The Pacific Northwest National Laboratory (PNNL) has recently developed a next-generation cylindrical millimeter-wave imaging system. This system is based on linear sparse multistatic imaging arrays. Data from this system can be focused using 3D FFT-based reconstruction algorithms, which are reasonably efficient and can be performed in near real time, or by back-projection methods that are versatile and more accurate but are computationally intensive and require lengthy post-processing. Cylindrical Fast Backprojection (CFBP) is a novel image reconstruction algorithm developed at PNNL that radically increases the efficiency of backprojection and is ideally suited to microwave and millimeter-wave imaging systems based on scanned linear arrays such as body scanners in common use for aviation security screening. This method achieves its gains in efficiency by separating a full backprojection into a sequence of three steps, range focusing, vertical focusing, and lateral focusing, with intermediate results used to avoid repetitive multidimensional computation. The method is called cylindrical fast backprojection due to the use of two-dimensional stored results, or look-up tables, that have cylindrical symmetry about the linear array. The method is well suited to cylindrically scanned linear arrays but is equally valid for linear arrays scanned to form planar or arbitrary apertures. This paper describes the CFBP algorithm and validates its performance using simulated data.
An active 3D microwave / millimeter-wave shoe scanner was previously developed at the Pacific Northwest National Laboratory (PNNL) using two linear arrays scanned over a rectilinear aperture. The radar system chirps a frequency sweep from 10-40 GHz. These frequencies allow imaging through optically opaque material such as leather, rubber, plastics, and other dielectrics. The system was designed to detect concealed items in the soles of shoes while allowing people to leave their shoes on through a security checkpoint. To shrink the footprint of the system, a new iteration of the design has been developed that scans the two linear arrays over a circular aperture. This new footprint opens the possibility of it being installed in the floor of a cylindrical millimeter-wave body scanner. The backprojection-based multilayer dielectric image reconstruction developed at PNNL can easily handle arbitrary spatial sampling, accommodating the new rotational shoe scanner design. Commonly, the fast Fourier transform (FFT) is used to efficiently compute the range response from the data collected by the system as a preprocessing step to the backprojection algorithm. It was found that converting to range using the discrete Fourier transform (DFT) directly has some advantages over the FFT. For example, nonlinear and non-uniform frequency sweeps can easily be compensated for during the computation of the DFT and only the range bins of interest need to be computed and their spacing can be chosen arbitrarily. Because the range conversion step of the image reconstruction is the fastest part of the process there is very little speed penalty for using the DFT over the FFT and it can even increase the speed of image reconstruction when the ranges of interest are fewer than the total span that is calculated in the FFT.
Active microwave and millimeter-wave imaging is commonly used for security screening at airport checkpoints and other protected facilities. This paper explores an imaging system concept that may improve screening convenience, reduce cost, and enable alternative operational concepts by allowing a person to walk naturally through the system. Millimeter-wave imaging systems require data to be acquired over a 2D spatial aperture to form a high-resolution image. This requirement is usually met using mechanical scanners or large antenna arrays that provide a 2D aperture and provide strict control over the position of the array in relation to a motionless target. The new concept explored in this paper replaces the mechanical scan with motion of the passenger. The complex motion of the passenger is expected to be optically tracked as he or she passes by stationary linear vertical millimeter-wave arrays and can be modeled using skeletal animation. Multiple linear arrays illuminate the passenger from a wide variety of angles to provide full coverage of the body. The radar data are then correlated with the skeletal animation model by employing generalized synthetic aperture focusing or back-projection techniques. These methods accurately reconstruct the image by integrating the measured response multiplied by the conjugate of the expected response from a point scatterer anywhere within a 3D image volume. This process yields an optimally focused image and can be applied to situations involving complex target motion. This paper describes this concept in detail and provides numerous simulation-based imaging results to explore the effectiveness of the proposed methods.
Active millimeter-wave imaging is in widespread use for security screening and other applications. The Pacific Northwest National Laboratory (PNNL) has developed a variety of microwave and millimeter-wave imaging systems and technology, including the cylindrical imaging technology that forms the basis of the L3/Leidos ProVision system. Since 2016, PNNL has been actively participating in a working group that is developing a proposed American National Standards Institute (ANSI) standard (N42.59) that will be used to evaluate and verify performance of active millimeterwave imaging systems used for security screening of humans. The standard is developing image quality tools (IQTs) that will be used to assess a variety of imaging metrics, such as lateral resolution, contrast, and depth resolution. Depth resolution is vital for high-performance microwave and millimeter-wave imaging because it enables precise focusing over a full 3D volume, and allows for differentiation of reflections from multiple surfaces, such as a layer of clothing over the human body. In this paper, depth resolution is analyzed using theoretical simulations and experimental 3D imaging studies. Presented results examine depth resolution using IQTs developed from a thin partially transparent film placed in front of a metallic surface, creating reflections that are laterally aligned and at variable separation. Coherence between these reflections is investigated as it complicates the interpretation of the imaging results.
Active millimeter and microwave imaging techniques can be used to create a high-resolution 3D image volume of a target’s reflectivity. Millimeter-wave imaging is commonly used for personnel security screening and numerous other applications. Backprojection based image reconstruction techniques form a 3D complex-valued volume. The complex-valued volume is commonly reduced to a real-valued volume by taking the magnitude. For anomaly detection and cross image registration of an object it is beneficial to generate an accurate representation of an object’s surface. Extracting a smooth and accurate surface from a magnitude only image is challenging. One difficulty is the magnitude image has limited resolution in the depth direction which normally limits precision to a moderate fraction of the depth resolution. Another difficulty is that the magnitude of the image depends heavily on the geometry and orientation of the object being imaged. The phase information in a complex-valued image volume provides a means to decouple the magnitude of the image from the geometry of an object and provide precision much finer than the depth resolution would indicate. This enables the generation of a smooth and accurate point cloud representation of the surface of an imaged object. A method to extract a point cloud from the phase information in a 3D complex-valued millimeter-wave image volume is developed and results with simulated and experimental data are presented
Active three-dimensional (3D) microwave and millimeter-wave imaging techniques have been extensively developed for concealed threat detection at the Pacific Northwest National Laboratory (PNNL), most notably the cylindrical millimeterwave imaging method currently in use for airport screening. Typically, a linear array is mechanically scanned over a cylindrical or planar aperture in order to form a high-resolution 3D image. A linear array mounted on a low-cost encoderdriven rail system was desired for rapid data collection and evaluation of concealed threat detection on a stationary target. A rail system to sweep out a planar aperture was quickly developed, however, due to the low-cost implementation of the rail system and encoder, resulting images were lower quality than expected. It was determined that the position information provided by the rail system encoder was not accurate enough to generate an image of the desired quality. Instead of using a traditional encoder wheel with the rail system, optical motion tracking was used to record 3D position information of the linear array synced with the radar as it was manually scanned over a nominally planar aperture. While optical motion tracking can provide position information with sub-millimeter level accuracy, it doesn’t guarantee that the scanned aperture is strictly planar or uniformly sampled. Reconstruction techniques necessary to incorporate 3D position information and compensate for an irregular imaging aperture are developed. Experimental results showing the benefit of precise optical motion tracking for a manually scanned linear array are presented.
The Pacific Northwest National Laboratory (PNNL) has recently developed an active 3D microwave/millimeter-wave shoe scanner. This system is designed to detect threats concealed within the soles of common footwear. The system was designed in response to the security incident involving Richard Reid, known as the “Shoe Bomber”. The system operates over the 10-40 GHz frequency range. Waves in this band readily pass through common shoe materials, such as leather, rubber, plastics, foams, and synthetic and natural cloth materials. The shoe scanner system consists of a linear array positioned underneath a low loss dielectric window that the person is directed to stand upon. The linear array is positioned so the antenna propagation is vertical, and the array axis is horizontal across the width of the shoes. A linear mechanical scan translates the arrays along the length of the shoes. A frequency-modulated continuous wave (FM-CW) transceiver is used to collect the signal scattered from the scene. The data collected from the system is fully 3D covering two spatial and one frequency dimensions. The system presents several challenges for efficient image reconstruction, including the dielectric window, multi-row linear arrays, and focusing close to the antenna elements. The dielectric window presents a significant challenge for image reconstruction since the waves will travel through an inhomogeneous layered media. In this paper, an efficient back-projection reconstruction algorithm is presented that overcomes these challenges. Experimental imaging results are shown that demonstrate high-resolution imaging performance for this new scanner.
Active three-dimensional (3D) microwave and millimeter-wave imaging is useful for a variety of applications including concealed weapon detection, in-wall imaging, non-destructive evaluation, and others. High-resolution imaging is usually performed using a fixed two-dimensional planar or cylindrical aperture that is defined using a two-dimensional array or precise mechanical scanning of a transceiver or sequentially-switched linear antenna array. For some applications, it is more convenient to manually translate a linear array over the scene of interest, or equivalently, move the target in front of the linear array to scan an effective aperture. Manually scanning the array or target creates several challenges for accurately focusing, or reconstructing, an image of the target. The motion of the array or target must be known accurately, typically with precision of 0.05-0.1 wavelengths. Additionally, the image reconstruction algorithm needs to be able to compensate for aperture shapes which are highly non-uniformly sampled, and which are not of a specific canonical shape such as planar or cylindrical. This paper explores high-resolution 3D microwave imaging of a moving target by using optical motion capture to track the moving target and develops highly versatile image reconstruction techniques that account for the irregular motion. Several experimental results are shown for moving targets in front of a fixed linear array.
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