Sevket Demirci

A Study on Millimeter-Wave Imaging of Concealed Objects: Application Using Back-Projection Algorithm

Millimeter-wave (MMW) imaging is a powerful tool for the detection of objects concealed under clothing. Several factors including difierent kinds of objects, variety of covering materials and their thickness, accurate imaging of near-fleld scattered data afiect the success of detection. To practice with such considerations, this paper presents the two-dimensional (2D) images of difierent targets hidden under various fabric sheets. The W-band inverse synthetic aperture radar (ISAR) data of various target-covering situations are acquired and imaged by applying both the focusing operator based inversion algorithm and the spherical back-projection algorithm. Results of these algorithms are demonstrated and compared to each other to assess the performance of the MMW imaging in detecting the concealed objects of both metallic and dielectric types.

PRACTICAL ALGORITHMS TO FOCUS B-SCAN GPR IMAGES: THEORY AND APPLICATION TO REAL DATA

It is well known in B-scan ground penetrating radar (GPR) imagery that the underground scatterers generally exhibit defocused, hyperbolic characteristics. This is mainly due to the data collection scheme and the finite beam width of the main lobe of the GPR antenna. To invert this undesirable effect and obtain focused images, various migration or focusing algorithms have been developed. In this paper, we survey the performance of our recent focusing algorithms, namely; hyperbolic summation (HS) and frequency-wavenumber (w-k) based synthetic aperture radar (SAR) focusing. The practical usage of these focusing methods were tested and examined on both simulated and measured GPR data of various buried targets. The simulation data set is obtained by a physical optics shooting and bouncing ray (PO-SBR) technique code. Measurements were taken by a stepped frequency continuous wave (SFCW) radar set-up. Scattered C-band field data were measured from a laboratory sand box and from outdoor soil environment. The proposed focusing methods were then applied to the B-scan GPR images to enhance the resolution quality within these images. The resultant GPR images obtained with the proposed algorithms demonstrate enhanced lateral resolutions.

Short-range ground-based synthetic aperture radar imaging: performance comparison between frequency-wavenumber migration and back-projection algorithms

Abstract Two popular synthetic aperture radar (SAR) reconstruction algorithms, namely the back-projection (BP) and the frequency wavenumber ( ω − k ) algorithms, were tested and compared against each other, especially for their use in ground-based (GB) SAR applications directed to foreign object debris removal. For this purpose, an experimental setup in a semi-anechoic chamber room was accomplished to obtain near-field SAR images of objects on the ground. Then, the 90 to 95 GHz scattering data were acquired by using a stepped frequency continuous-wave radar operation. The performances of the setup and the imaging algorithms were then assessed by exploiting various metrics including point spread function, signal-to-clutter ratio, integrated side-lobe ratio, and computational complexity. Results demonstrate that although both algorithms produce almost accurate images of targets, the BP algorithm is shown to be superior to the ω − k algorithm due to its some inherent advantages specifically suited for short-range GB-SAR applications.

The millimeter-wave imaging of concealed objects

In this study, two different applications are performed. An application of concealed object detection is firstly studied through millimeter-wave inverse synthetic aperture radar (ISAR) imaging. For this purpose, near-field W-band ISAR measurements of one centimeter metallic and dielectric cube targets covered by distinct textile materials are carried out in an anechoic chamber room. Then, the images are reconstructed by using back-projection ISAR imaging technique. Then, the performance of the technique is quantified from the resulted images by using the accuracy and quality metrics. An application of a concealed weapon is also handled using millimeter-wave tomography imaging. The tomography imaging of concealed weapon located behind styropor foam wall is performed. Then, the images are reconstructed by using a method which calculates polarization currents on the object surface. The results obtained are finally shown.

On the imaging applications of ground penetrating radar

This paper presents a brief overview on the imaging aspects of ground penetrating radar (GPR). Different experiments in three fields of investigation are conducted to demonstrate the feasibility of GPR for the imaging of subsurface materials. First, the GPR signatures of electrically and dimensionally different objects located inside the sand pool are obtained and processed into a focused image. The resulted image reveals that even the targets which have inherently low-level returns can be located by GPR. The second experiment is aimed to detect the water leaks from the plastic pipes via GPR technique. It is shown that additional water around the leak manifest itself as reflecting regions in the image, thereby giving information about the leak location. Furthermore, the GPR measurement of concrete column is performed to locate and image the stirrups within the columns. The results dictate the usability of GPR for concrete column analysis in measuring the position and size of stirrups.

Back-projection algorithm for ISAR imaging of near-field concealed objects

In this study, the detection of metal objects concealed under clothes is investigated through millimeter-wave inverse synthetic aperture radar (ISAR) imaging. A metallic gun is selected as the target and W-band ISAR measurements for a two-dimensional scanning geometry are carried out in our anechoic chamber measurement facility. The ISAR images of concealed object are reconstructed using the near-field back-projection algorithm. The obtained images indicate the efficient detection and identification of the gun covered with different textile types.

A new and practical formulation of bistatic Inverse Synthetic Aperture Radar imaging and verification of the formulation using numerical examples

A compact formulation of bistatic inverse synthetic aperture radar (ISAR) imaging is provided and presented thru numerical simulation of various perfect scatterers. The imaging formulation is derived for small-angle and small-bandwidth conventions. After completing the formulation, its relationship to monostatic set-up is examined and discussed. To validate the algorithm, the images of point target models are then simulated and presented for different bistatic geometries. It is shown that for a range of bistatic angle, the imaging algorithm successfully works and has the advantage of reduced bandwidth over the monostatic case at the cost of worse resolution.

The millimeter-wave ISAR imaging of concealed objects

The detection of concealed objects like weapon, explosives, and other dangerous items became very important problem for defending people against terrorist attacks. Recent time, many methods have been developed but number of methods are not efficient to effectively solve this problem. The main aim of the research in this work is devoted to the detection of concealed dangerous objects by increasing sensitivity and resolution in obtained images. In this paper, we propose to use inverse synthetic aperture radar (ISAR) technology for the detection of hidden items in the millimeter-wave length. The millimeter-wave ISAR imaging of concealed cubes and a gun are studied. The theoretical background employed for the reconstruction of measurement results is briefly described. The millimeter-wave ISAR measurement setup is presented. Then, measurements carried out for different scenarios are explained. Finally, the reconstructed results are shown for all conducted measurements.

Ground Penetrating Radar Image Focusing using Frequency-Wavenumber based Synthetic Aperture Radar Technique

In the classical B-Scan ground penetrating radar (GPR), the collected data represented in the image domain, depict undesired hyperbolic effects and have low resolution features. In this work, we present frequency-wavenumber based Synthetic Aperture Radar (SAR) focusing technique to solve this problem. The formulation of algorithm is given and applied to both the simulation data obtained by a physical optics shooting and bouncing ray (PO-SBR) code and to the measured GPR data from real soil environment. Simulation and the measurement results demonstrate that proposed algorithm is very effective in achieving well-focused GPR images with high lateral resolutions.

The Development and Comparison of Two Polarimetric Calibration Techniques for Ground-Based Circularly Polarized Radar System

Two techniques are described for the calibration of ground-based (GB) circularly polarized (CP) full polarimetric radars. The techniques are based on the point target calibration approach that uses various types of canonical reflectors with different orientations. Specifically, the calibration methods for linearly polarized (LP) radar proposed by Wiesbeck et al. and Gau et al. are selected and adapted to CP with suitable reflectors. The applicability of the techniques is examined through C-band scatterometric and synthetic aperture radar (SAR) measurements in an anechoic chamber. For the scatterometric mode, comparisons of calibrated channel imbalances with theoretical values show agreement within ±0.3 dB in amplitude and ±5◦ in phase. The crosstalk between the channels is also reduced by ∼ 5 to 30 dB after calibration. For the SAR mode, calibrated scattering matrix of a vertical wire target exhibits significant elimination of distortions between channel amplitudes and phases. The effect of calibration on target parameter retrieval is also investigated through the Cloud-Pottier eigenvector-based decomposition. Both calibration techniques are shown to yield improved accuracy of entropy-alpha (H-ᾱ) distributions and orientation angle (β̄) values.