Gianluca Gennarelli

A Microwave Resonant Sensor for Concentration Measurements of Liquid Solutions

This paper presents the design, fabrication, and characterization of a microwave resonator as a tool for concentration measurements of liquid compounds. The sensing device is a rectangular waveguide cavity tuned at 1.91 GHz, which exploits the fundamental TE101 mode in a transmission-type configuration. The coupling structure is optimized by means of a finite element code so as to achieve a high Q-factor. According to the type of substance inside the mixture, its concentration is conveniently related to changes of the S21 scattering parameter (transmission coefficient) in terms of: 1) resonance frequency; 2) 3-dB bandwidth; and 3) amplitude at the resonance frequency. Experimental tests on liquid solutions in controlled conditions are presented to evaluate the performance of the device.

A Linear Inverse Scattering Algorithm for Radar Imaging in Multipath Environments

This letter deals with the electromagnetic imaging in the presence of multipath propagation of interest for through-wall and urban sensing scenarios. The 2-D tomographic approach here presented combines a linear inverse scattering model, based on the Kirchhoff approximation, with the finite-difference time-domain (FDTD) technique. In particular, FDTD is exploited to evaluate the incident field and Greens function in noncanonical scenarios, so that the kernel of the linear integral equation is completely built. After, an inversion scheme based on the truncated singular value decomposition is applied to obtain a regularized solution of the problem. Numerical results demonstrate that the proposed approach yields well-focused images free of multipath ghosts, thus allowing to discriminate the actual target position. Moreover, it permits to highlight the capabilities offered by multipath exploitation such as improved crossrange resolution and detection of targets in the non-line-of-sight region of the radar.

SAR Imaging Algorithms and Some Unconventional Applications: A unified mathematical overview

This article deals with two significant aspects related to synthetic aperture radar imaging (SAR-I) of relevant theoretical and applicative interest. The first objective regards the analysis of the most-used SAR-I approaches under the unified mathematical framework provided by the Porter-Bojarski integral equation. The second objective is to provide an updated overview on how SAR-I research is generalizing previous algorithms to deal with unconventional scenarios.

RF/Microwave Imaging of Sparse Targets in Urban Areas

This letter deals with the localization of sparse targets in urban areas. The problem is tackled in the framework of 2-D linear inverse scattering accounting for the complexity of the scenario, and a sparse optimization scheme is exploited as an effective technique capable of providing high-resolution images. In the proposed approach, due to the noncanonical scenario to be considered, the kernel of the relevant integral equation is computed numerically via the finite-difference time-domain method. The achievable reconstruction capabilities are assessed by means of a numerical analysis, which compares the results based on sparse optimization to those provided by the truncated singular values decomposition algorithm.

Radar Imaging Through a Building Corner

Through-wall imaging (TWI) requires dealing with targets embedded in a complex obscuring environment such as the walls of a building. This obscuring layout is often composed by many simple elements (possibly interacting) such as slabs, corners, and T-like structures. Most of the existing literature on TWI has focused on slab-like walls, which is reasonable when the targets are relatively far from corners. This paper instead concerns the TWI in the more challenging situation where the targets are in close proximity (inside and/or outside) of a building corner. The aim is to gain insight into how propagation through the corner impacts on the imaging problem. To keep the study simple, a preliminary analysis is presented for a 2-D geometry under the linearized Born approximation. First, the Greens function, as well as the kernel of the relevant scattering operator, is evaluated by using a high-frequency analytical approach based on the geometrical optics and the uniform theory of diffraction. This allows one to take into account the multipath propagation phenomena and provide thus an expression of the scattering operator more accurate than that viable under the assumption of a simple slab wall. Then, the imaging is achieved by solving the relevant linear inverse scattering problem with a regularizing truncated-singular-value-decomposition algorithm. The filtering introduced by the inversion procedure, which is dependent on the considered background scenario, is highlighted and linked to the achievable performance while imaging targets both internal and external with respect to the corner. Finally, reconstruction results obtained from synthetic data are reported to assess the approach.

Real-Time Through-Wall Situation Awareness Using a Microwave Doppler Radar Sensor

This paper deals with the development of a short-range radar suitable for the detection of humans behind visually opaque structures such as building walls. The system consists in a continuous wave Doppler radar operating in the S-band of the electromagnetic spectrum in order to ensure an adequate signal penetration through the walls. Based on the interaction of the electromagnetic waves with human targets, a phase modulation of the radar signal arises due to their movements and tiny periodic chest displacements associated with the respiratory activity. A simple and effective radar data processing algorithm is proposed to detect, in real-time, the presence of one or several human subjects in the through-wall scene. Such an algorithm automatically provides also an indication on whether the subjects are static or moving in the environment. As shown by experimental tests carried out in an indoor scenario, the proposed sensing device and related signal processing yields prompt and reliable information about the scene thus confirming its practical value.

Multiple Extended Target Tracking for Through-Wall Radars

Tracking moving targets hidden behind visually opaque structures as building walls is a crucial issue in many surveillance, rescue, and security applications. The electromagnetic waves at the low microwave frequency range penetrate into common building materials and thereby enable the radar to expose behind the wall scene. However, due to complexity of the scattering scenario, the radar signal undergoes multipath propagation phenomena. These typically manifest themselves as environmental clutter which may impair detection and tracking of true targets. In this paper, a signal processing strategy is proposed to track multiple extended targets in a scene by means of a wide-band monostatic through-wall radar. The system collects data sets at regular time steps which are first processed by a microwave tomographic technique. Then, a detection/tracking stage is implemented in order to track the position and dynamics of targets in real time. An extended target-tracking approach is applied to properly exploit at the tracking stage the information related to extended nature of targets. The effectiveness of the proposed signal processing chain is assessed by numerical tests based on full-wave data pertaining to an indoor scenario.

On the Achievable Imaging Performance in Full 3-D Linear Inverse Scattering

This communication is concerned with the performance achievable within a 3-D inverse scattering approach in relationship with the measurement configuration. The problem is undertaken under the Born approximation, and three configurations are considered, namely a multimonostatic, a single view/multistatic and a multiview/multistatic, all of them in the multifrequency framework. The analysis is worked out with the tools of the diffraction tomography and of the singular value decomposition of the relevant operator. Tomographic reconstructions based on full-wave synthetic data are shown.

Radar Imaging Through Cinderblock Walls: Achievable Performance by a Model-Corrected Linear Inverse Scattering Approach

We address the problem of imaging targets located behind an inhomogeneous wall made with cinderblocks. The problem, which has relevance in through-wall-imaging applications, is characterized by the presence of multipath propagation phenomena usually producing artifacts and distortions in the retrieved images, if not suitably accounted for in the scattering model. The strategy here adopted to mitigate this issue is to employ a linearized scattering model based on the Born approximation, where the kernel of the relevant integral equation is evaluated numerically by means of the finite-difference time-domain method. In this way, the complexity of the background scenario is accurately taken into account. The inversion is successfully performed by the truncated singular value decomposition algorithm so as to regularize the inverse problem. The achievable imaging capabilities are analyzed in terms of resolution limits, and most notably, resolution can be effectively enhanced, owing to multipath exploitation. Numerical tests based on synthetic data are reported to assess the reconstruction performance in the case of canonical objects.

Passive Multiarray Image Fusion for RF Tomography by Opportunistic Sources

The large diffusion of wireless infrastructures in public and private areas is currently stimulating research on surveillance radar systems capable of exploiting network transmissions as potential sources of opportunity. Since these sources are generally narrowband, we propose in this letter a single-frequency approach for imaging targets by using passive arrays deployed around the scattering scene. Single-frequency data allow casting the imaging as an inverse source problem, which avoids the need to retrieve information about the sources prior to imaging. The drawbacks of the highly coarse resolution and blinding effects due to the sources are overcome by employing a multiarray image fusion strategy in conjunction with a change detection scheme for imaging moving targets. The proposed approach is tested via numerical experiments based on full-wave synthetic data corresponding to an indoor scenario.