Linear Synthetic Aperture Research Articles

Bridge Monitoring Strategies for Sustainable Development with Microwave Radar Interferometry

The potential of a coherent microwave radar for infrastructure health monitoring has been investigated over the past decade. Microwave radar measuring based on interferometry processing is a non-invasive technique that can measure the line-of-sight (LOS) displacements of large infrastructure with sub-millimeter precision and provide the corresponding frequency spectrum. It has the capability to estimate infrastructure vibration simultaneously and remotely with high accuracy and repeatability, which serves the long-term serviceability of bridge structures within the context of the long-term sustainability of civil engineering infrastructure management. In this paper, we present three types of microwave radar systems employed to monitor the displacement of bridges in Japan and Italy. A technique that fuses polarimetric analysis and the interferometry technique for bridge monitoring is proposed. Monitoring results achieved with full polarimetric real aperture radar (RAR), step-frequency continuous-wave (SFCW)-based linear synthetic aperture, and multi-input multi-output (MIMO) array sensors are also presented. The results reveal bridge dynamic responses under different loading conditions, including wind, vehicular traffic, and passing trains, and show that microwave sensor interferometry can be utilized to monitor the dynamics of bridge structures with unprecedented spatial and temporal resolution. This paper demonstrates that microwave sensor interferometry with efficient, cost-effective, and non-destructive properties is a serious contender to employment as a sustainable infrastructure monitoring technology serving the sustainable development agenda.

A novel method for extracting geometric parameter information of buildings based on CSAR images

ABSTRACT Compared with the traditional linear synthetic aperture radar (LSAR), circular SAR (CSAR) system can observe 360° of the observation area. In turn, more abundant scattering information in all directions of the observation area can be obtained, making it easier to observe the target information. Based on the observation advantages of CSAR, a new method for extracting geometric parameters of buildings is proposed in this paper. In this method, the scattering features of buildings on CSAR sub-aperture images and full-aperture images are extracted, and the corresponding relationship between the scattering features and building geometric parameters is deduced, and then the building geometric parameters are calculated. Through the above corresponding relationship, the geometric parameters of the building are calculated. At the same time, in this method, the advantages of CSAR omnidirectional imaging are used to extract the geometric parameters of buildings from multiple sub-aperture images. This method realizes the high-precision extraction of building geometric parameters by CSAR imaging for the first time. Finally, the experimental verification is carried out by introducing the measured data of the Ku-band autonomously measured, which confirms the effectiveness and accuracy of the proposed method.

基于线性约束最小方差准则和合成孔径连续声束合成的内镜超声成像算法

基于线性约束最小方差准则和合成孔径连续声束合成的内镜超声成像算法

Photoacoustic tomography in a clinical linear accelerator for quantitative radiation dosimetry

Cancer is the second leading cause of death in the United States. Approximately half of all cancer patients receive radiation therapy, in which linear accelerators are used to deliver high doses of x-ray radiation to tumors, inducing cell death. X-ray energy deposition causes pressure changes that produce acoustic signals due to the photoacoustic effect. Here, clinical x-ray beams were directed at test objects made of antimonial lead and other metallic materials within a water tank. Photoacoustic signals were measured using a calibrated broadband hydrophone and validated using simulations in k-Wave. Linear and two-dimensional synthetic apertures were formed by mechanically scanning the x-ray source and test object within a single plane. Tomographic images of test objects, reconstructed from measured photoacoustic signals, show good agreement with object geometry. X-ray doses incurred by the test objects are mapped based on the reconstructed acoustic pressure sources and Gruneisen parameter of the material employed. Potential applications to in vivo dosimetry for x-ray and proton therapy, potentially enabling safer and more effective treatments, are discussed.Cancer is the second leading cause of death in the United States. Approximately half of all cancer patients receive radiation therapy, in which linear accelerators are used to deliver high doses of x-ray radiation to tumors, inducing cell death. X-ray energy deposition causes pressure changes that produce acoustic signals due to the photoacoustic effect. Here, clinical x-ray beams were directed at test objects made of antimonial lead and other metallic materials within a water tank. Photoacoustic signals were measured using a calibrated broadband hydrophone and validated using simulations in k-Wave. Linear and two-dimensional synthetic apertures were formed by mechanically scanning the x-ray source and test object within a single plane. Tomographic images of test objects, reconstructed from measured photoacoustic signals, show good agreement with object geometry. X-ray doses incurred by the test objects are mapped based on the reconstructed acoustic pressure sources and Gruneisen parameter of the material...

Creating synthetic aperture images using experimental and simulated backscattering of solid elastic cubes

To study aspect and material dependent backscattering of objects, we used cubes as their geometry has distinct exposure points for different backscattering mechanisms. We insonified two solid cubes, one brass and the other steel, in underwater tank scale experiments, measuring their backscattering using circular, linear, and cylindrical synthetic aperture sonar. The right angles of the edges allow for enhanced backscattering by Rayleigh waves, which couple in at material-specific angles, since the waves are retroreflected by the cube’s edges [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 105, 700-710 (1998)]. In concert with the experiments, we model the backscattering from our cubes using Kirchhoff-Integration based simulations. The simulation isolates the specular responses, simplifying the complex responses from the cube’s geometry and allowing us to identify specific effects, such as splitting from the corner reflections of a cube when its top ridge is tilted. Using Fourier based back-projection algorithms, we reconstructed target images from both the experimental and simulation results. Three dimensional images were also recreated from the cylindrical data. Using a combination of experimental and simulation results, we identified aspect dependent mechanisms. [Work supported by ONR.]

Autofocusing circular synthetic aperture sonar imagery using phase corrections modeled as generalized cones.

Circular synthetic aperture sonar (CSAS) is a coherent aperture synthesis technique that utilizes backscattered acoustic information from an encircled scene to generate information rich, high-resolution imagery. The aperture length required for image synthesis is much longer than in its linear synthetic aperture sonar counterpart and can result in challenging phase delay and navigation estimation constraints. Residual uncorrected phase errors manifest as focus aberrations in reconstructed CSAS imagery. This paper demonstrates that phase error in image patches can be approximated as an aspect variant linear phase shift representable as a generalized cone in wave-number space. If the geometry of the generalized cone is known, it can be applied as the spectral phase of an inverse filter for aberration correction. A method is derived for reconstructing the error cone geometry from independent estimates of its local curvatures, which are found via a series of one-dimensional line searches that maximize the focus of CSAS sub-aperture images. This approach is applied to real and simulated CSAS data containing aperture distortions, and the results successfully demonstrate estimation and correction of the underlying focus aberrations.

Correction of Dechirp Distortion in Long-Distance Target Imaging with LFMCW-ISAR

The combined linear frequency modulation continuous wave (LFMCW) and inverse synthetic aperture radar (ISAR) can be used for imaging long-distance targets because of its long-distance and high resolution imaging abilities. In this paper, we find and study the dechirp distortion phenomenon (DDP) for imaging long-distance targets by a dechirp-on-receive LFMCW radar. If the targets are very far from the radar, the maximum delay-time is not much smaller than a single sweep duration, and the dechirp distortion is triggered since the distance of the target is unknown in a LFMCW-ISAR system. DDP cannot be ignored in long-distance imaging because double images of a target appear in the frequency domain, which reduces resolution and degrades image quality. A novel LFMCW-ISAR signal model is established to analyze DDP and its negative effects on long-distance target imaging. Using the proportionately distributed energy of double images, the authors propose a method to correct dechirp distortion. In addition, the applicable scope of the proposed method is also discussed. Simulation results validate the theoretical analysis and the effectiveness of the proposed method.

Frequency response of ordnance replicas across multiple scales

Broad-band, multi-aspect backscatter data obtained using small-scaled and full-scaled replicas of Unexploded Ordnance (UXO) are reported. Data were collected using either linear or circular rail systems. The experiments were performed in: (1) NSWC PCD's small scale test bed (less than 1/16 scaled) which has a simulated bottom, (2) NSWC PCD's Facility 383 9-million gallon fresh water test pool which has a 5-ft sand bottom, and (3) the Gulf of Mexico during GULFEX12 off Panama City, Florida. Data were processed using linear and circular synthetic aperture sonar techniques to generate both images and plots of target strength as functions of frequency and aspect angle. The results from all three experiments are compared to each other and with predictions from Finite-element (FE) analysis. These comparisons are used to assess the utility of alternative methods for generating sonar data from bottom targets of sufficient fidelity to study scattering phenomena and support development of automated target recognition. [Work supported by ONR Code 32 and SERDP.]

Synthetic Aperture Focusing Technique and Contrast Source Inversion Algorithm Applied in Elastic Wave Imaging

In this paper the linear synthetic aperture focusing technique (SAFT), the nonlinear contrast source inversion (CSI) algorithm and the CSI algorithm with extended approaches, such as frequency approach and regularization approach, are applied to reconstruct a complicated target immersed in water from the experimental elastic wave data. The inversion results from these datasets using multi-bistatic measurement, which are obtained in the acoustic wave case and the pressure and vertically polarized shear (P-SV) wave case, verify the CSI algorithm with extended approaches is an accurate and promising method for elastic wave imaging.

SAR Imaging with Noise Radar

Two ground-based coherent imaging systems designed on the basis of noise radar technology (NRT) using continuous waveform with a linear synthetic aperture and pulse coherent noise waveform with a circular synthetic aperture are presented. A short description of noise waveform synthetic aperture radar (SAR) theory and data processing algorithms are given. Experimental results obtained show that the quality of the imagery is comparable to that of frequency-modulation continuous-wave (FMCW) or stepped frequency radar systems. Experimental validation of the technique at different locations provides confidence in the ability of noise waveform SAR to achieve high-resolution imaging at low cost. The exploitation of the interferometric phase combining pairs of images spaced in time is shown to allow the detection of displacements in the scene with a submillimeter accuracy.

3-D E-CSAR imaging of a T-72 tank and synthesis of its SARr reconstructions

The results of three-dimensional (3-D) imaging of a T-72 tank using its angular azimuthal (turntable) and linear elevation synthetic aperture data at X band are presented. This is achieved using an accurate and computationally efficient wavefront (Fourier-based) reconstruction algorithm for elevation and circular (E-CSAR) data. The E-CSAR 3-D images are then used to synthesize 2-D spotlight and stripmap slant plane synthetic aperture radar (SAR) images of the target at a desired range and squint angle. For this purpose, a procedure is introduced that incorporates the spatially varying azimuthal and elevation Doppler signatures of individual reflectors on the target as well as the mean range, azimuth, and elevation of the flight path. Results using the E-CSAR images of the T-72 tank are provided.

Filtering of multichannel SAR images

An explicit form of the linear multichannel synthetic aperture radar (SAR) intensity filter, which preserves radiometry while optimally reducing speckle is derived, together with a compact expression for the theoretical gain in equivalent numbers of looks (ENLs). The filter can be applied to mixed data types, which is demonstrated using a combination of ERS and JERS satellite data, and confirms the filter performance predicted by the theory. Tests indicate that a simplified form of the filter, which neglects correlation between images, gives an ENL only slightly less than optimal, while being much easier to implement. Exact analysis of the effect of estimating filter weights shows that the linear increase in ENL with the number of images predicted for the ideal filter does not occur. In practice, the ENL is affected by the window size used to estimate the weights and saturates as the number of images increases. An efficient recursive form of the filter is described, which is most naturally applied to multitemporal data for the practically important case where the current image is uncorrelated with previous images in a data sequence.

Estimation of modal eigenvalues from an arbitrary distribution of receivers in a planar array

Acoustic propagation in shallow-water areas is strongly influenced by the acoustic properties of the sediment layers. Many approaches have been proposed for obtaining the acoustic properties of the sediment layers. A simple and powerful approach has been the inversion algorithms based on modal eigenvalues [Rajan et al., J. Acoust. Soc. Am. 82, 998–1017 (1987)]. The performance of this approach has been demonstrated in field experiments. In these field experiments, the data were obtained on a linear synthetic aperture horizontal array. In recent years there has been an effort to extend the procedure to cover wider areas so that the range-dependent properties of the sediment can be determined. A means of achieving this objective is to collect the acoustic data on a planar array of receivers from which the range-dependent properties of the sediment can be obtained. Schemes for extracting modal eigenvalues of the propagating modes from data acquired on a curvilinear track with nonuniform grid spacing will be presented and the performance of the schemes evaluated using synthetic and field data.

Time-of-flight diffraction tomography for NDT applications

An ultrasonic imaging system based on a direct time-space reconstruction algorithm has been developed. The radio-frequency signals are collected along a linear synthetic aperture with point sources and receivers in separate positions. This allows the investigation of defects with different incident angles and the final image is less sensitive to the defect characteristics of size, shape and orientation. Theoretical formulae for the lateral and the axial resolution are derived and the actual system performance is evaluated with a computer testing program. Theoretical models of ultrasonic scattering from simple artificial defects embedded in metals, based on the Born approximation and the geometrical theory of diffraction, are employed to simulate signals from some simplified objects covering a broad ka range, with ka up to about 5. Comparisons between theoretical, simulated and experimental lateral and axial resolution are reported, with lateral resolutions found by experiment of 1.34γ, by simulation λ compared with the theoretical limit of 0.36λ.

Square-law detected synthetic aperture radar

A narrow synthetic radar beam may be generated from square-law detected radar pulses provided the return pulse is incoherent. The processing would provide the same range resolution as a linear synthetic aperture system, but would reduce the azimuth resolution by the ratio of the transmitted pulse length to the carrier wavelength.