Cebrián García

Near Field Characterization of an Imaging System Based on a Frequency Scanning Antenna Array

This contribution describes the concept of frequency scanning for imaging applications. The designed frequency scanning antenna array, which works in Ku frequency band, is used for the sectoral illumination of the scene under investigation, enabling the recovery of information restricted to the selected area. While the proposed idea simplifies beam scanning techniques with respect to mechanical or electrically-controlled scanning systems, the price-to-pay is the limitation on the amount of scattered field information. The near field radiation of the frequency scanning antenna array is characterized through an equivalent source model to assess accurately the incident field used in the imaging algorithm. This analysis also enables the evaluation of the frequency steering performance of the antenna in the near field, validating its use for the proposed application.

Submillimeter-Wave Frequency Scanning System for Imaging Applications

A new submillimeter-wave imaging system based on the frequency scanning concept is presented. Using significantly less information than beam-steering techniques, frequency scanning-based imaging systems are able to provide angular information in range-based radar systems which do not allow mechanical or electronic beam steering. Several strategies have been adopted to overcome the low dynamic range due to the propagation losses and non-specular reflection on the object-under-test. Moreover, the system is not only capable of detecting the placement of the objects, but also their profile. In this sense, an imaging application showing the system capabilities for the detection of objects concealed under clothes is presented. Results using 1-D FSAA allowing profile imaging are shown, proving the validity of the proposed system.

Antenna Diagnostics Using Arbitrary-Geometry Field Acquisition Domains

An extension of the sources reconstruction method (SRM) for handling with arbitrary-geometry field acquisition domains is presented. The amount of information that is needed to retrieve the original sources (field-sampling requirements, number of field components to be considered) is discussed as well as the influence of the noise in the measurements and the resolution limitations. The discussion is supported by a numerical simulation using a cubic field acquisition domain.

Inverse Fast Multipole Method for Monostatic Imaging Applications

A 3-D imaging technique for monostatic radar cross section measurement is presented. The method is based on an existing formulation conceived for bistatic geometries which takes into account the scattered field polarization and is accelerated using the fast multipole method. The synthetic aperture radar (SAR) image processing includes acquired field, which is of interest for polarimetric SAR imaging. In addition, the proposed technique overcomes the limitation of fast-Fourier-transform-based inverse techniques on canonical domains (planar, cylindrical, and spherical). Validation with simulations and measurements is presented. Calculation time is also drastically reduced using graphics processing unit implementation.

Influence of contour smoothness and electric size on the profile reconstruction of metallic objects using hybrid optimization

The main purpose of this article is to analyse the influence of the smoothness and electric size on the reconstruction of a metallic object contour in a 2D case. A multistage hybrid method based on Evolutionary Algorithms (EA) and an Interior-Point algorithm is used for the profile reconstruction. The purpose of this approach is to reduce the required amount of information as well as improving the convergence capabilities, specially when the object has a large electric size. The inverse problem is recast as a nonlinear optimization problem minimizing the difference between the measured and the estimated scattered field on an observation domain. The reconstruction of scatterers with different shapes and sizes is carried out to verify the accuracy of the method. In addition, two different EA are examined: Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). Numerical results show that a high reconstruction accuracy can be achieved, even when the Signal to Noise Ratio (SNR) is low.

The Sources Reconstruction Method for Antenna Diagnostics and Imaging Applications

The advances on communications systems require more challenging antenna designs, in‐ creasing the complexity of design, manufacturing, and testing stages. Different sources of er‐ ror may affect the antenna final prototype, degrading its performance, and then requiring a re-design step to identify and correct these manufacturing errors. In general, this re-design step requires invasive techniques based on trial-error procedure, whose technical and eco‐ nomical costs are directly related to the antenna complexity. For this reason, the develop‐ ment of fast and accurate non-invasive methods for antenna faults detection (antenna diagnostics) has been of great interest in the last decade.

Measurement setup for profile reconstruction in the 90 GHz frequency band

A measurement setup for profile reconstruction of metallic targets in the 90 GHz frequency band is proposed and tested. Inverse Synthetic Aperture Radar (iSAR) formulation is used to calculate the radar reflectivity from the measured scattered field. This reflectivity gives a spatial image of the Objet Under Test (OUT) which provides information about the profile of the OUT. Measurements are done using a planar measurement setup for mm- and sub-mm frequency bands. Aiming to test the system resolution, a metallic plate with four different size holes has been chosen as OUT. Profile reconstruction results using this measurement setup are shown.

The adaptive cross approximation algorithm applied to a volumetric method-of-moments for electromagnetic analysis of inhomogeneous bodies

The study of the electromagnetic fields inside inhomogeneous arbitrary-shaped bodies requires the use of volumetric-based numerical techniques. Among different possibilities (Finite-Difference Time-Domain, FDTD, Finite Element Method, FEM), the following contribution is focused on the Volumetric Method-of-Moments (V-MoM) [1],[2].