Luiz C. Trintinalia

Joint time-frequency ISAR using adaptive processing

A new joint time-frequency inverse synthetic aperture radar (ISAR) algorithm that combines ISAR processing with the joint time-frequency signal representation is presented as a means of extracting the nonpoint-scattering features from the standard ISAR image. The adaptive Gaussian representation, applied to the range aids of the ISAR image, is used as the time-frequency processing engine. This technique uses Gaussian basis functions to adaptively parameterize the data and, as a consequence, the point-scattering mechanisms and resonance phenomena can be readily separated based on the width of the Gaussian bases. The adaptive joint time-frequency ISAR algorithm is tested using data generated by the moment-method simulation of simple structures and the chamber measurement data from a scaled model airplane. The results show that nonpointscattering mechanisms can be completely removed from the original ISAR image, leading to a cleaned image containing only physically meaningful scattering centers. The nonpoint-scattering mechanisms, when displayed in the frequency-aspect plane, can be used to identify target resonances and cutoff phenomena.

Scattering center parameterization of wide-angle backscattered data using adaptive Gaussian representation

We present a scattering center extraction algorithm to parameterize the backscattered data from complex targets collected over large angular apertures. This parameterization is based on a scattering center model of the target, but includes an aspect-dependent amplitude function for each scattering center. A two-dimensional (2-D) adaptive Gaussian representation (AGR) algorithm is used to extract the position and the amplitude function associated with each scattering center. The algorithm is tested with data generated by the Xpatch radar simulation code as well as chamber measurement data. The results show that a very good compression ratio can be achieved, resulting in a compact scattering center model of the target. Once such model is available, we can easily reconstruct range profiles and ISAR images at any aspect on the same plane with good accuracy.

ASAR-antenna synthetic aperture radar imaging

The antenna synthetic aperture radar (ASAR) imaging concept is introduced. We present the ASAR imaging algorithm to pinpoint the locations of secondary scattering off a platform from antenna radiation data. It is shown that a three-dimensional (3-D) ASAR image of the platform can be formed by inverse Fourier transforming the multifrequency, multiaspect far-field radiation data from an antenna mounted on the platform. This concept is demonstrated using the computed radiation data from the code Apatch, which employs the shooting and bouncing ray (SBR) technique. Furthermore, we develop a fast ASAR algorithm specially tailored for the SBR approach. By taking advantage of the ray tracing information within the SBR engine, we demonstrate that the fast approach can result in the same quality of image as the frequency-aspect algorithm at only a fraction of the computation time.

Integral equation modeling of multilayered doubly-periodic lossy structures using periodic boundary condition and a connection scheme

This paper presents a boundary integral formulation to analyze multilayered doubly-periodic lossy structures with arbitrary geometry. The formulation is based on the moment method using first-order triangular patch basis functions. Each individual layer is analyzed separately using the simple free-space Greens function. After discretization, periodic boundary conditions are imposed on each region and a connection scheme is used to connect the regions. Metallic patches between layers or on the periodic boundary are also included in the model. Several examples are presented showing both the flexibility and the accuracy of the method.

Extraction of waveguide scattering features using joint time-frequency ISAR

A new joint time-frequency ISAR algorithm that combines the conventional ISAR processing with the joint time-frequency signal representation is presented. The adaptive spectrogram, applied to the range axis of the ISAR image, is used as the time-frequency processing engine. The algorithm is tested using the chamber measurement data from a scale model airplane. The results show that the nonpoint scattering mechanisms due to the waveguide-like engine inlet can be seamlessly removed, leading to an enhanced ISAR image consisting only of point scatterers. Furthermore, the extracted inlet features are displayed in the frequency-aspect plane and show distinct waveguide cutoff features.

An improved triangular patch basis for the method of moments

We present a new set of triangular patch basis functions. It retains the same properties as the traditional RWG basis, but is capable of representing any linear current distribution over each triangle. With this improvement, the proposed basis is much less sensitive to the meshing scheme. Moreover, it provides a better representation of the actual current distribution, leading to a faster convergence of the moment method solution. All of this can be achieved with about the same computational complexity of the RWG basis.

Time-frequency representation of wideband radar echo using adaptive normalized Gaussian functions

We apply a signal representation scheme that uses adaptive normalized Gaussian functions, introduced by Qian and Chen (see Signal Processing, vol.36, no.1, p.11, 1994), to the analysis of wideband radar echo comprised of scattering centers and interior resonances. This scheme provides an adaptive adjustment of the time and frequency resolution to best match the signal, in contrast to the Gabor expansion and wavelet decomposition which use preselected, fixed resolution cells. This representation is economical and allows an easy and meaningful energy representation in the time-frequency domain.

Simple Excitation Model for Coaxial Driven Monopole Antennas

A new excitation model for the numerical solution of the electric field integral equation (EFIE) applied to arbitrarily shaped monopole antennas fed by coaxial lines is presented. This model yields a stable solution for the input impedance of such antennas with very low numerical complexity and without the convergence and high parasitic capacitance problems associated with the usual delta gap excitation.

Super-resolved parameterization of dispersive scattering mechanisms in the time-frequency plane

We implement a super-resolution algorithm that allows the full parameterization of waveguide-type dispersion mechanisms. In our algorithm, ESPRIT is used as the processing engine. Since super-resolution methods in their original form are not equipped to handle dispersive behavior, additional processing is required to parameterize the data. We use ESPRIT both in the time and frequency domain to extract the cutoff frequencies and the delays associated with each mode. A nonlinear sampling scheme is then used to come up with an improved estimate of the cutoff frequencies. Finally, a polynomial fit for the amplitude of each mode is performed to fully parameterize the data. The resulting curves can be displayed in the time-frequency plane with very high resolution.

Time-frequency processing of backscattered signal from a slotted waveguide array

Electromagnetic signals backscattered from targets provide information useful to classify and identify the targets. It was recently reported that the time-frequency processing of wideband backscattered signals gives us additional insights into the scattering physics of targets not available in either the time domain or the frequency domain alone. In the two-dimensional time-frequency plane, resonance frequencies, scattering centers and dispersive behavior can be clearly visualized simultaneously. The time-frequency analysis has already been applied to investigate the backscattering signals from grating structures. For 2-D finite dielectric gratings, it was shown that the Floquet frequencies due to the periodicity of the structure, the scattering centers due to the edges of the grating and the dispersive characteristics due to surface waves can be individually identified. In this work the backscattering from a 3-D rectangular slotted waveguide array will be analyzed by the use the short-time Fourier transform (STFT). The data, generated in the frequency domain, was obtained by the use of the moment method together with a connection scheme.<<ETX>>