Tadahiro Negishi

Multipath Exploitation in Non-LOS Urban Synthetic Aperture Radar

Multipath is exploited to image targets that are hidden due to lack of line of sight (LOS) path in urban environments. Urban radar scenes include building walls, therefore creating reflections causing multipath returns. Conventional processing via synthetic aperture beamforming algorithms do not detect or localize the target at its true position. To remove these limitations, two multipath exploitation techniques to image a hidden target at its true location are presented under the assumptions that the locations of the reflecting walls are known and that the target multipath is resolvable and detectable. The first technique directly operates on the radar returns, whereas the second operates on the traditional beamformed image. Both these techniques mitigate the false alarms arising from the multipath while simultaneously permitting the shadowed target to be detected at its true location. While these techniques are general, they are examined for two important urban radar applications: detecting shadowed targets in an urban canyon, and detecting shadowed targets around corners.

Measurements to Validate the UTD Triple Diffraction Coefficient

Measurement results to validate the UTD triple diffraction coefficient are presented. The experimental setup consists of multiple metallic objects, with triangular and rectangular profiles, located inside an anechoic chamber and illuminated by a sector antenna to reproduce a spherical wavefront with a transverse electromagnetic (TEM) incident field. Another sector antenna is moved vertically to collect electromagnetic fields across the second order UTD Incident Shadow Boundaries and in the triple diffraction transition region. The measured and theoretical fields are compared using a free space normalization. Such comparison is also validated by calculating the mean error, the standard deviation, and root mean square error that occur between the theoretical model and the measured field. The results show excellent agreement between the theoretical third order UTD solution, employing the novel triple diffraction coefficient, and the experimental results.

RF Tomography in Free Space: Experimental Validation of the Forward Model and an Inversion Algorithm Based on the Algebraic Reconstruction Technique

Radio-frequency tomography was originally proposed to image underground cavities. Its flexible forward model can be used in free-space by choosing an appropriate dyadic Greens function and can be translated in the microwave domain. Experimental data are used to validate a novel inversion scheme, based on the algebraic reconstruction technique. The proposed method is improved by introducing physical bounds on the solution returned. As a result, the images of the dielectric permittivity profiles obtained are superior in quality to the ones obtained using classical regularization approaches such as the truncated singular value decomposition. The results from three experimental case studies are presented and discussed.

Metamaterial Spheroidal Cavity to Enhance Dipole Radiation

Metamaterials have been considered for their potential to improve the radiation characteristics of sources. A novel geometry consisting of a semioblate spheroidal cavity containing two layers, one made of DPS and the other made of DNG metamaterial, and built underneath an aperture in infinite metallic plane is considered. An exact analytical solution is obtained and its numerical evaluation demonstrates a significant improvement of the directive radiation of a dipole source, with the appropriate combination of the ordinary double positive (DPS) and DNG layers.

A dyadic target model for multistatic SAR/ISAR imaging

Current SAR/ISAR imaging algorithms rely upon the assumption that the area under observation consists of a superposition of infinitesimally small isotropic scatterers (i.e., the point scatterer model). This approximation fails to capture the real-world scattering mechanisms occurring within the targets under illumination. This paper proposes an imaging technique based upon the assumption that targets may be modeled as a superposition of infinitesimally small dipoles. The orientation of each dipole is encapsulated in a dyadic contrast function. The image reconstruction, i.e., retrieval of the dyadic reflectivity function from measured data, will provide information describing the shape and the direction of predominant edges of the target.

Exploitation of dominant scatterers for sidelobe suppression in radar tomography

Multistatic SAR/ISAR can be described and generalized using the principles of radar (or radio frequency) tomography. In radar tomography, distributed transmitters and receivers sense a region of interest using suitable waveforms. Using the principles of linear scattering (Born approximation), a (linear) relation exists between the measured returns and the shape of targets, and an image can be formed by inverting such relation. Due to the limited illumination and observation points, each point target will exhibit a spatially-varying point spread function (psf). A dominant scatterer having a large psf will inevitably mask the return of weak scatterers nearby. To mitigate this masking effect, the image is analyzed to identify dominant scatters. Then, these scatterers are modelled as dipole sources and included in the imaging formation as part of illumination points (i.e., new transmitters). When dominant scatterers are modelled as transmitters, their respective sidelobes are removed from the image, so that weak targets can be identified. Simulations and results demonstrate this concept.

Near-Field Transmission Imaging by 60 GHz Band Waveguide-Type Microscopic Aperture Probe

Near-field imaging has been intensively investigated to observe the shape and the physical properties of objects, aiming at wide applications in the areas of science and engineering. In this research, by using 60 GHz band waveguide-type microscopic aperture probe, the characteristics of the near-field imaging in transmission mode have been studied by simulation and experiment. The probe is made of a WR-15 rectangular waveguide with end-shielded metal plate and a 0.5 mm-diameter aperture. In the simulation, at first, the electric field distribution at the aperture, at the rear (waveguide) and the front positions (free space) are presented. Second, the transmitted electric fields are presented for three cases: (a) scanning of a dielectric slit, (b) by varying the distance between the aperture and a dielectric sample, and (c) scanning of a dielectric groove. In the experiment, the lateral resolution with a two-slit and the depth resolution with grooves having various depths in rectangular format are described and the results show both resolutions to be much shorter than the wavelength. Finally, the scanned images of the letter N punched through a dielectric material and a leaf are demonstrated.

Experimental Validation of the Quadratic Forward Model for RF Tomography

An effective way to solve the inverse scattering from dielectric objects relies on the Born approximation, which allows to linearize the problem and retrieve a qualitative reconstruction of the targets in terms of location and extent. The limits of the validity of the linear model can be extended by considering a quadratic approximation of the operator relating the scattered field data to the unknown object function. The use of the quadratic operator allows on the one hand to recover additional spatial variations of the object profile and on the other hand to mitigate the local minima (false solution) problem typically affecting nonlinear inversion methods. In this letter, we present an experimental validation of the quadratic inverse model for dielectric objects in free space. The data processing confirms that the tomographic images based on the quadratic model are better resolved compared to the ones provided by the inversion of the linear Born model.

RF tomography under the quadratic approximation

In this work, we discuss imaging results obtained by applying a quadratic approximation to RF Tomography.

RF tomography in free space: Experimental validation of the forward model and of a Conjugate Gradient inversion algorithm

Experimental data are used to validate a novel inversion scheme, based on a Conjugate Gradient algorithm. The proposed inversion provides actionable reconstruction results at a fraction of the computational effort needed by classical regularization techniques. Additionally, Conjugate Gradient allows us to introduce physical bounds on the solution returned, if a re-orthogonalization technique is also applied. Experimental case studies are presented and discussed.