg mode scattered fields of the reinforced walls are computed for different wall parameters and incidence angles for an operation frequency range of 0.5-2.0 GHz and validated by FDTD simulation results.

Optimum Polarizations for Discrimination of a Foliage-Camouflaged Target, Using Genetic Algorithms

Many realizations of foliage around a hard target are run to obtain the statistical variations of foliage and target responses. This is accomplished by using a hybrid target-foliage model, developed for the investigation of the scattering behavior of metallic targets embedded inside a forest canopy. This model is based on the coherent scattering theory of wave propagation through the foliage and an iterative physical optics approximation of scattering from the target. The model is capable of accounting for the first-order near-field interactions between the hard target and the foliage. Fully polarimetric simulation results of a foliage-camouflaged metallic target having complex geometry are generated at 2 GHz, and a polarization synthesis optimization method for improving signal-to-clutter ratio is carried out by applying genetic algorithms.

Simulation of Through-Wall Microwave Imaging: Forward and Inverse Models

In this paper we use Finite Difference Time Domain (FDTD) numerical technique and Physical Optics (PO) approximation in conjunction with phase conjugation method, for computation of transmitted field through walls and image formation. Using this model, different refocusing techniques can be investigated. Numerical results for imaging of target behind a solid block wall and cinder block wall are provided.

Hybrid FDTD and ray optics approximation for simulation of through-wall microwave imaging

In this paper the phenomenology of the near-field microwave imaging through walls is examined. A hybrid finite difference time domain (FDTD) numerical technique and physical optics (PO) approximation in conjunction with the phase conjugation method is used, for the computation of the transmitted field through different walls and image formation. Numerical results, delineating features of wideband and wide angle wave propagation and imaging through walls are presented

Refocusing through single layer building wall using synthetic aperture radar

Through-wall imaging using synthetic aperture array technique is studied by employing ultra-wideband antennas and for wide incidence angles. The effect of the building walls on the target image distortions is investigated by simulations and measurements. It is shown that using the idea of match filtering, the effect of the wall can be compensated for and point target response can be reconstructed. A controlled experiment within the laboratory environment is performed to verify the methods, presented. It is shown that for an ultra-wideband system operating over frequency band of 1-3 GHz highly distorted images of two point targets in close proximity of each other behind a wall can be resolved after refocusing. A dual-frequency synthetic method is also presented that can improve the cross- range resolution of the refocused image.