Joseph T. Case

Millimeter-wave detection of localized anomalies in the space shuttle external fuel tank insulating foam

The Space Shuttle Columbias catastrophic accident emphasizes the growing need for developing and applying effective, robust, and life-cycle-oriented nondestructive testing (NDT) methods for inspecting the shuttle external fuel tank spray on foam insulation (SOFI). Millimeter-wave NDT techniques were one of the methods chosen for evaluating their potential for inspecting these structures. Several panels with embedded anomalies (mainly voids) were produced and tested for this purpose. Near-field and far-field millimeter-wave NDT methods were used for producing images of the anomalies in these panels. This paper presents the results of an investigation for the purpose of detecting localized anomalies in several SOFI panels. To this end, continuous-wave reflectometers at single frequencies of 33.5, 70, or 100 GHz representing a relatively wide range of millimeter-wave spectrum [Ka-band (26.5-40 GHz) to W-band (75-110 GHz)] and utilizing different types of radiators were employed. The resulting raw images revealed a significant amount of information about the interior of these panels. However, using simple image processing techniques, the results were improved in particular as it relates to detecting the smaller anomalies. This paper presents the results of this investigation and a discussion of these results.

Optimum Two-Dimensional Uniform Spatial Sampling for Microwave SAR-Based NDE Imaging Systems

Microwave imaging systems for nondestructive evaluation, based on 3-D synthetic aperture radar (SAR) techniques, utilize either a real aperture, composed of many antennas mounted next to one another, or a synthetic aperture, generated by raster scanning a single antenna. To obtain a quality SAR image, the spatial sampling must be dense enough to accurately sample the electric field reflected from a target. Conversely, the quantity of spatial samples may be optimally reduced, resulting in reduced system complexity and required resources for systems employing real apertures and reduced imaging time for synthetic aperture systems. In the literature, it has been reported that the optimum sampling step size is equal to the theoretical resolution, as per the Nyquist rate. It has also been reported that an image generated using a sampling step size equal to the theoretical resolution may not possess the same spatial resolution as predicted. Also, as expected and reported, resolution is dependent upon the distance between the target and the aperture, aperture dimensions, and antenna beamwidth. However, existing formulations of SAR resolution do not account for all of the physical characteristics of a measurement (e.g., 2-D limited-size aperture, electric field decreasing with distance from the measuring antenna, etc.). This paper presents a theoretical formulation of resolution and a study into optimum uniform spatial sampling by analyzing simulated 3-D SAR images according to metrics representing image quality, namely, half-power resolution and RMS error between practically sampled images and an ideally sampled image. The results of this simulation demonstrate optimum sampling given design requirements that fully explain resolution dependence on sampling step size. Also, it is found that there is additional widening of the 2-D spectral estimation of the data due to the aperture-limited nature of the measurements, which further influences the choice of sampling step size. Subsequently, the simulated results are compared to experimental results corroborating the efficacy of the formulation. Finally, design curves and procedures are proposed for selecting sampling step size as per resolution requirements.

Piecewise and Wiener Filter-Based SAR Techniques for Monostatic Microwave Imaging of Layered Structures

In this paper, two new techniques for microwave imaging of layered structures are introduced. These techniques were developed to address the limiting issues associated with classical synthetic aperture radar (SAR) imaging techniques in generating focused and properly-positioned images of embedded objects in generally layered dielectric structures. The first method, referred to as piecewise SAR (PW-SAR), is a natural extension of the classical SAR technique, and considers physical and electrical properties of each individual layer and the discontinuity among them. Although this method works well with low loss dielectric media, its applicability to lossy media is limited. This is due to the fact that this method does not consider signal attenuation. Moreover, multiple reflections within each layer are not incorporated. To improve imaging performance in which these important phenomena are included, a second method was developed that utilizes the Greens function of the layered structure and casts the imaging approach into a deconvolution procedure. Subsequently, a Wiener filter-based deconvolution technique is used to solve the problem. The technique is referred to as Wiener filter-based layered SAR (WL-SAR). The performance and efficacy of these SAR based imaging techniques are demonstrated using simulations and corresponding measurements of several different layered media.

Rapid Rotary Scanner and Portable Coherent Wideband Q-Band Transceiver for High-Resolution Millimeter-Wave Imaging Applications

Millimeter-wave imaging techniques, based on synthetic aperture focusing (SAF), have been successfully used for nondestructive testing (NDT) of various composite and aerospace structures. Most current imaging mechanisms utilize raster scanning platforms, whereby the imaging system is scanned in a rectangular grid over the structure-under-test (SUT). Most raster scanning platforms, although relatively simple in design and construction, are inherently slow. Furthermore, SAF techniques necessitates the use of vector-measuring instruments such as a vector network analyzer (VNA), which are typically: 1) bulky; 2) cannot be mounted on scanning platforms; 3) are not suitable for in-field use; and 4) expensive. These factors limit the effectiveness of these millimeter-wave imaging techniques in applications where frequent and rapid inspection of large structures is required. Hence, there is a great demand for rapid mechanical scanning systems combined with portable wideband transceivers in order to increase the utility of these imaging techniques, and provide a real solution to many practical NDT applications. To this end, a unique rotary scanner system, capable of scanning a relatively large area in a relatively short span of time, was designed and constructed. In addition, a custom-designed portable transceiver system operating in the frequency range of 35-45 GHz (Q-band) was developed and incorporated into the rotary scanner system for producing coherent (amplitude and phase) and accurate data suitable for synthetic aperture imaging and the 10-GHz bandwidth allows the generation of relatively high-resolution millimeter-wave holographical images. This paper presents the design of the rotary scanning system, the associated Q-band transceiver and the integration of the two systems via a custom-designed software. To illustrate the efficacy of the complete imaging system, SAF of several complex structures produced using the proposed system, are presented and discussed.

Optimum 2-D Nonuniform Spatial Sampling for Microwave SAR-Based NDE Imaging Systems

Microwave and millimeter-wave synthetic aperture radar (SAR)-based imaging techniques, which are used for nondestructive evaluation (NDE), have shown tremendous usefulness for the inspection of a wide variety of complex composite materials and structures. An important practical issue associated with these imaging techniques is the required criteria associated with the physical gathering of the imaging data. In previous work on uniform sampling optimization, it was shown that the (uniform) spatial sampling density should be higher than the Nyquist density if preservation of spatial resolution, as defined by the half-power width of an ideal point target, is of interest. Conversely, nonuniform sampling has shown to provide effective signal reconstruction even for average spatial sampling densities below the Nyquist density-a distinct advantage over uniform sampling. This paper presents a comprehensive study into the optimization of nonuniform sampling for microwave SAR-based NDE imaging using three typical reconstruction techniques for nonuniformly sampled data, including natural interpolation, area-weighted Fourier integration, and conjugate gradient residual error minimization methods. To study the efficacy of these reconstruction methods, simulations of a point target were performed for a range of target distances and a range of average spatial sample separations. Resulting SAR images from the reconstruction techniques are then analyzed and compared according to two metrics: error between an image and an ideal image and resolution as determined by the half-power width of a point target. This is followed by experimental results corroborating the simulation results. Finally, nonuniform sampling requirements, given a minimum metric performance, are generalized for a given imager aperture size.

Nonuniform Manual Scanning for Rapid Microwave Nondestructive Evaluation Imaging

Wideband synthetic aperture radar (SAR) technique is a robust imaging tool for microwave and millimeter-wave imaging such as nondestructive evaluation applications. In this paper, we present an alternative method to conventional raster scanning involving manually selected and nonuniformly distributed measurement positions, enabling the production of complete SAR images—potentially using only a fraction of the conventionally required measured data. The user is kept informed throughout the scanning process by a stream of real-time SAR images. Finally, data reconstruction algorithms are used offline to produce high-quality images with considerably lower background noise and image artifacts as compared to the real-time images. We also introduce a novel reconstruction method that uses the components of the SAR algorithm to advantageously exploit the inherent spatial information contained in the data, resulting in a superior data reconstruction and final SAR image. This paper presents the measurement methodology along with the images obtained from three different specimens of increasing geometrical complexity.

Quantitative and qualitative comparison of SAR images from incomplete measurements using compressed sensing and nonuniform FFT

In this paper the performance of two wideband synthetic aperture radar (SAR) imaging methods from incomplete data sets are compared quantitatively and qualitatively. The first approach uses nonuniform fast Fourier transform (NUFFT) SAR to form images from nonuniform spatial and frequency data points. The second approach benefits from the emerging compressed sensing (CS) methodology to recover raw data from undersampled measurements. The results of our experimental tests show that CS has a better performance in terms of error and image contrast while NUFFT SAR has lower computational complexity.

Modified Waveguide Flange for Evaluation of Stratified Composites

Nondestructive evaluation of stratified (layered) composite structures at microwave and millimeter-wave frequencies is of great interest in many applications where simultaneous determination of the complex dielectric properties and thicknesses of multiple layers is desired. Open-ended rectangular waveguide probes, radiating into such structures, are effective tools for this purpose. The technique utilizes a full-wave electromagnetic model that accurately models the complex reflection coefficient as a function of frequency and material properties. While the electromagnetic model assumes an infinite waveguide flange (or ground plane), the measurements are conducted using a finite-sized flange. Consequently, the results of the electromagnetic model and those from measurements may not be sufficiently alike for accurate dielectric property and thickness evaluation. This paper investigates the effect of using an open-ended waveguide with a standard finite-sized flange on the error in evaluating the complex dielectric properties of a composite structure. Additionally, we present the design of a novel flange that markedly reduces this undesired effect by producing very similar electric field properties, at the flange aperture, to those created by an infinite flange. Finally, the efficacy of the design for evaluating the dielectric properties of a layered composite structure is demonstrated as well.

Novel Reflectometer for Millimeter-Wave 3-D Holographic Imaging

Small, portable, and wideband millimeter-wave imaging systems are used in many nondestructive testing and imaging applications, as these systems are capable of producing high-resolution images of the interior of composite structures. Typically, systems capable of producing holographic 3-D images incorporate expensive and bulky wideband heterodyne coherent reflectometers or commercial vector network analyzers. In many nondestructive testing applications, evaluation of electrical property distribution of an object is not of interest; instead, the geometrical distribution of the object is studied. In such cases, the use of coherent reflectometers that provides referenced information about the magnitude and phase of a reflected signal is not required. Consequently, simpler reflectometers, capable of producing 3-D holographical images of objects are of great value since they reveal significant information about the object. Moreover, 3-D holographical images enable production of image slices at different depths. This paper presents a novel wideband, small, and low-cost reflectometer capable of producing holographic 3-D millimeter-wave images. The design of the reflectometer, as well as several examples of produced images in diverse materials, is provided.

MILLIMETER WAVE HOLOGRAPHICAL INSPECTION OF HONEYCOMB COMPOSITES

Multi‐layered composite structures manufactured with honeycomb, foam, or balsa wood cores are finding increasing utility in a variety of aerospace, transportation, and infrastructure applications. Due to the low conductivity and inhomogeneity associated with these composites, standard nondestructive testing (NDT) methods are not always capable of inspecting their interior for various defects caused during the manufacturing process or as a result of in‐service loading. On the contrary, microwave and millimeter wave NDT methods are well‐suited for inspecting these structures since signals at these frequencies readily penetrate through these structures and reflect from different interior boundaries revealing the presence of a wide range of defects such as isband, delamination, moisture and oil intrusion, impact damage, etc. Millimeter wave frequency spectrum spans 30 GHz–300 GHz with corresponding wavelengths of 10−1 mm. Due to the inherent short wavelengths at these frequencies, one can produce high spatial...