Ioannis Aliferis

Time‐domain processing of electromagnetic data for multiple‐target detection

This research represents a synthesis of ultra‐wideband antenna hardware, employed for multiple target detection, using time‐reversal processing and the decomposition of the time‐reversal operator to obtain target location. Real‐data examples are presented using a symmetric and asymmetric configuration consisting of two scatterers. A new entropy‐based criterion for stopping the time‐reverse sequence is employed.

Recent nonlinear inversion methods and measurement system for microwave imaging

The paper reviews different inversion methods in connection with different measurement systems to solve nonlinear inverse scattering problems for various applications where the discrepancy between the measured and computed scattered field is minimized at each step of an iterative procedure. First, the reconstruction of the complex permittivity profile (permittivity and conductivity profiles) of inhomogeneous buried objects using polarization diversity (TM, TE and cascaded TE-TM polarizations) is considered for nondestructive testing, geophysical, civil engineering, or humanitarian applications. Second, a boundary-oriented method based on contour deformations is exposed for reconstructing the shape of metallic objects for radar imaging purposes.

Inversion algorithms and measurement systems for microwave tomography of buried objects

The detection and identification of buried inhomogeneities using electromagnetic waves are of crucial importance for many nondestructive testing applications. The paper deals with two different inversion algorithms for microwave tomography. First, a qualitative algorithm based on diffraction tomography for detecting the buried object, and second, a quantitative algorithm for reconstructing the complex permittivity profile of the object. The backscattered field is measured at different frequencies over a probing line above or in contact with the soil at different receiver locations and for difference positions of a transmitting antenna (multistatic configuration). The two reconstruction algorithms process the backscattered field generated by the buried inhomogeneities and incorporate the incident near-field radiation pattern transmitted in the soil by the broadband antenna. Tomographic reconstructions of buried objects are presented for a situation of practical interest using bow-tie antennas in the frequency band 0.3-1.3 GHz.

Shape-gradient optimization applied to the reconstruction of 2-D and 3-D metallic objects

A shape-gradient optimization based on contour deformations by means of Level Set method is used for reconstructing 2-D and 3-D perfectly conducting objects electromagnetic imaging. A frequency hopping technique is applied during the inverse scattering procedure for first locating the objects and reconstruct them roughly, then with higher frequencies to allow finer details to be retrieved. The numerical results clearly show that the inversion algorithm yields to accurate reconstruction of one or several objects even with noise-contaminated data and limited coverage of the incident fields.

Detection of debris (FOD) on runways in W-band: Relevance and validity domain of two-dimensional approaches

For sake of simplicity and execution speed, many imaging algorithms are based on two-dimensional approaches. We present a study on the relevance and validity domain of two-dimensional approaches to detect debris on airport runways in W-band (75-110 GHz), taking into account polarization (both 2D-TM and 2D-TE). The study deals, first with objects in free-space, and second placed on runway in far field configuration using plane wave illumination or horn antenna.

Amplitude and Phase Measurements of Scattered Fields for Quantitative Imaging in the W-band

This communication investigates a bi-static scattering measurement system for mm-Wave imaging purposes in the W-band (75-110 GHz). The main benefit of using millimeter waves is the high resolution provided by short wavelengths (2.7 to 4 mm). The specific interest in the W-band stems from the increasing number of applications. The system provides multi-frequency, multi-view (multiple receiver positions) data at a single incidence, as the emitter is fixed. In the context of future imaging applications, we seek high accuracy in the measurements. For this, we compare the experimental results obtained from two PEC cylinders with the analytic solution. Raw measurement data have been calibrated during postprocessing in order to account for misalignment errors and noise of the measurement system. Therefore the large cylinder is used as the reference target for deriving the calibration coefficients and the small cylinder is used as a test object. Averaged values of the signal-to-noise ratio over the W-band are 15.5 dB and 10 dB for the large and the small cylinder, respectively. Measurement results are used for the first time in W-band for quantitative imaging.

Quantitative millimeter-wave imaging in W-band

In this paper, we present quantitative image reconstructions (i.e. permittivity and conductivity profiles) from synthetic and experimental data. First, we perform the qualitative imaging for localizing the target in order to determine the investigation domain. Second, we solve the inverse-scattering problem via an iterative algorithm based on a Conjugate Gradient (CG) method.

Millimeter-wave imaging of foreign object debris (FOD) based on two-dimensional approach

In this paper, we present the results of image reconstructions of foreign object debris (FOD) based on 2D algorithms in the W-band (75-110 GHz). To test the feasibility of using 2D reconstruction algorithms, we first studied the relevance of two-dimensional assumption in the working frequency band for different typical debris. Thanks to the short wavelength (2.7 to 4 mm), most debris can be considered as two-dimensional. The reconstruction of complex permittivity profile for dielectric (lossy or lossless) and perfectly conducting targets is carried out using 2D-TM and 2D-TE algorithms using characteristic elements (VV, VH, HV and HH) of the scattering matrix with polarimetric measurements in multistatic configuration.

Millimeter-wave imaging: Quantitative reconstructions from experimental data in the W-band

The conjugate gradient algorithm presented here, and applied to measured far-field scattered data shows its effectiveness in the reconstruction of metallic objects. This method can be, in principle, applied to any kind of lossy dielectric objects with near-field as well as far-field data.