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Multiple-input multiple-output (MIMO) imaging systems in the terahertz frequency range have a high potential in the field of non-destructive testing (NDT). With such systems it is possible to detect defects in composite materials, for example cracks or delaminations in fiber composites. To investigate mass-produced products it is necessary to study the objects in close to real-time on a conveyor without affecting the production cycle time. In this work we present the conception and realization of a 3D MIMO imaging system for in-line investigation of composite materials and structures. To achieve a lateral resolution of 1 mm, in order to detect such small defects in composite materials with a moderate number of elements, precise sensor design is crucial. In our approach we use the effective aperture concept. The designed sparse array consists of 32 transmitters and 30 receivers based on planar semiconductor components. High range resolution is achieved by an operating frequency between 220 GHz and 260 GHz in a stepped frequency continuous wave (SFCW) setup. A matched filter approach is used to simulate the reconstructed 3D image through the array. This allows the evaluation of the designed array geometry in regard of resolution and side lobe level. In contrast to earlier demonstrations, in which synthetic reconstruction is only performed in a 2D plane, an optics-free full 3D recon- struction has been implemented in our concept. Based on this simulation we designed an array geometry that enables to resolve objects with a resolution smaller than 1mm and moderate side lobe level.

This paper will discuss the development of a millimeter-wave (mm-wave) receiver module used in a sparse array passive imaging system. Using liquid crystal polymer (LCP) technology and low power InP low noise amplifiers (LNA), enables the integration of the digital circuitry along with the RF components onto a single substrate significantly improves the size, weight, power, and cost (SWaP-C) of the mm-wave receiver module compared to previous iterations of the module. Also comparing with previous generation modules, the operating frequency has been pushed from 77 GHz to 95 GHz in order to improve the resolution of the captured image from the sparse array imaging system.

“Clad steel” refers to a thick carbon steel structural plate bonded to a corrosion resistant alloy (CRA) plate, such as stainless steel or titanium, and is widely used in industry to construct pressure vessels. The CRA resists the chemically aggressive environment on the interior, but cannot prevent the development of corrosion losses and cracks that limit the continued safe operation of such vessels. At present there are no practical methods to detect such defects from the exposed outer surface of the thick carbon steel plate, often necessitating removing such vessels from service and inspecting them visually from the interior. In previous research, sponsored by industry to detect and localize damage in pressurized piping systems under operational and environmental changes, we investigated a number of data-driven signal processing methods to extract damage information from ultrasonic guided wave pitch-catch records. We now apply those methods to relatively large clad steel plate specimens. We study a sparse array of wafer-type ultrasonic transducers adhered to the carbon steel surface, attempting to localize mass scatterers grease-coupled to the stainless steel surface. We discuss conditions under which localization is achieved by relatively simple first-arrival methods, and other conditions for which data-driven methods are needed; we also discuss observations of plate-like mode properties implied by these results.

In US, there are over 1482 dry cask storage system (DCSS) in use storing 57,807 fuel assemblies. Monitoring is necessary to determine and predict the degradation state of the systems and structures. Therefore, nondestructive monitoring is in urgent need and must be integrated into the fuel cycle to quantify the “state of health” for the safe operation of nuclear power plants (NPP) and radioactive waste storage systems (RWSS). Innovative approaches are desired to evaluate the degradation and damage of used fuel containers under extended storage. Structural health monitoring (SHM) is an emerging technology that uses in-situ sensory system to perform rapid nondestructive detection of structural damage as well as long-term integrity monitoring. It has been extensively studied in aerospace engineering over the past two decades. This paper presents the development of a SHM and damage detection methodology based on piezoelectric sensors technologies for steel canisters in nuclear dry cask storage system. Durability and survivability of piezoelectric sensors under temperature influence are first investigated in this work by evaluating sensor capacitance and electromechanical admittance. Toward damage detection, the PES are configured in pitch catch setup to transmit and receive guided waves in plate-like structures. When the inspected structure has damage such as a surface defect, the incident guided waves will be reflected or scattered resulting in changes in the wave measurements. Sparse array algorithm is developed and implemented using multiple sensors to image the structure. The sparse array algorithm is also evaluated at elevated temperature.

Detection, localization, and characterization of impact damage in composites using in situ transducers are important objectives for the aerospace industry to both reduce maintenance costs and prevent failures. A network of piezoelectric transducers spatially distributed over an area of interest is one practical configuration for utilizing guided waves to accomplish these objectives. Detecting and localizing barely visible impact damage with such a sparse array has been demonstrated in prior work, and improvements in localization were demonstrated by incorporating fairly crude estimates of scattering patterns in the imaging algorithms. Here we obtain more estimates of scattering patterns from a simulated defect by employing baseline subtraction of wavefield data recorded in a circle centered at the scatterer. Scattering patterns are estimated from the wavefield residual signals before and after simulated damage is introduced and the estimated scattering patterns are then incorporated into sparse array imaging via the minimum variance imaging method. Images created with different scattering patterns are compared and the efficacy of the methodology is assessed.

A linear ultrasound array system usually has a larger pitch and is less costly than a phased array system, but loses the ability to fully steer the ultrasound beam. In this paper, we propose a system whose hardware is similar to a large-pitch linear array system, but whose ability to steer the beam is similar to a phased array system. The motivation is to reduce the total number of measurement channels M (the product of the number of transmissions, nT, and the number of the receive channels in each transmission, nR), while maintaining reasonable image quality. We combined adjacent elements (with proper delays introduced) into groups that would be used in both the transmit and receive processes of synthetic transmit aperture imaging. After the M channels of RF data were acquired, a pseudo-inversion was applied to estimate the equivalent signal in traditional STA to reconstruct a STA image. Even with the similar M, different choices of nT and nR will produce different image quality. The images produced with M=N2/15 in the selected regions of interest (ROI) were demonstrated to be comparable with a full phased array, where N is the number of the array elements. The disadvantage of the proposed system is that its field of view in one delay-configuration is smaller than a standard full phased array. However, by adjusting the delay for each element within each group, the beam can be steered to cover the same field of view as the standard fully-filled phased array. The LPSSTA system might be useful for 3D ultrasound imaging.

There are many structures serving vital infrastructure, energy, and national security purposes. Inspecting the components and areas of the structure most prone to failure during maintenance operations by using non- destructive evaluation methods has been essential in avoiding costly, but preventable, catastrophic failures. In many cases, the inspections are performed by introducing acoustic, ultrasonic, or even thermographic waves into the structure and then evaluating the response. Sometimes the structure, or a component, is not accessible for active inspection methods. Because of this, there is a growing interest to use passive methods, such as using ambient noise, or sources of opportunity, to produce a passive impulse response function similar to the active approach. Several matched field processing techniques most notably used in oceanography and seismology applications are examined in more detail. While sparse array imaging in structures has been studied for years, all methods studied previously have used an active interrogation approach. Here, structural damage detection is studied by use of the reconstructed impulse response functions in ambient noise within sparse array imaging techniques, such as matched-field processing. This has been studied in experiments on a 9-m wind turbine blade.

A new promising architecture of microwave personnel screening system is analyzed in this paper with numerical simulations. This architecture is based on the concept of inverse aperture synthesis applied to a naturally moving person. The extent of the synthetic aperture is formed by a stationary vertical linear antenna array and by a length of subject’s trajectory as he moves in the vicinity of this antenna array. The coherent radar signal processing is achieved by a synchronous 3D video-sensor whose data are used to track the subject. The advantages of the proposed system architecture over currently existing systems are analyzed. Synthesized radar images are obtained by numerical simulations with a human torso model with concealed objects. Various aspects of the system architecture are considered, including: advantages of using sparse antenna arrays to decrease the number of antenna elements, the influence of positioning errors of body surface due to outer clothing. It was shown that detailed radar images of concealed objects can be obtained with a narrow-band signal due to the depth information available from the 3D video sensor. The considered ISAR architecture is considered perspective to be used on infrastructure objects owing to its superior qualities: highest throughput, small footprint, simple design of the radar sub-system, non-required co-operation of the subject.

Conventional photoacoustic computed tomography (PACT) image reconstruction methods assume that the object and surrounding medium are described by a constant speed-of-sound (SOS) value. In order to accurately recover fine structures, SOS heterogeneities should be quantified and compensated for during PACT reconstruction. To address this problem, several groups have proposed hybrid systems that combine PACT with ultrasound computed tomography (USCT). In such systems, a SOS map is reconstructed first via USCT. Consequently, this SOS map is employed to inform the PACT reconstruction method. Additionally, the SOS map can provide structural information regarding tissue, which is complementary to the functional information from the PACT image. We propose a paradigm shift in the way that images are reconstructed in hybrid PACT-USCT imaging. Inspired by our observation that information about the SOS distribution is encoded in PACT measurements, we propose to jointly reconstruct the absorbed optical energy density and SOS distributions from a combined set of USCT and PACT measurements, thereby reducing the two reconstruction problems into one. This innovative approach has several advantages over conventional approaches in which PACT and USCT images are reconstructed independently: (1) Variations in the SOS will automatically be accounted for, optimizing PACT image quality; (2) The reconstructed PACT and USCT images will possess minimal systematic artifacts because errors in the imaging models will be optimally balanced during the joint reconstruction; (3) Due to the exploitation of information regarding the SOS distribution in the full-view PACT data, our approach will permit high-resolution reconstruction of the SOS distribution from sparse array data.

Active imaging systems in the millimeter wave region have proven to offer good results for security applications. Especially a coherent signal detection results in a high dynamic range. Several techniques and systems were published in the last years. The drawback of an active illumination of the measurement object is the effect of shading and poor illuminated areas due to specular reflections from smooth surfaces. The visibility of an object depends on its surface roughness and its relative positioning to the imaging sensor. Especially in personnel screening, the human skin behaves as a smooth mirror for millimeter waves. This paper describes the incorporation of multipath signals in the imaging process to enhance the illumination properties of active imaging systems. The proposed multipath concept is demonstrated with an active multistatic imaging system working from 70 to 80 GHz for security applications.

Sequentially-switched linear arrays are an enabling technology for a number of near-field microwave imaging applications. Electronically sequencing along the array axis followed by mechanical scanning along an orthogonal axis allows dense sampling of a two-dimensional aperture in near real-time. The Pacific Northwest National Laboratory (PNNL) has developed this technology for several applications including concealed weapon detection, groundpenetrating radar, and non-destructive inspection and evaluation. These techniques form three-dimensional images by scanning a diverging beam swept frequency transceiver over a two-dimensional aperture and mathematically focusing or reconstructing the data into three-dimensional images. Recently, a sparse multi-static array technology has been developed that reduces the number of antennas required to densely sample the linear array axis of the spatial aperture. This allows a significant reduction in cost and complexity of the linear-array-based imaging system. The sparse array has been specifically designed to be compatible with Fourier-Transform-based image reconstruction techniques; however, there are limitations to the use of these techniques, especially for extreme near-field operation. In the extreme near-field of the array, back-projection techniques have been developed that account for the exact location of each transmitter and receiver in the linear array and the 3-D image location. In this paper, the sparse array technique will be described along with associated Fourier-Transform-based and back-projection-based image reconstruction algorithms. Simulated imaging results are presented that show the effectiveness of the sparse array technique along with the merits and weaknesses of each image reconstruction approach.

Periodic inspection of aerospace structures, while essential for ensuring their safety, incurs significant costs over a structure’s life and also can result in significant loss of service. Structural health monitoring (SHM), which is also referred to as in situ nondestructive evaluation (NDE), offers the promise of more frequent assessments of structural integrity with little or no loss of service; however, such systems are not in common use. Here we consider a combined SHM and NDE approach to inspection of composite, plate-like components where the SHM system detects sites of possible damage and the follow-up NDE method utilizes the in situ SHM sensors to facilitate the inspection. The specific SHM approach considered is that of a sparse guided wave array using simple transducers that are spatially distributed on the structure. The NDE approach is non-contact guided wavefield imaging whereby one or more of the SHM transducers is used as a source and full wavefield data are recorded over the area of interest. This method has the advantage over conventional ultrasonic methods of being non-contact and requiring minimal surface preparation. Sparse array and wavefield data from a composite specimen with simulated sites of damage are presented here to illustrate the concept. Damage is simulated via glued-on steel plate pieces at multiple locations, and localization is performed using delay-and-sum imaging. A small, single site of simulated damage is well-localized, whereas larger and multiple sites of damage are not; however, their presence is readily detected. The follow-up wavefield imaging using a single sparse array transducer as a source is able to not only locate the sites of damage, but is able to provide a reasonable estimate of their sizes.

Passive terahertz imaging technology, which provided blackbody thermal imaging, showed great potential for security applications during the last years. Different research teams in different countries spent a great effort on it for obvious reasons, and first commercial passive THz human imaging system was available. Yet, development of passive THz-imaging system was always limited by the frame rate, spatial resolution and detector cell of it. In this paper, we presented the concept and experimental set-up for a quasi-optical system used in a near-field 0.14THz passive terahertz imaging system. The quasi-optical system was composed of a double-mirror beam-waveguide system and a special feed-horns array. The design of quasi-optical system was based on near-field Gauss optics theory. The double-mirror beam waveguide system was made up of an off-axis ellipsoid mirror and a flat mirror. The influence of angle-scanning of quasi-optics system in near-field THz-imaging was investigated theoretically. Meanwhile, a spatial sparse array-arrangement of feed-horns was applied in quasi-optics system, that the adjacent feed-horns were stagger in the horizontal direction. The influence of this special feed-horns array arrangement on the near-field imaging was also discussed in theory. The quasi-optical system met the requirements of the near-field imaging. The problem of low frame rate of traditional sparse array arrangement was solved by this special array arrangement. And the spatial imaging range of the passive imaging system was also increased. The experimental results of this terahertz imaging system were consistent with our earlier theoretical results. When the detective distance was 3-5m, the experimental spatial resolution of near-field THz imaging was 5cm, and a 5Hz of frame rate was obtained. The quasi-optics system had demonstrated its ability to scan persons and various kinds of objects. We hoped a commercial, user-friendly terahertz imaging system, which could be used for security purposes, would be achieved in the future. And as a beginning step towards this goal, our theoretical and experimental results would be the basis of the improvement of near-field passive terahertz imaging system.

Personnel screening is demanded nowadays for securing air traffic as well as critical infrastructures. The millimeter-waves are able to penetrate clothes and detect concealed objects, making them an attractive choice for security screening. Imaging methods based on multistatic architecture can ensure high quality imagery in terms of resolution and dynamic range. Following the advances in semiconductor technology, fully electronic solutions delivering real-time imaging are becoming feasible. Furthermore, the continuously increasing capabilities of digital signal processing units allow for the utilization of digital-beamforming techniques for image reconstruction, thus offering new opportunities for imaging systems to use sophisticated operation modes. Based on these modern technologies, an advanced realization addressing personnel screening in E-band with planar multistatic sparse array design is demonstrated.

In this paper, a novel imaging technique is assessed with a structural health monitoring (SHM) system based on a sparse array of piezoceramic sensors and actuators for in-plane inspection. The imaging approach used in this system is based on the Time-of-Flight (ToF), and the knowledge of the velocity of ultrasonic waves in the structure. While this technique assumes non-dispersive wave propagation, the proposed imaging technique exploits the dispersion of waves as it is based on the phase velocity. The signal measured at a given sensor is correlated with a theoretical prediction of a propagated burst in the structure and, combining the results for multiple sensors, an image of the reflectors in the structure is obtained. This paper presents the implementation of the novel imaging technique in an existing system, including considerations for physical access to the signals and their conditioning. The performance of the existing imaging approach is compared with the novel imaging technique proposed for two test cases. The first assessment is conducted on a simple aluminum plate where magnets are used to simulate a defect. Then, the assessment of the novel imaging techniques is conducted on riveted plates with simulated cracks of different lengths. Imaging results are presented for a number of damage detection scenarios on these structures. The novel imaging technique is shown to improve imaging localization, resolution and robustness, while allowing fast implementation.

This paper proposes an adaptive Unscented Kalman Filter (UKF) algorithm for Acoustic Emission (AE) source localization in plate-like structures in noisy environments. Over all, the proposed approach consists of four main stages: 1) feature extraction, 2) sensor selection based on a binary hypothesis testing, 3) sensor weighting based on a well-defined weighting function, and 4) estimation of the AE source based on the UKF. The performance of the proposed algorithm is validated through pencil lead breaks performed on an aluminum plate instrument ed with a sparse array of piezoelectric sensors. To simulate highly noisy environment, two piezoelectric transducers have been used to continually generating high power white noise during testing. Keywords: Acoustic Emission (AE), Adaptive Unscented Kalman Filter (UKF), Binary Hypothesis Test, Lamb waves, inflight structural health monitoring. 1. INTRODUCTION The continual aging of aircraft fleets resulted in new safety requirements and necessitated upgrading the exciting maintenance technologies

Diffuse optical topography remains a valid tool in functional near infrared spectroscopy (fNIRS) since it avoids solving the forward and inverse computational problems, which are encountered in diffuse optical tomography. Topography is particularly useful when a sparse array of optodes is used and depth specificity is not the primary interest. We have developed an easy toolbox for diffuse optical topography (“EasyTopo”) based on a standard template of brain atlas. EasyTopo approximates the cortical layer of the brain as a hemispherical surface. Therefore, the stereotaxic coordinates of the brain surface and the co-registered fNIRS measurements (channels) are converted into the spherical coordinates, where 2D angular interpolation of the channel-wise data is implemented to obtain a topographic image of brain activation in the latitude-longitude space. Then, the interpolated image is projected back onto the brain surface in the original 3D stereotaxic coordinates. Compared with the existing 3D topography methods, EasyTopo is more computationally efficient and does not require any data extrapolation. Another advantage of EasyTopo is that the data between two spatially adjacent channels are interpolated along their included angles (i.e., along the angular direction) rather than along a straight line going under the brain surface. The former geometry in principle matches better with the realistic brain structure than the latter one. EasyTopo has been validated with both simulation and human experiments. Now this toolbox is publically available.

Time of flight diffraction and imaging (TOFDI) is based on time of flight diffraction (TOFD); it adds cross-sectional imaging to examine the bulk of a sample. Multiple wave modes are generated by a pulsed laser beam, ablative source and are received by a sparse array of non-contact electromagnetic acoustic transducers (EMATs). A B-scan is formed from multiple data captures (A-scans), with time and scan axes, and colour representing amplitude. B-scans may contain horizontal lines from surface waves propagating directly from emitter to receiver, or via a back-wall reflection, and angled lines after reflection off a surface edge. A Hough transform (HT), modified to deal with the constraints of a Bscan, can remove such lines. A parabola matched filter has been developed to identify features in the B-scan caused by scattering from point-like features, reducing them to peaks. The processed B-scan is processed further to form a crosssectional image, enabling detection and positioning of multiple defects. Phase correlation of camera images is used to track the relative position between transducer and sample to sub-pixel precision.

Finite element model updating based on a multi-scale data is demonstrated on a skewed in-service highway bridge. The multi-scale data approach provides an evidence-based method to create a bound of model class to ensure that the optimum model retains physical connectivity to the real structure. The need for this hybrid approach to model selection comes from the challenges of applying model updating to online structural health monitoring (SHM) strategy based on output-only measurements of in-service highway bridges, which should consider various uncertainties. In vibration-based FE model updating methods, the optimum model is selected by minimizing the error between modal parameters of the model and the real structure. A major drawback of model selection is the probability that the optimum model has no physical connectivity with the real structure. This is because a large set of updating parameters is required to increase accuracy. In this paper, an evidence-based approach to model selection using temperature-induced tilts is implemented in which the cyclical static behavior of a bridge is used to create a bound of possible models. This approach has a strong potential to be applicable to large civil structures with sparse array of sensors.

Photoacoustic imaging is a hybrid imaging modality capable of producing images based on optical contrast, but with depth penetration and resolution similar to ultrasound imaging. In this work, a staring, sparse approach to 3D photoacoustic imaging was used to image a number of objects. The photoacoustic system described in this paper improved upon a previous generation that contained 30 commercial transducers, by incorporating 60 custom-built transducers into a compact hemispherical array. Imaging was performed by acquiring an experimental estimate of the imaging operator and solving a linear system model to provide an estimate of the object. The imaging operator contained 18,000 voxels, each at 0.5-mm isotropic resolution. The dimensions of the imaging operator were 30 mm × 30 mm × 2.5 mm. In the first experiment, a black wire was arranged in a triangular shape and imaged in a 0.3% Intralipid TM solution. The second experiment utilized a rotating human hair, where the hair was imaged at different angular positions. Both objects were successfully captured with reasonable accuracy, though image artifacts were present in both sets of images. The experimental results demonstrated that objects of substantial geometrical complexity could be reconstructed using measurements from only 60 transducers with prior knowledge of the imaging operator.

Near range microwave imaging systems have broad application prospects in the field of concealed weapon detection, biomedical imaging, nondestructive testing, etc. In this paper, the techniques of MIMO and sparse line array are applied to near range microwave imaging, which can greatly reduce the complexity of imaging systems. In detail, the paper establishes two-dimensional near range MIMO imaging geometry and corresponding echo model, where the imaging geometry is formed by arranging sparse antenna array in azimuth direction and transmitting broadband signals in range direction; then, by analyzing the relationship between MIMO and convolution principle, the paper develops a method of arranging sparse line array which can be equivalent to a full array; and the paper deduces the backprojection algorithm applied to near ranging MIMO imaging geometry; finally, the imaging geometry and corresponding imaging algorithm proposed in this paper are investigated and verified by means of theoretical analysis and numerical simulations.

Reflective band sensors are often signal to noise limited in low light conditions. Any additional filtering to obtain spectral information further reduces the signal to noise, greatly affecting range performance. Modern sensors, such as the sparse color filter CCD, circumvent this additional degradation through reducing the number of pixels affected by filters and distributing the color information. As color sensors become more prevalent in the warfighter arsenal, the performance of the sensor-soldier system must be quantified. While field performance testing ultimately validates the success of a sensor, accurately modeling sensor performance greatly reduces the development time and cost, allowing the best technology to reach the soldier the fastest. Modeling of sensors requires accounting for how the signal is affected through the modulation transfer function (MTF) and noise of the system. For the modeling of these new sensors, the MTF and noise for each color band must be characterized, and the appropriate sampling and blur must be applied. We show how sparse array color filter sensors may be modeled and how a soldier's performance with such a sensor may be predicted. This general approach to modeling color sensors can be extended to incorporate all types of low light color sensors.

Guided wave imaging has shown great potential for structural health monitoring applications by providing a way to visualize and characterize structural damage. For successful implementation of delay-and-sum and other elliptical imaging algorithms employing guided ultrasonic waves, some degree of mode purity is required because echoes from undesired modes cause imaging artifacts that obscure damage. But it is also desirable to utilize multiple modes because different modes may exhibit increased sensitivity to different types and orientations of defects. The well-known modetuning effect can be employed to use the same PZT transducers for generating and receiving multiple modes by exciting the transducers with narrowband tone bursts at different frequencies. However, this process is inconvenient and timeconsuming, particularly if extensive signal averaging is required to achieve a satisfactory signal-to-noise ratio. In addition, both acquisition time and data storage requirements may be prohibitive if signals from many narrowband tone burst excitations are measured. In this paper, we utilize a chirp excitation to excite PZT transducers over a broad frequency range to acquire multi-modal data with a single transmission, which can significantly reduce both the measurement time and the quantity of data. Each received signal from a chirp excitation is post-processed to obtain multiple signals corresponding to different narrowband frequency ranges. Narrowband signals with the best mode purity and echo shape are selected and then used to generate multiple images of damage in a target structure. The efficacy of the proposed technique is demonstrated experimentally using an aluminum plate instrumented with a spatially distributed array of piezoelectric sensors and with simulated damage.

Sparse arrays are highly attractive for implementing two-dimensional arrays, but come at the cost of degraded image quality. We demonstrate significantly improved performance by exploiting the coherent ultrawideband nature of singlecycle THz pulses. We compute two weighting factors to each time-delayed signal before final summation to form the reconstructed image. The first factor employs cross-correlation analysis to measure the degree of walk-off between timedelayed signals of neighboring elements. The second factor measures the spatial coherence of the time-delayed delayed signals. Synthetic aperture imaging experiments are performed with a THz time-domain system employing a mechanically scanned single transceiver element. Cross-sectional imaging of wire targets is performed with a onedimensional sparse array with an inter-element spacing of 1.36 mm (over four λ at 1 THz). The proposed image reconstruction technique improves image contrast by 15 dB, which is impressive considering the relatively few elements in the array. En-face imaging of a razor blade is also demonstrated with a 56 x 56 element two-dimensional array, showing reduced image artifacts with adaptive reconstruction. These encouraging results suggest that the proposed image reconstruction technique can be highly beneficial to the development of large area two-dimensional THz arrays.

This paper describes the modeling of human task performance using a passive interferometric millimeter wave (MMW) imaging sensor. The model is based on a previous model developed for concealed weapon identification using an active terahertz imager. Both models leverage the task performance modeling approach developed by the US Army Night Vision and Electronic Sensors Directorate. Key developments for this model include modeling of the effects of an interferometric antenna array, including sparse arrays, and a novel optical upconversion and processing stage being developed by the University of Delaware. Sparse interferometric arrays do not fully sample the spatial frequency extent of the image and as a result, can have degraded spatial frequency response over a fully populated array. The spatial frequency response of the sparse array can have a dramatic effect on image quality. Image quality is empirically related to task performance through the use of perception experiments. Possible applications of this model include system trade studies, concealed weapon identification, and navigation in fog and brown out conditions.

Sparse aperture imaging systems are capable of producing high resolution images while maintaining an overall light collection area that is small with respect to a fully filled aperture yielding the same resolution. However, conventional sparse aperture systems pay the penalty of reduced contrast at mid-band spatial frequencies. The modulation transfer function (MTF), or normalized autocorrelation, provides a quantative measure of both the resolution and contrast of an optical imaging system. Numerical MTF calculations were thus used to examine mid-band contrast recovery through the systematic increase of autocorrelation redundancy in a Golay-9 sparse array. In a Golay-9 sparse aperture arrangement, three sets of three sub-apertures can be shown to lie at unique radii from the center of the array. In order to increase the mid-frequency contrast we then have two options. The first, and most influential, is to increase the size of the sub-apertures located at the intermediate radius from the array origin. This directly increases autocorrelation redundancy at mid-band frequencies. The second option, though less effective, is to increase the relative mid-band frequency response by attenuating the outer most sub-apertures. We will demonstrate that by increasing the diameters of the mid-radii sub-apertures, mid-band contrast can be increased by over 45%, compared to uniform sub-aperture diameter arrays. We will also demonstrate that attenuating the outer most sub-apertures can further increase mid-band contrast recovery, but only by less than 1%. The effects on array fill factor will also be discussed.

High-resolution imaging can be achieved by optical aperture synthesis. Using a sparse array of smaller aperture, one can synthesize an aperture with the resolution of the equivalent filled aperture while reducing the size and weight of system. The environment and vibration factors of synthetic aperture arrays lead to errors in phase measurement and cause a severe degradation in the image quality. Real time phase calibration of synthetic aperture arrays is very important to produce images of consistently good quality. Redundant spacings calibration allows continuous calibration of the instrument with errors without requiring the model of the source.

In such applications as fast 3D imaging with 2D arrays and point-of-care imaging with an ultra portable devices, periodic sparse arrays(PSA) can be efficiently used to increase the effective aperture size with less number of active elements than the conventional method. Generally, PSA can be represented as sub-arrays distributed uniformly in P -element intervals, each with L consecutive elements, where L < P. Since the continuous wave beam pattern in the far-field is given by Fourier transform of aperture function, the beam pattern of PSA is a multiplication of beam patterns of the upsampled dense array by the ratio of P and L -elements sub-array. A recent method to design a PSA pair provides analytically the values of P and L for transmit and receive arrays to eliminate the dominant grating lobes, which occur at the same position on both transmit and receive. In this work, we present a method to design a PSA pair with improved performance by further suppressing the residual grating lobes of PSA. It can be accomplished by properly shading amplitude of the transmit and receive sparse arrays. This shading window function is also obtained by signal analysis of aperture functions. The beam patterns of various PSA pairs based on the proposed design method are evaluated through computer simulations. The simulation results show that the residual grating lobes are reduced by about 10dB more in all cases. Consequently, our method can be used to improve the performance of beam pattern or enhance the periodicity of sparse array.

In support of the U.S. Army's need for intelligence on the configuration, content, and human presence inside enclosed areas (buildings), the Army Research Laboratory is currently engaged in an effort to evaluate RF sensors for the "Sensing Through The Wall" initiative (STTW).Detection and location of the presence of enemy combatants in urban settings poses significant technical and operational challenges. This paper shows the potential of hand held RF sensors, with the possible assistance of additional sources like Unattended Aerial Vehicles (UAV), Unattended Ground Sensors (UGS), etc, to fulfill this role. In this study we examine both monostatic and multistatic combination of sensors, especially in configurations that allow the capture of images from different angles, and we demonstrate their capability to provide comprehensive information on a variety of buildings. Finally, we explore the limitations of this type of sensor arrangement vis-a-vis the required precision in the knowledge of the position and timing of the RF sensors. Simulation results are provided to show the potential of this type of sensor arrangement in such a difficult environment.

Passive imaging for security and other applications has reached an important level of development. Ka and W band imaging systems are now commercial products, thanks to cheap and stable amplifiers. While deployment numbers are still modest, improvements to these systems will come from engineering and algorithm development, and not fundamental research. What research should focus on are system volume reduction and resolution improvements. Fundamental research has several potential paths to solve these problems. Silicon Germanium CMOS can build acceptable millimeter wave amplifiers, and while SiGe noise figure is higher than GaAs, the ability to integrate RF and back end processing will push us closer to a CCD-like sensor. Antimony Arsenide features higher mobility than GaAs, with very low flicker noise and operation above 200 GHz which will reduce aperture size for equivalent resolution. More focus is needed in material characterization, particularly for clothing and common commercial materials. Finally, Sparse Array technology may build flat, conformal structures with high resolution and relatively low detector count. This paper will briefly discuss the time line of past innovations, and explore the advantages and challenges of the new technologies that will drive this field forward.

Corrosion is detrimental to the structural integrity of many critical components, and ultrasonic methods are routinely used in the field to make thickness measurements at points of interest. However, is often difficult to assess the true extent of corrosion damage because of the likelihood of missing small corroded areas and the difficulty in mapping the extent of large corroded areas without an extensive number of time consuming measurements. Guided ultrasonic waves have the potential to both detect corrosion as early as possible and reduce the subsequent inspection time. This paper presents results from a study using Lamb waves to quantify the area extent of corrosion in an aluminum plate specimen. A sparse array of ultrasonic transducers was attached to an aluminum plate, and broadband excitation methods were used to generate both symmetric and anti-symmetric Lamb wave modes. As has been demonstrated in previous studies, the through transmission response recorded from each transmit-receive pair may be analyzed to determine if a defects exists and approximately determine its location. This paper presents a method to determine the exact location and quantify the extent of the corroded area using an acoustic wavefield imaging method. Lamb waves are generated using one of the permanently attached transducers as a source, and the acoustic wavefield is captured on the surface of the plate using an air-coupled transducer as a receiver. Full wavefield data are recorded as the receiver is scanned over the specimen surface, and wavefield images are processed to remove the strong incident wave and enhance the weaker scattered waves. The amplitude at the crest of the leading Lamb mode (S0) is analyzed to produce spatial images of defective areas. Measured length and area results from these images compare very favorably with actual defect sizes. Results are also presented for scattering from a through hole with a simulated crack.

Millimeter and Sub-mm-wave imagers/sounders are considered for future meteorological geostationary satellite missions. A novel interferometric Geostationary Atmospheric Sounder (GAS) has been developed and a concept demonstrator is under construction. The concept is a response to the requirements of observations for nowcasting and short range forecasting in 2015-2025, as determined by EUMETSAT for post-MSG operational satellites observations. Prioritized parameters include vertical profiles of temperature and humidity with high temporal and horizontal resolution (15 min and 30 km) under all weather conditions. Frequency bands around 53GHz, 118GHz, 183GHz, 380GHz have the highest user priority and are all supported by GAS. The instrument relies on an innovative configuration of interferometer elements which enables the use of a sparse array and simplifies calibration.

Acoustic emission (AE) testing is potentially a highly suitable technique for structural health monitoring (SHM) applications due to its ability to achieve high sensitivity from a sparse array of sensors. For AE to be deployed as part of an SHM system it is essential that its capability is understood. This is the motivation for developing a forward model, referred to as QAE-Forward, of the complete AE process in real structures which is described in the first part of this paper. QAE-Forward is based around a modular and expandable architecture of frequency domain transfer functions to describe various aspects of the AE process, such as AE signal generation, wave propagation and signal detection. The intention is to build additional functionality into QAE-Forward as further data becomes available, whether this is through new analytic tools, numerical models or experimental measurements. QAE-Forward currently contains functions that implement (1) the excitation of multimodal guided waves by arbitrarily orientated point sources, (2) multi-modal wave propagation through generally anisotropic multi-layered media, and (3) the detection of waves by circular transducers of finite size. Results from the current implementation of QAE-Forward are compared to experimental data obtained from Hsu-Neilson tests on aluminum plate and good agreement is obtained. The paper then describes an experimental technique and a finite element modeling technique to obtain quantitative AE data from fatigue crack growth that will feed into QAE-Forward.

This paper presents new software (and simulations) that would phase a space based free flyer sparse array telescope. This particular sparse array method uses mirrors that are far enough away for sensors at the focal point module to detect tip tilt by simply using the deflection of the beam from each mirror. Also the large distance allows these circle six array mirrors to be actuated flats . For piston the secondary actuated mirrors (one for each large mirror segment of these widely spaced sparse array mirrors distributed on a parabola) are moved in real time to maximize the Strehle ratio using the light from the star the planet is revolving around since that star usually has an extremely high SNR (Signal to Noise Ratio). There is then no need for a 6DOF spider web of laser interferometric beams and deep dish mirrors (as in the competing Darwin and JPL methods) to accomplish this. Also the distance between the six 3 meter aperture mirrors could be large (kilometer range) guaranteeing a high resolution and also substantial light gathering power (with these 6 large mirrors) for imaging the details on the surface of extrasolar terrestrial type planets. In any case such a multisatellite free flyer concept would then be no more complex than the European cluster which is now operational. This is a viable concept and a compelling way to image surface detail on extra solar earthlike planets. It is the ideal engineering solution to the problem of space based large baseline sparse arrays . Significant details of the software requirements have been recently developed. In this paper the Fortran code needed to both simulate and operate the actuators in the secondary mirror for this type of sparse array is discussed.

The anntenna area of distributed micosateliites radar system is usually smaller than the minimum SAR antenna area constraint, and there are range-Doppler ambiguities. So one key focus of this signal processing research is to develop processing approaches that exploit the added degrees of freedom of a spatially diverse formation to resolve inherent ambiguities of this sparse aperture distributed micosateliites radar system. In the paper, a beamform processing method of three-dimensional sparse arrays is proposed to spatially null Doppler ambiguities. This method first filters out each unambiguous frequency point from ambiguous Doppler channels of a few different phase centers by using spatial filter, this processing can be regard as a space-time processing, then combines all unambiguous frequency point to a whole unambiguous Doppler band, after that, does imaging processing. Theoretical derivation, performance analysis, and simulation of this method are discussed in the paper.

This paper describes a novel real-time algorithm for optically phasing sub-apertures of a sparse-array telescope system based on recursive estimation of the sub-aperture placement that maximizes a fringe-contrast metric. The sub-apertures are phased in pairs using broad spectral band flood-illumination of the sparse-array, while blocking reflections from all but two sub-apertures. The resulting Young’s geometry at the pupil produces an interference pattern that is characterized to determine spatial-frequency filters that are utilized to generate a contrast metric from the fringe patterns. This contrast metric is shown to generate a near-Gaussian variation as a function of optical path-length difference (OPD), with the maximum contrast occurring at zero OPD. The functional relationship between fringe contrast and sub-aperture position based on a common-path, laser-based relative-piston measurement system is developed into an estimator for maximization of fringe contrast (and therefore phasing of sub-apertures). The recursive algorithm produces real-time estimates of the zero OPD value of the relative position that improves as additional data is acquired.

This paper presents the development of the control system architecture for vibration mitigation and autonomous phasing of sub-apertures on a sparse-array test bed. The paper begins with a brief description of the telescope system under consideration, including the actuation system providing 3 degree-of-freedom rigid body correction to each sub-aperture, and the metrology system, comprised of a white-light-based low-bandwidth absolute position sensing system and a high-bandwidth, laser-based relative position sensing system. The control problem posed by the telescope is described, including a discussion of the performance requirements the control system must meet, which include asymptotic set point tracking, broadband and tonal disturbance rejection, and tracking of non-stationary objectives. The use of system identification techniques in development of an accurate model of the input-output dynamics of the system is presented. The overall control system architecture including discussions on aspects such as tolerance of sensor dropouts, and the design of these control systems based on the identified model is presented. The paper presents the results of the application of this control system approach to the experimental system, demonstrating performance of the controlled system.

This paper presents an overview of the development and capabilities of a space-traceable testbed developed for investigation of research issues related to deployable space telescopes. The Air Force Research Laboratory (AFRL) is developing the Deployable Optical Telescope (DOT), which upon completion will be a fully-deployable, sub-scale, space-traceable ground testbed for development and demonstration of critical technologies for the next-generation of space-optics systems. The paper begins with an overview of the DOT project’s technology goals, including the specific performance objectives of the various technologies that are being incorporated into the DOT testbed. The paper presents an overview of the DOT design, including the central integrating structure, deployable primary mirror petals, deployable secondary tower, deployment mechanisms, lightweight mirror segments, metrology, and control systems. The paper concludes with a report on the current status of DOT activities as well as a view of the future research that is planned for the project.

Telescopes and imaging interferometers with sparsely filled apertures can be lighter weight and less expensive than conventional filled-aperture telescopes. Sparse-aperture systems can be characterized by their fill factor, which is the ratio of the area of a given aperture to the area of a filled aperture having comparable theoretical resolution. We show that the modulation transfer function (MTF) at the midrange spatial frequencies tends to be proportional to the fill factor. Using signal-to-noise ratio (SNR) expressions for various sources of noise, we derive the relationship between the integration time, needed to achieve a given SNR, and the fill factor. For example, for a fixed, sparse array, the integration time is proportional to the inverse cube of the fill factor for photon noise, to the inverse square of the fill factor for readout noise, and to the inverse fourth-power of the fill factor for dark current.

Interferometric SAR (IFSAR) can be shown to be a special case of 3-D SAR image formation. In fact, traditional IFSAR processing results in the equivalent of merely a super- resolved, under-sampled, 3-D SAR image. However, when approached as a 3-D SAR problem, a number of IFSAR properties and anomalies are easily explained. For example, IFSAR decorrelation with height is merely ordinary migration in 3-D SAR. Consequently, treating IFSAR as a 3-D SAR problem allows insight and development of proper motion compensation techniques and image formation operations to facilitate optimal height estimation. Furthermore, multiple antenna phase centers and baselines are easily incorporated into this formulation, providing essentially a sparse array in the elevation dimension. This paper shows the Polar Format image formation algorithm extended to 3 dimensions, and then proceeds to apply it to the IFSAR collection geometry. This suggests a more optimal reordering of the traditional IFSAR processing steps.

The transducer driving function for Bessel beam has circular symmetry and can be generated by annular or 2-D arrays. In 2-D array, the elements are divided into a number of rings. Taking advantage of the circular symmetry, it was shown that arranging the elements in a hexagonal pattern instead of ordinary rectangular pattern could produce almost the same field pattern with 14% less elements. Our aim here is to eliminate some of the elements of the hexagonal array and obtain a hexagonal sparse array while maintaining the quality of the generated field. In our proposed method, starting from the outer most ring, a specific number of the elements of the ring are randomly selected and turned off. The field pattern of the resulting sparse arrays is simulated and compared to the field of the array with all of its elements active. If the relative mean square error is lower than a specific threshold value, more elements of the ring are turned off. This procedure is then repeated for the next ring until reaching the central ring. Our simulations for hexagonal sparse arrays show that for an error threshold of 4%, an acceptable Bessel beam can be generated only with 22% of the transducer elements used in the original hexagonal arrays. Generated beam still shows its non-diffracting property over a limited distance.

A novel high-speed 3D imaging system, base don projecting a speckle pattern onto an object and imaging the resulting pattern from multiple angles, is proposed that uses a unique single lens camera system with a rotating off-axis aperture. The rotating aperture allows adjustable non-equilateral spacing between defocused images to achieve greater depth of field and higher sub-pixel displacement accuracy. High resolution ultra fast processing is achieved by recursively correlating image pairs down to diffraction limited image size of the optics. Correlation errors are eliminated during processing by a novel technique based on the multiplication of correlation table elements from one or more adjacent regions. The Gaussian nature of the projected speckle pattern is used to reveal image disparity to sub-pixel accuracy. Processing is accomplished by compressing the images into sparse array format before they are correlated. This system is ideally suited for hardware implementation and circumvents many of the inherent limitations of multi- camera system using FFT spectral based correlation.

The need for multiple, cost-effective deployment of undersea acoustic mine-field reconnaissance and mine-hunting systems requires the development of light-weight, low-power, high- resolution sonars. These imaging sonars should be portable enough for use in a diver's hands or in a remote imaging sonar on an unmanned undersea vehicle in shallow waters. Under the support of the DARPA Sonoelectronics program, a system that can simultaneously focus many signals using multiple CCD/CMOS, programmable time-delay beamforming circuits connected to a sparsely-populated 2D acoustic array, is being developed. Using this approach, a real-time image could be formed to search for mine-like objects. To make a sparse array practical, the combination of low insertion loss and wide bandwidth performance is critical to realizing acceptable imaging performance with low illumination levels. In this system, a 20 cm diameter 2 MHz sparse array providing a 0.21 degree main beamwidth is proposed. Using this array, a real-time 2D cross-sectional scan of 128 X 128 pixels is achievable with a down-range and cross-range resolution of approximately 1 cm at a range of 3 m. In this paper, the signal-to-clutter ratios as a function of array bandwidth and sparse-array elements placement are presented. Furthermore, the performance of a CCD/CMOS subarray processor will be reported.

The Redstone Technical Test Center (RTTC) has the requirement to project dynamic, infrared (IR) imagery to sensors under test. This imagery must be of sufficient quality and resolution so that, sensors under test will perceive and respond just as they do to real-world scenes. In order to achieve this fidelity from a pixelized infrared resistor emitter array, non-uniformity correction (NUC) is necessary. An important step in performing NUC is to calibrate the IR projection system so as to be capable of projecting a radiometric uniform IR image. The quality of the projected image is significantly enhanced by proper application of this calibration. To properly implement non- uniformity correction, it is necessary to accurately measure the radiometric emission of each element, or display pixel (emitter pixel), in the emitter array. This paper presents mathematical models and image-processing techniques required to successfully calibrate a non-uniform emitter projection system to absolute temperature. RTTC has developed a high- speed, reliable, and flexible means of digitally processing IR images captured from an emitter array. This method of evaluating IR imagery is also useful in performing sensor and overall projection system characterization. The purpose of this paper is to present the methods for correcting the absolute temperature non-uniformity of an IR resistor array.

Experimental results are presented for active vibration control of the Air Force Research Laboratory's UltraLITE Precision Deployable Optical Structure (PDOS), a ground based model of a sparse array, large aperture, deployable optical space telescope. The primary vibration suppression technique employs spatio-temporal filtering, in which a small number of sensors are used to produce modal coordinates for the structural modes to be controlled. The spatio-temporal filtering technique is well suited for the control of complex, real-world structures because it requires little model information, automatically adapts to sensor and actuator failures, is computationally efficient, and can be easily configured to account for time-varying system dynamics. While controller development for PDOS continues, the results obtained thus far indicate the need for an integrated optical/structural control system.

A spatio-temporal filter (STF) based active vibration suppression technique is presented. The STF approach is intended for use for stability and jitter compensation for the UltraLITE Precision Deployable Experiment -- a ground demonstration of a sparse array, deployable, large aperture, optical space telescope concept. This technique is well suited for control of complex, real-world structures because it requires little model information, autonomously accommodates sensor and actuator failures, is computationally efficient and the controller is easily updated to account for time varying system dynamics. An overview of the STF approach is given and experimental active vibration suppression results obtained on the Mirror Mass Simulator testbed at AFRL, Kirtland AFB are presented.

In this paper we address a new approach to the remote sensing imaging problems for radar/sonar array imaging system stated and treated as ill-posed inverse problems of restoration the extended object reflected signals distorted in a stochastic scattering medium. The developed approach is based on combining the Bayesian estimation technique for signal restoration problems with constrained regularization technique for inversion of the signal formation operator of the stochastic measurement channel. To reduce the generic ill-posed imaging problem to its radar/sonar system oriented numerical version the experiment design methodology is applied. This results in the projection-dependent scheme for measured data that originates from limited number of sensors of a sparse array. Next, to alleviate the limitations on the absence of prior knowledge ofthe object signal the model-based assumptions for the desired image space are introduced. Model-based fusion of such diverse information on data sets and image space in a generalized constrained array imaging inverse problem is the first issue addressed in the paper. Optimal/suboptimal solution of this problem in the mixed Bayesian-regularization setting that results in the development of numerical technique for extended object imaging in scattering random media with improved spatial resolution is the second issue this paper addresses. Some computer simulation results are also provided to illustrate the proposed approach.

A two-step adaptive beamforming approach in subarray is introduced in this paper. A main beam is formed by conventional digital beamforming algorithm. The space positions of M jammings are estimated and then M beams pointing at the directions of M jammings are formed. An adaptive processing of M+1 order is complemented with an adaptive algorithm. This algorithm has excellent performances in jamming cancellation, stability, and computational complexity.

In this paper, a simple configurable systolic array structure for signal processing is proposed. This array is configurable not only for its dimension, but also for different kinds of signal processing algorithms. The basic connection pattern used is a spiral structure. Through a carefully designed switch, a desired array dimension can be easily configured. The configurability for various kinds of signal processing operations is achieved by designing a simple configurable processing element whose configuration is set by a control signal. This systolic array can be applied to the following areas: matrix multiplication, FIR filtering, convolution and correlation, DFT and IDFT. Examples are included to demonstrate how to use the systolic array for each of these operations. With some modification of the PE structure, more applications in signal processing could be expected. This systolic array design gives a good trade-off among simplicity, modularity, flexibility and configurability.

An integrated optics, controls, and structures modeling tool has been developed to analyze the performance of complex electro- optical (EO) sensing systems. Hosted within an object-oriented graphical environment (Khoros) developed by the University of New Mexico, complex systems such as active ground-based telescopes, airborne spectrometers, and space-based sparse array telescopes can be simulated and rapidly evaluated. The TAOS model integrates data products from existing codes such as MATLAB, CODE 5, NASTRAN, and others to allow multi-disiplinary parametric analysis of system performance. Because the model includes accurate physical optics and radiometric representations, almost any function of an optics system can be quickly generated and studied. In addition, degrading effects of dynamic structures, use of compensating control systems, and effects of the observing environment (wind load, boundary layer, and seeing) can also be included. Use of this simulation tool on NASA programs such as the Space Telescope Imaging Spectrometer has reduced design schedules by factors of three. Other typical analysis applications include the study of atmospheric compensated imaging systems using combined adaptive optics/post-processing techniques, simulation of hyper-spectral imagers, and methods for achieving coherent phasing of telescope arrays. This paper also provides a progress report on TAOS modeling of the European Southern Observatory (ESO) Very Large Telescope (VLT).