Mark McClure

Ultra-wide-band synthetic-aperture radar for mine-field detection

A full-wave model is developed for electromagnetic scattering from buried and surface land mines (both conducting and plastic), taking rigorous account of the lossy, dispersive, and potentially layered properties of soil. The (polarimetric) theoretical results are confirmed via synthetic-aperture radar (SAR) measurements, performed using the US Army Research Laboratorys BoomSAR, with which fully polarimetric ultra-wide-band (50-1200 MHz) SAR imagery is produced. The SAR system is used to acquire a large database of imagery, including a significant distribution of naturally occurring clutter. Several techniques are used for mine detection with such data, including several detectors that are based on target features gleaned from the modeling, as well as a matched-filter-like detector that directly incorporates the target signatures themselves. In addition, the theoretical model is used to predict wave phenomenology in various environments (beyond the limited range of parameters that can be examined experimentally). Since the efficacy of radar-based subsurface sensing depends strongly on the soil properties, we perform a parametric study of the dependence of such on the target RCS, and on possible landmine resonances.

Matching pursuits with a wave-based dictionary

The method of matching pursuits utilizes a nonlinear iterative procedure to project a given waveform onto a particular dictionary. For scattering problems, the most appropriate dictionary is composed of wave objects that are consistent with the underlying wave phenomenology. A signal scattered from most targets of interest can be decomposed in terms of wavefronts, resonances, and chirps-and each of these subclasses can be further subdivided based on characteristic wave physics. Here, we investigate the efficacy of applying the method of matching pursuits with a wave-based dictionary for the processing of scattering data. The performance of this algorithm is examined for scattering data with and without additive noise.

On the superresolution identification of observables from swept-frequency scattering data

A superresolution signal processing algorithm is used for the identification of wavefronts from the fields scattered from several canonical targets. Particular wave objects that are examined are single and multiple edge diffraction, scattering from flat and curved surfaces, cone diffraction, and creeping waves. The scattering data are computed numerically via the method of moments (MoM) and are processed using a modified matrix-pencil algorithm. General properties of superresolution processing of such data-independent of the particular algorithm used-are assessed through an examination of the Cramer-Rao (C-R) bounds for basic scattering scenarios.

Wide-area detection of land mines and unexploded ordnance

Advanced electromagnetic modelling tools are discussed, focused on sensing surface and buried land mines and unexploded ordnance, situated in a realistic soil environment. The results from these forward models are used to process scattered-field data, for target detection and identification. We address sensors directed toward the wide-area-search problem, for which one is interested in detecting a former mine field or bombing range. For this problem class we process data measured from an actual airborne radar system. Signal-processing algorithms applied include Bayesian processing and a physics-based hidden Markov model.

Time-domain wave-oriented data processing of scattering by nonuniform truncated gratings

In a companion paper [ J. Opt. Soc. Am. A11, 2675 ( 1994)] we investigated wave-oriented processing techniques that extract from frequency-domain (FD) scattering data for nonuniform truncated thin-wire or strip gratings the wave phenomenology that ties features in data to scattering mechanisms that are responsible for these features. The resulting observable-based parameterization (OBP) should be useful for construction of wave-based algorithms aimed, through forward and backward propagation routines, at design, classification, and imaging. The processing strategy projects the data, which are assembled on an elevated track at height z parallel to the grating-plane coordinate x, onto the (x, kx) phase-space subdomain by means of windowed Fourier transforms; here kx is the spatial spectral wave number corresponding to x. The (x, kx) phase-space distributions and their information content for various grating configurations are analyzed in detail and are related to previously derived analytic scattering models based on truncated local Floquet modes (FM’s) and on FM-modulated edge diffractions. We explore time-domain (TD) wave-oriented processing techniques for scattering by truncated gratings produced by a pulsed incident plane wave. By spatial–temporal resolution, the TD data base adds to the FD processing options. Time (t)–frequency (ω) phase-space distributions implemented through windowed transforms extract from TD data new TD phenomenology, TD-OBP, that differs fundamentally from that in the frequency domain: strongly dispersive TD-FM’s with prolonged time-varying frequency response ωm(t), where m is the FM index, and temporally well-resolved weakly dispersive impulselike edge diffractions. These phenomenologies extracted from the TD scattering data are interpreted in terms of, and shown to be in agreement with, previously developed analytic TD scattering models. The (t, ω) processing is applied to pulsed plane-wave scattering data produced for various grating configurations by previously calibrated numerical reference codes and reveals the changes in (t, ω) footprints that are attributable to departures from strict canonical truncated periodicity. Short-pulse TD excitation is found to resolve the element structure within each scattering cell directly, in contrast to (t, ω) processing, in which element-structure information is tied indirectly to the FM excitation strengths.

Wave-based matching-pursuits detection of submerged elastic targets

Matching pursuits is a nonlinear algorithm which iteratively projects a given signal onto a complete dictionary of vectors. The dictionary is constructed such that it is well matched to the signals of interest and poorly matched to the noise, thereby affording the potential for denoising, by adaptively extracting an underlying signature from a noisy waveform. In the context of wave scattering and propagation, there are basic constituents that can be used to construct most measured waveforms. A dictionary of such constituents is used here, in the context of wave-based matching-pursuit processing of acoustic waves scattered from submerged elastic targets. It is demonstrated how wave-based matching pursuits can be utilized for denoising as well as to effect a detector, the latter being parametrized via its receiver operating characteristic (ROC). Results are presented using measured aspect-dependent (orientation-dependent) scattered waveforms, for the case of a submerged elastic shell.

Frequency-domain scattering by nonuniform truncated arrays: wave-oriented data processing for inversion and imaging

We previously presented an asymptotic diffraction theory for time-harmonic and transient scattering by arbitrarily illuminated truncated nonuniform thin-wire gratings [ J. Opt. Soc. Am. A11, 1291 ( 1994)]. We parameterized and interpreted the results in terms of scattered truncated Floquet modes (FM’s) and Floquet-modulated edge diffraction, which generalize the constructs of the conventional geometric theory of diffraction (GTD). We also demonstrated that numerical implementation of the FM–GTD algorithm yields results that compare very well with data computed from rigorously based numerical reference solutions. We enlarge the previous frequency-domain numerical data base for gratings to scattering by truncated arrays whose elements are arbitrarily oriented strips rather than thin-wire filaments and also to arrays whose element locations depart from truncated periodicity in a random rather than an orderly manner. We show that the FM–GTD parameterization of the scattered field remains applicable under these generalized conditions. With a view toward inversion and imaging, our principal purpose is the application of space-wave-number phase-space processing techniques to extract the footprints of truncated nonuniform periodicity from the scattered-field data. Because the processing is tied to the wave physics, we refer to this procedure as wave-oriented data processing. Implementation involves projection onto appropriate phase-space subdomains and the generation of space–wave-number phase-space distributions by windowed Fourier transforms. It is found that this form of processing in the frequency domain highlights effects of truncation and perturbed periodicity but is not very sensitive to the structure of the array elements (i.e., wires versus strips). In a companion paper [ J. Opt. Soc. Am. A11, 2685 ( 1994)] we perform phase-space processing in the time domain, show how the time-domain FM-GTD phenomenology is revealed through time-frequency distributions, and show also how short-pulse excitation enhances the sensitivity with respect to element structure by means of spatial–temporal resolution.

Time-domain design-oriented parametrization of truncated periodic strip gratings

Asymptotic methods are used to develop an algorithm that parametrizes time-domain plane-wave interaction with a truncated grating of periodically spaced, perfectly conducting strips in free space. By distinctly displaying the edge effects as well as the truncated Floquet mode contributions from the body of the grating, the model contains the necessary building blocks for time-domain, finite-grating design. Short-pulse, plane-wave diffraction results computed from the model are shown to agree well with numerical reference data, and salient properties of the time-domain Floquet-mode constituents in the model are highlighted via time-frequency representations.<<ETX>>

Wave-based target identification via the method of matched pursuits

Matched-pursuits is a nonlinear algorithm which iteratively projects a given signal onto a complete dictionary of vectors. The dictionary is constructed such that it is well matched to the signals of interest and poorly matched to the noise, thereby affording the potential for denoising, by adaptively extracting an underlying signature from a noisy waveform. In the context of wave scattering and propagation, there are basic constituents that can be used to construct most measured waveforms. A dictionary of such constituents is used here, in the context of wave-based matched-pursuit processing of acoustic waves scattered from submerged elastic targets. It is demonstrated how wave-based matched- pursuits can be utilized for denoising as well as to effect a detector, the latter being parametrized via its receiver operating characteristic.

Multiresolution signature-based SAR target detection

A full-wave electromagnetic-scattering model is utilized to effect a land-mine detector via a multiresolution template- matching-like algorithm. Detection is performed on fully polarimetric ultra-wideband (50 - 1200 MHz) synthetic aperture radar (SAR) imagery. Multiresolution template matching is effected via discrete-wavelet transform of the SAR imagery and the parametric target-signatures (templates). Detector results are presented in the form of receiver operating characteristics.