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Due to its high resolution and deep penetration depth, microwave ultrawideband radar is a promising tool for the non-destructive testing (NDT) of transportation infrastructure. Microwave radiation can also be used to reconstruct the dielectric properties of objects and therefore can be used to detect an air cavity or metallic rust in concrete. We used contrast source inversion (CSI) as one of the most promising inverse scattering schemes. To resolve the observation domain limitations of NDT, radar imaging method, also known as range points migration (RPM) method, was first incorporated into the inverse scattering algorithm based on CSI as a region of interest (ROI) estimator, which substantially improves the accuracy of complex permittivity reconstruction. In addition, the ROI optimizing scheme based on the CSI cost function is used to enhance ROI accuracy. The effectiveness of the proposed methods is validated via finite-difference time-domain (FDTD)-based numerical simulation, which assumes typical NDT model. [ABSTRACT FROM AUTHOR]

A cylindrical antenna with a conductively loaded slot (CACLS) is designed for broadband imaging radar applications. Through design considerations, such as conductive loading, cylinder termination, and the insertion of an electromagnetic (EM) wave absorber into the cylinder, the proposed CACLS is shown to be able to radiate temporally short and nondispersive pulses. Based on the final design of the antenna, parametric studies pertaining to the time-domain radiation performances depending on the dimension of the cylinder are performed. The CACLS with the determined design parameters is then fabricated and its performances verified through antenna measurements and a radar experiment. In addition to its excellent pulse radiation and reception performance, the realized antenna has a compact and physically robust structure, which makes it potentially suitable for many broadband imaging radar applications. [ABSTRACT FROM AUTHOR]

We present a novel approach to millimeter-wave imaging that combines a new dynamic antenna array concept with active illumination by means of noise signals and interferometric image formation. The concept is based on a sparse linear array of receiving antennas that is dynamically rotated about its centroid, combined with three noise transmitters that illuminate the scene. Cross-correlations of the scattered signals received by individual array elements provide spatial frequency samples of the scene, from which the scene can be reconstructed via inverse Fourier transform. As the spatial frequency sampled by an antenna pair is dependent on the antenna baseline vector, dynamic rotation of the linear array generates sampling curves in the spatial frequency domain, providing a denser sampling function and a more accurate scene reconstruction. While traditional static interferometric imaging arrays utilize nearly an order of magnitude fewer antenna elements than phased arrays, our dynamic array concept relies only on a sparse linear array, further reducing the number of hardware elements required. We discuss the design of a 38 GHz dynamic antenna array imaging system and present measurements of multiple reflecting objects. [ABSTRACT FROM AUTHOR]

Fast detection of hazardous objects in millimeter-wave personnel screening can increase the operating efficiency of secure environments. In airports, accurate automatic detection and classification of body-worn objects can reduce the number of pat-downs while keeping the transportation environment safe and secure. Since many benign objects, such as paper and leather, are lossy materials, being able to characterize them and set them aside from the secondary checks will significantly reduce the screening process time. In this article, we introduce an effective image-processing-based method for characterizing lossy materials that might be concealed under clothing at checkpoints. The method has been combined with a modified version of the previously developed algorithm that can be used specifically to characterize lossless materials. The proposed algorithm can automatically distinguish lossless materials from lossy ones and calculate their thickness and permittivities. Starting from the radar reconstructed image showing a cross section of the body, we extract the nominal body contour using the Fourier series, separate body, and object responses, categorize the object as lossy or lossless based on the depression and protrusion of the body contour, and, finally, predict possible values for the object’s permittivity and thickness. Our resulting classification is good, implying fewer nuisance alarms at checkpoints. [ABSTRACT FROM AUTHOR]

The spectrum of millimeter-wave frequencies is becoming increasingly crowded due to the advances in fifth-generation (5G) communications and automotive radar, leading to greater interest in spectrum coexistence for functions like sensing and communications. While most research has focused on minimizing interference between sensing and communications, we take a different approach that leverages dense communications signal environments for passive sensing applications. We present a millimeter-wave imaging system that captures information in the spatial frequency (Fourier) domain, similar to concepts used in radio astronomy. In contrast to traditional millimeter-wave sensing and imaging approaches, Fourier-domain image formation is improved as more incoherent signals are included in the environment. In particular, multiple millimeter-wave communications signals generate spatiotemporal incoherence, which enables Fourier-domain sampling and image formation. We present a method for characterizing the imaging efficacy of communications signals in an environment based on new spatiotemporal coherence metrics. We discuss simple solutions to problems of increasing spatial coherence in cases where transmit resources are limited. Finally, we demonstrate a 38 GHz millimeter-wave imaging system capturing 5G communications signals from a set of independent transmitters, demonstrating the ability to image multiple objects in a scene. [ABSTRACT FROM AUTHOR]

A novel detection scheme with fractional orbital angular momentum (OAM) beams is proposed in this communication, so as to predict the rotational velocity of the discrete body of revolution (DBoR). At first, the echo model of DBoR is derived under the incidence of integer OAM beams. The theoretical derivation shows that some integer OAM modes may not work for the detection of DBoR due to echo cancellation. Then the echo model with fractional OAM modes is also derived. It shows that the echo frequency spectrum with the fractional OAM always includes effective integer OAM modes. Consequently, a novel detection scheme with fractional OAM for DBoR is proposed. It should be noted that all these theoretical derivations are based on ideal scattering points. Finally, 3-D full-wave simulations of a practical DBoR target are conducted with the method of momentum (MoM) and its fast algorithm. Numerical results demonstrate the effectiveness of the proposed detection scheme. It can be used as an efficient tool to analyze the rotation characteristics for DBoR targets. [ABSTRACT FROM AUTHOR]

Target classification is of great value when the number of transceivers in a sensing system is relatively small. This article proposes a high-accuracy and efficient classification method with machine learning techniques on frequency-modulated-continuous-wave (FMCW) radar. We first establish the novel mapping relationship from physical space to range-Doppler (PS-RD) image based on wave propagation theory, by which four physical features that effectively capture the kinematic and geometrical characteristics of targets, including speed, total reflectivity (ToRe), area, and incidence angle, are extracted from range-Doppler (R-D) image. Then, a multilayer perceptron (MLP) with a single hidden layer is employed to realize the classification. Since the above-mentioned four physical features, derived from wave physics, are chosen as the input of the neural network, our classifier does not work in a black-box way. The computational complexity of the whole classifier is the same as that of a 2-D fast Fourier transform (FFT), which guarantees a real-time operation. As an example, the proposed classifier is applied to the automotive radar system, where road targets are to be classified into five categories, including pedestrian, bike, sedan, truck/bus, and other static objects. Real-world data obtained from 77 GHz FMCW radars are provided for validation, where the proposed physics-assisted classifier turns out to outperform the state of the art in automotive radar application. The overall accuracy of the real data is about 99% even with complex multiple-target cases. [ABSTRACT FROM AUTHOR]

The computationally efficient solution of fully nonlinear microwave inverse scattering problems (ISPs) is addressed. An innovative system-by-design (SbD)-based method is proposed to enable, for the first time to the best of the authors’ knowledge, an effective, robust, and time-efficient exploitation of an evolutionary algorithm (EA) to perform the global minimization of the data-mismatch cost function. According to the SbD paradigm as suitably applied to ISPs, the proposed approach is found on: 1) a smart reformulation of the ISP based on the a priori information on the imaged targets for defining a minimum dimensionality and representative set of degrees of freedom (DoFs) and 2) the artificial intelligence (AI)-driven integration of a customized global search technique with a digital twin (DT) predictor based on the Gaussian process (GP) theory. Representative numerical and experimental results are provided to assess the effectiveness and the efficiency of the proposed approach also in comparison with competitive state-of-the-art inversion techniques. [ABSTRACT FROM AUTHOR]

In this study, we propose a generalized algorithm for far-field radar cross section determination by using 3-D synthetic aperture imaging with arbitrary antenna-scanning surfaces. This method belongs to a class of techniques called image-based near-field-to-far-field transformation. The previous image-based approaches have been formulated based on a specific antenna-scanning trajectory or surface, such as a line, plane, circle, cylinder, and sphere; the majority of these approaches consider 2-D radar images to determine the azimuth radar cross section. We generalize the conventional image-based technique to accommodate an arbitrary antenna-scanning surface and consider a 3-D radar image for radar cross section prediction in both the azimuth and zenith directions. We validate the proposed algorithm by performing numerical simulations and anechoic chamber measurements. [ABSTRACT FROM AUTHOR]

The rapid development of artificial electromagnetic (EM) materials provides unprecedented freedom to manipulate EM waves. In recent years, reflective spectrum conversion technology based on EM materials has been widely studied due to its multiharmonic generation capability. The possibility of its application in wireless communication is explored. However, the research on spectrum conversion is mainly based on periodic modulation waveforms. The modulation effect of nonperiodic for EM materials has not been studied so far. In this article, a nonperiodic time-varying active frequency selective surface (AFSS) is proposed to manipulate the spectral distribution of harmonics. By dynamically controlling the amplitude of the reflected echo, a continuous Doppler spectrum is generated. On this basis, the imaging characteristics of nonperiodic time-varying AFSS are analyzed in detail, and the defocusing phenomenon of the target image is discovered. Finally, the radar imaging experiments for nonperiodic time-varying AFSS are conducted to verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

Chipless RFID tag decoding has some inherent degrees of uncertainty because there is no handshake protocol between chipless tags and readers. This article initially compares the outcome of different pattern recognition methods to decode some frequency-based tags in the mm-wave spectrum. It will be shown that these pattern recognition methods suffer from almost 2%–5% false decoding rate. To overcome this misdecoding problem, two novel methods of making images of the chipless tags are presented. The first method is making 2-D images based on side-looking aperture radar concepts, and the second one is making virtual 2-D images from the 1-D backscattering signals. Then, a 2-D decoding algorithm is suggested based on a convolutional neural network to decode those tag images and compare the results. It is shown that this combined decoding method has very high accuracy, and it almost eliminates any ambiguity and false decoding problems. This is the first time a deep learning method is used with image construction methods to decode chipless RFID tags. [ABSTRACT FROM AUTHOR]

We are concerned with the scattering of 3-D Gaussian beam propagators from a perfectly electric conductor wedge in uniform translation. The incident Gaussian beam propagators are the building blocks of the phase-space beam summation method, which is a general framework for analyzing the propagation and scattering of waves. In this article, we use the asymptotic plane wave (PW) scattering from a moving wedge. By applying asymptotic analysis to the PW decomposition of the incident fields, we obtain the closed-form expressions for the wave potentials and the corresponding time-dependent electromagnetic vector operators. The asymptotic scattered potentials consist of relativistic reflected, shadowing, and diffraction waves. These waves are investigated and parameterized to include the velocity-dependent features and wave phenomena corresponding to the wedge speed. Numerical examples are included. [ABSTRACT FROM AUTHOR]

Nearfield (NF) 3-D imaging provides an effective solution of objects’ radar cross section (RCS) within a compact range. This article proposes a precise RCS extrapolation via NF 3-D imaging with adaptive parameter optimization Bayesian learning (APOBL), i.e., first, in the process of NF 3-D imaging, objects’ scattering centers may vary with the observation angle, while it is hard for the existing Bayesian learning via presetting parameters to reach an optimal estimation. For this issue, we present a parameter self-adaption solution, improving precision, and stability. Second, we also apply a block-based optimization idea in Bayesian-learning-based 3-D imaging, ensuring NF 3-D imaging quality. Third, in the process of RCS extrapolation, we apply a weighted Green’s function operator into the 3-D imaging-based NF-to-far-field (NF–FF) compensation, further ensuring high precision. The simulation and experiment results verify that the proposed method has an advantage in precision over the existing 3-D imaging-based RCS extrapolation methods. [ABSTRACT FROM AUTHOR]

In order to improve the speed of passenger screening while preserving the effective capability to detect more sophisticated threats, airport security imaging systems must be able to accurately characterize concealed body-worn objects. In addition to improving the passenger experience, this system capability will enhance airport security for the traveling public. This article presents a real-time, fully automatic algorithm for the wideband millimeter-wave (mm-wave) radar reconstruction of the nominal human body contours, even in the presence of an affixed weak dielectric object or when a portion of the body cross section is not captured by the imaging scanner. The algorithm extracts the main contours from a noisy collection of 3-D reconstructed reflectivity and approximates the nominal human body cross section via fitting a low-order angular Fourier series. This important step is essential for precise characterization of concealed body-worn explosives. A ranking algorithm is developed as a metric for the nominal body reconstruction accuracy. We verify the developed algorithm by applying it to the actual images of the high definition–advanced imaging technology (HD-AIT) system, a laboratory prototype mm-wave scanning system developed recently by the U.S. Department of Homeland Security (DHS). The reconstructed body contours may be used to estimate the electric permittivity of the concealed person-worn objects. [ABSTRACT FROM AUTHOR]

Radar imaging of typical planet landforms can be used for planetary exploration such as landform recognition and geometry measurement. This communication realizes high-resolution imaging simulations of typical planetary landforms, such as craters, hills, and volcanoes, by using parametric scattering center (SC) models. Based on the geometric models of these typical landforms, parametric SC models are constructed from scattered solutions of the physical optics (POs) method. Because the SC models take geometric parameters as model parameters, these models can simulate the scattered waves of landforms of different sizes. These SC models are validated by the PO results. In addition, the landform types can be recognized and their geometrical parameters are further extracted by SC model matching. The extracted geometric parameters show good agreement with real landforms, demonstrating that the SC models of typical planetary landforms have potential applications in planetary geomorphologic exploration. [ABSTRACT FROM AUTHOR]

We present a new approach to the classification of scenes in the microwave and millimeter-wave bands that leverages a novel dynamic antenna array concept to capture distinct features in the spatial frequency information of the scene. The spatial frequency information of a scene is obtained through its Fourier transform, and by sampling a subset of this information, key features can be extracted and used for image classification. We demonstrate that a dynamic antenna array can synthesize spatial frequency filters, and that scene classification can be accomplished using the filtered signals without full image reconstruction. We develop a new dynamic antenna array concept using only two antennas to generate a ring-shaped spatial frequency filter and explore the use of this concept for the classification of ground scenes. Natural ground scenes tend to have smoother spatial frequency signals, while, in contrast, features such as buildings and roadways result in sharp broadband spatial frequency responses. Using this design, we demonstrate the ability to classify between two classes of ground scenes: those with man-made structures (buildings, roads, etc.) and those without (natural scenes). We demonstrate the ability of the spatial filters synthesized by the proposed dynamic antenna array to achieve a classification accuracy of 0.971 with an empirical true positive rate of 0.982. The method is broadly applicable to microwave and millimeter-wave sensing at any range. [ABSTRACT FROM AUTHOR]

This article deals with the classical question of estimating the achievable resolution in terms of the configuration parameters in inverse source problems. In particular, the study focuses on the case of a planar surface magnetic current which is to be reconstructed from near-field observed over a bounded rectangular aperture parallel to the source domain. Resolution formulas are well known for far-field or Fresnel-zone configurations, and also for near-field cases, when data are full-view or a measurement aperture is an unbounded plane. For the case of bounded near-field observations, the resolution estimations mainly rely on some asymptotic arguments in order to mimic far-field reasoning. Here, the plan is to improve these results and to work out a resolution estimation that precisely captures the spatially varying behavior entailed by the near-field and aspect-limited configuration. To this end, the pertinent radiation operator is inverted by an adjoint inversion scheme (a back-propagation-like method) and the corresponding point-spread function is analytically estimated. Numerical examples show that the derived resolution estimation clearly points out the role of the geometrical parameters of the configuration and it is more accurate than other literature results. [ABSTRACT FROM AUTHOR]

Inspired by the circular-aperture synthetic aperture radar (Circular-SAR) imaging regime that can provide higher azimuth image resolution, the imaging regime employing the multiple-input–multiple-output (MIMO) arc array is proposed. Moreover, no efficient imaging algorithm is now available for these MIMO arc array setups. This article proposes a modified Omega-K algorithm for near-field azimuth image reconstruction based on the geometry transformation from the single-frequency MIMO arc array to the equivalent MIMO linear array. Note that this geometry transformation relies on the calculation of the phase difference and the definition of the corresponding compensation function. Compared with the single-input–single-output (SISO) arc scan requiring more elements, simulations with good agreement were performed to validate the azimuth image reconstructions of single/multiple targets. Although the computing efficiency of the proposed algorithm for MIMO arc array is lower than that of the traditional Omega-K algorithm for the SISO arc scan, it still has an obvious advantage relative to the traditional backprojection (BP) algorithm that is always taken as the gold standard. Furthermore, three realistic imaging geometries, including the metal strips at different radii, the tilted metal plate, and the human body torso cross section, were simulated to test the fidelity of the proposed reconstruction algorithm. Proof-of-principle experiments were also carried out based on an MIMO arc array with the help of one arc scanner. Finally, both simulation and experimental results validate the effectiveness of the proposed algorithm on azimuth image reconstruction quality. [ABSTRACT FROM AUTHOR]

A backprojection-based synthetic aperture radar (SAR) processing approach for fast and accurate imaging of moving objects and stationary background is proposed. This divide-and-conquer backprojection approach allows for efficient detection of moving objects, estimation of their positions and velocities, and overlaying their focused images over the stationary background. Two algorithms are presented to exploit the tradeoff between the complexity and performance. The full 4-D phase space imaging algorithm provides the optimal performance under low signal-to-clutter conditions, while the adaptive detection and imaging leads to reduced complexity in more favorable scenarios. We show that in a single-channel SAR the conventional straight-line data collection path does not resolve the target’s velocity, therefore a curved path is assumed in the formulation and numerical examples. [ABSTRACT FROM AUTHOR]

Switchable active frequency selective surface (AFSS) absorber/reflector can work in opposite scattering state and be widely applied in the stealth field. At present, abundant researches on material design have been conducted to obtain excellent electromagnetic (EM) properties. However, there are few studies on the modulation effect of switchable AFSS absorber/reflector on the EM wave. In this article, a time-domain digital-coding AFSS absorber/reflector is proposed to manipulate the spectral distribution of harmonic. The accurate control of each harmonic is achieved through AFSS modulating parameters. Then, a simple controller based on FPGA is designed. By coding “0” and “1” elements with controlled sequences, the AFSS can be switched flexibly in both absorbing and reflection states. On this basis, the imaging characteristics of time-domain digital-coding AFSS absorber/reflector are further analyzed, and distance transformation phenomena are described in detail. The synthetic aperture radar (SAR) experiments are performed to demonstrate the required function of distance transformation. [ABSTRACT FROM AUTHOR]

A planar periodic leaky-wave antenna (PLWA) with continuous backward-to-forward beam-scanning capability and consistent gain is presented. The PLWA is made of a new type of periodically arranging meander line structures, in which the capacitive coupling between neighboring units plays the key role in achieving a high scanning rate. In detail, the beam-scanning rate of the proposed PLWA is efficiently tuned by varying the gap between two adjacent units, without considerably changing the working frequency band. To mitigate the open stopband (OSB) and to improve the impedance matching, several inductive open stubs are introduced into a single meander line unit cell to enhance the radiation. In the experiment, a prototype of the proposed PLWA was fabricated and measured. The simulated and measured beams show good agreement in terms of scanning range and radiation performance. A continuous beam scanning from −60° to +58° through broadside in the frequency band of 5.95–7.1 GHz is observed, and hence, a 102.6°/GHz scanning rate is realized in practice. Besides, the PLWA shows an average gain level of 11.96 dBi with a variation lower than 2 dB and a sidelobe below −10 dB at all the measured frequencies. The proposed PLWA may have potential applications in radar, microwave imaging, and wireless communication due to its compact structure, easy to fabricate, and dispersionless performance. [ABSTRACT FROM AUTHOR]

An attributed scattering center (ASC) model provides concise and physically relevant features for complex targets and plays an important role in inverse scattering problems. Extracting ASC features from a radar image is designed to solve a highly nonlinear and nonconvex optimization problem, which requires an accurate yet complex parameter initialization process and generates intensive computation and storage cost. This article presents an ASC feature extraction method with a genetic algorithm (GA) (AFEM-GA). The proposed AFEM-GA utilizes an improved real-coded GA to search for the global optimum for each ASC feature in the radar frequency-aspect angle domain in parallel, without complex parameter initialization and with low computation and storage costs. Moreover, an improved relaxation algorithm is also proposed for multiple ASC feature extractions to avoid error accumulation. Experiments based on both numerical computed data and real data verify the accuracy and robustness of the proposed AFEM-GA and show its potential in radar image interpretation. [ABSTRACT FROM AUTHOR]

We present the design, fabrication, and experimental characterization of a 2-D, dynamically tuned, metasurface aperture, emphasizing its potential performance in computational imaging applications. The dynamic metasurface aperture (DMA) consists of an irregular, planar cavity that feeds a multitude of tunable metamaterial elements, all fabricated in a compact, multilayer printed circuit board process. The design considerations for the metamaterial element as a tunable radiator, the associated biasing circuitry, as well as cavity parameters are examined and discussed. A sensing matrix can be constructed from the measured transmit patterns, the singular value spectrum of which provides insight into the information capacity of the apertures. We investigate the singular value spectra of the sensing matrix over a variety of operating parameters, such as the number of metamaterial elements, number of masks (aka tuning states), and number of radiating elements. After optimizing over these key parameters, we demonstrate computational microwave imaging of simple test objects. [ABSTRACT FROM AUTHOR]

A fast back-projection (FBP) imaging algorithm is proposed to generate 3-D uniformly balanced images of an object embedded in a layered structure based on a combined near-field monostatic and bistatic synthetic aperture radar (SAR) system. The spectral layered Green’s function is used in the FBP algorithm. Like conventional real-time imaging algorithms, the complexity of the imaging function is reduced via the stationary phase method, and the fast Fourier transform is implemented to accelerate the computation. Unlike conventional real-time imaging algorithms, the singularity problem associated with Green’s function and antenna pattern is solved in this proposed algorithm by using a point spread function (PSF). For a real-time imaging system, besides fast image generation, an efficient data acquisition is needed. A combined monostatic and bistatic SAR configuration is used here to decrease the data collection time. The performance of this proposed FBP method is demonstrated by measurements of a stratified environment. [ABSTRACT FROM AUTHOR]

Radar-based microwave imaging (MWI) systems have shown encouraging results in early detection of breast cancer; however, there exist remaining challenges. Resolution is one of the most important issues not being addressed with clear theoretical relations in medical MWI systems. In this analytical study, we thoroughly clarify the relation of resolution to the most fundamental aspects of an MWI system, namely, limited-view versus full-view array geometry, monostatic versus multistatic configuration, single-frequency versus wideband operation, and near-field versus far-field imaging. This goal is achieved by expressing the inverse scattering problem in the spatial frequency domain and utilizing K-space representation. Besides, to analyze the MWI system more accurately, the sidelobe level (SLL) of its point-spread function (PSF) is taken into account. We show that generally, the resolution limits of a multistatic configuration are the same as its monostatic counterpart and, hence, have no superiority in terms of resolution. In addition, in imaging with a full-view array, the signal bandwidth has no effect on the resolution. Thus, the single-frequency operation provides the same resolution as the wideband operation. We also demonstrate that the effect of using multistatic configuration or wideband operation is to reduce the SLL of the PSF and, hence, improve the image quality. [ABSTRACT FROM AUTHOR]

When electromagnetic (EM) wave illuminates a target, complex structures of the target produce different scattering mechanisms, which compose the whole scattered field of the target. Decomposing and extracting different scattering mechanisms will improve the knowledge of EM scattering for complex targets, and will be of great significance for feature recognition and scattering characteristics control. In this article, we present a method to decompose and extract different scattering mechanisms such as specular scattering, multiple scattering, and creeping waves from the scattered field. The theoretical basis of the method is that the wave corresponding to different scattering mechanisms reaches the receiver at different time. Thus, we acquire the whole scattered field through simulation or experimental test at first. Then, we perform the signal processing with time-frequency analysis to obtain the arrival time of different scattering mechanisms. According to the time slot difference, the time-domain signal at each frequency can be decomposed and different scattering mechanisms can be extracted. Several examples validate the proposed method. [ABSTRACT FROM AUTHOR]

In this article, new assumptions are introduced into the well-known synthetic storm technique (SST) proposed by Drufuca and further developed by Matricciani, to address the orbital diversity in satellite links. The new approach receives as input information on collocated rain rate recordings and instantaneous estimates of the storm direction and speed; and assumes that the rain event is transversely constant to the storm movement. The achieved improvements are evaluated by analyzing two example measured rain attenuation events “seen” by two Ka-band beacon receivers looking at two different satellites spaced around in azimuth. A study is made of the cross correlation coefficient between rain attenuation series for these two links. Moreover, a methodology is sketched as to how multiple non-GEO satellite constellation availability (for global Internet access at Ka-band) could be evaluated in the frame of the SST. [ABSTRACT FROM AUTHOR]

This article investigates whether a vortex wave detection method with a rotational antenna can improve the receiving signal-to-noise ratio (SNR) of stealth targets. To be specific, a vortex wave detection system with a rotational antenna is established. Considering both bistatic and monostatic models, it demonstrates that the vortex wave can be efficiently synthesized by applying the fast Fourier transformation algorithm to all receiving pulses. The SNR of synthetic orbital angular momentum (OAM) is compared with that of circular synthetic aperture radar (SAR) because the synthetic OAM and circular SAR systems have similar architectures and are described by similar mathematical models. Experiments confirm that the synthetic OAM method can improve the receiving SNR of a specific complex target with different polarizations. The SNR improvement by OAM for general complex targets is then demonstrated in simulations. Furthermore, the detection probability and the quality of 2-D detection results can also be improved by the proposed synthetic OAM method. [ABSTRACT FROM AUTHOR]

This communication presents a freehand imaging system, which takes full advantage of a compact multiple-input multiple-output (MIMO) radar to provide the real-time results by means of freehand movements. The system is based on a commercial radar-on-chip, equipped with four receivers and two transmitters, and an optical tracking system (i.e., motion capture system) for retrieving its position and attitude. The reduced size of the radar enables its handheld operation, making possible to create a synthetic aperture by moving the scanner over the area of interest. Due to the new degrees of freedom with respect to previous implementations based on quasi-monostatic topologies, a novel calibration strategy for the system is required in order to mitigate positioning errors. After calibration, the simulations and measurements are accomplished revealing that the novel freehand multistatic setup significantly outperforms the monostatic setup in terms of scanning speed and image quality. Multimedia material is provided to illustrate this performance. [ABSTRACT FROM AUTHOR]

In this article, an ultrawide band circularly polarized crossed dipole antenna is proposed for near-field synthetic aperture radar imaging. The presented antenna comprises two orthogonal dipoles with elliptical arms vacated by rotated elliptical slot. The slots and a modified feeding configuration enhance both impedance and axial ratio bandwidth. The antenna has 4.75:1 impedance bandwidth (1.6–7.6 GHz) and 4.1:1 axial ratio bandwidth (1.8–7.1 GHz). To reduce the coupling in quasi-monostatic imaging scenarios, a planar circularly symmetric structure is used that decreases coupling about 10 dB. Due to importance of antenna time-domain behavior in imaging systems, transient response is evaluated in near-fields and also far-fields, which shows a good fidelity factor. The electric field distribution in a region of near-field area is almost uniform in different frequencies that improve imaging. The far-field features like radiation pattern, gain, and axial ratio are also satisfactory. Finally, an oil well monitoring system using the proposed antenna is simulated and based on the results, existed perforations in the well wall are detected. Compared to previous antennas, this antenna has several advantages such as higher impedance and axial ratio bandwidth, planar structure, low front to side ratio which mitigates mutual coupling, and satisfactory transient response. [ABSTRACT FROM AUTHOR]

The coherence factor (CF) is defined as the ratio of coherent power to incoherent power received by the radar aperture. The incoherent power is computed by the multi-antenna receiver based only on the spatial variable. In this respect, it is a 1-D CF, and thereby the image sidelobes in down-range cannot be effectively suppressed. We propose a 2-D CF by supplementing the 1-D CF by an incoherent sum dealing with the frequency dimension. In essence, we employ both spatial diversity and frequency diversity which, respectively, enhance imaging quality in cross-range and range. Simulations and experimental results are provided to demonstrate the performance advantages of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper presents a novel anisotropic metasurface (AMS)-based integrated antenna system to facilitate dual-polarized fan-beam patterns with independent beam-scanning ability. In the proposed configuration, the antenna subsystem comprises a specially designed differentially fed microstrip patch antenna (source antenna) along with a polarization rotator (PR). The differential antenna arrangement is realized using a square shaped microstrip patch capacitively coupled with two metallic strips, which are connected to a specially designed wideband hybrid coupler using two metallic vias for the microstrip feeding. This source antenna is loaded with a PR by placing it at a height of $0.35\lambda _{0}$ from the radiating aperture. The PR comprised two identical metallic layer patterns, separated by two substrates. Depending upon the relative orientation of these layers, the polarization of the linearly polarized impinging spherical electromagnetic (EM) waves (originating from the source antenna) either remains preserved or gets rotated by 90° while transmitting through this PR. After realizing the antenna subsystem, a novel AMS lens is designed and placed above the PR. The proposed AMS lens is realized by integrating two cylindrical MS (CMS) lenses, $\text {CMS}_{Y}$ and $\text {CMS}_{X}$. This integration provides a unique phase profile to the AMS lens, thus introducing appropriate phase correction to the incident orthogonally polarized spherical EM waves along their respective polarization direction. The proposed configuration of AMS lens, thus, results into formation of the independent dual-polarized fan-beam radiation beams. Finally, the steering of the generated fan-beams along the direction of their respective polarization is facilitated using the proposed system by translating the AMS lens parallel to the source antenna. [ABSTRACT FROM AUTHOR]

Knowledge of the spatial–temporal variation of rain fields is required for the planning and optimization of wide-area high-frequency terrestrial and satellite communication networks. This article presents data and a method for characterizing multi-resolution statistical/dynamic parameters describing the spatial–temporal variation of rain fields across ocean climate in North-Western Europe. The data are derived from the NIMROD network of rain radars. The characterizing parameters include 1) statistical distribution of point 1 min rainfall rates, 2) spatial and temporal correlation functions of rainfall rate, and 3) the probability of rain/no rain. The main contributions of this article are the assessment of the impact of varying spatial and temporal integration lengths on these parameters, their dependencies on the integration volumes and area sizes, and the model for both temporal and spatial correlation parameters. [ABSTRACT FROM AUTHOR]

The amplitude and phase distortions of radar echo signal will cause the emergence of undesirable ghost scattering points, which degrade the quality of terahertz (THz) radar images. In this paper, a physics-based procedure is presented to predict the atmospheric attenuation and dispersion characteristics at THz frequencies, which is mainly based on the line-by-line calculation method with a specific modification in phase-shift prediction. The line-by-line parameters provided by the high-resolution transmission spectroscopic database and atmospheric condition parameters obtained from the Air Force Geophysics Laboratory reference atmospheric constituent profiles are adopted to predict the atmospheric transmittance and the phase shift for specific transfer paths. The results are compared with measured data to demonstrate the accuracy, while the proposed procedure is used to analyze the impacts of atmospheric transfer characteristics on radar imaging via the high-resolution range profile simulation. The signal distortion is interpreted in terms of paired echoes to illustrate the importance of frequency band selection for high-resolution imagery at THz frequencies. [ABSTRACT FROM AUTHOR]

Radar cross-section (RCS) images are valuable for understanding the 3-D target scattering center distribution of objects. However, 3-D RCS imaging requires more complex measurement systems and much more data points compared to conventional 2-D methods. A reduction of measurement effort can be achieved by using a tomographic technique based on amplitude-phase-spectrum estimation, scale-invariant feature transform image registration, and constant false alarm rate detection. Besides an amplitude spectrum, the proposed method also obtains the phase spectrum of the scattering center distribution, which allows the application of polarimetric decompositions. The generation of full-polarimetric 3-D images is demonstrated based on measurement data obtained in a compact antenna test range. Since all relevant scattering mechanisms can be identified, the approach proved to be useful for an advanced understanding of the scattering mechanisms occuring at radar targets. [ABSTRACT FROM AUTHOR]

We present a novel imaging method for compact low-profile imagers in millimeter wave (mmW) and terahertz (THz) frequencies. The method borrows the principles of computerized tomography to generate 2-D or 3-D high-resolution images using simplified RF front ends. To perform imaging, the proposed scheme implements the projection-slice theorem. Specifically, it records the Radon transform (RT) of the desired field-of-view (FOV) and reconstructs the image using the Fourier slice theorem. To achieve the illumination of the FOV, the proposed imaging configuration uses a rotating linear antenna array that produces a highly directive fan beam. The antenna elements are further reduced using multistatic sparse array techniques without compromising the image reconstruction process. By dramatically reducing the number of active elements, the new method could allow high-resolution mmW/THz imaging systems to be embedded in low profile and lightweight platforms, including unmanned aerial vehicles or CubeSats. In this paper, we present the basic concept of the novel imaging scheme and simulation results verifying the proof of concept. Finally, we present a modified imaging antenna array that alleviates interference from objects and sources outside the FOV. [ABSTRACT FROM AUTHOR]

Through a cinder block wall imaging using a standard back projection method is studied by employing backscattering in the frequency range of 1–4 GHz. First, using the finite-element method, numerical backscatterings of metallic and dielectric targets behind a cinder block wall are calculated and the effects of the wall on images are investigated. It is shown that the target image is shifted, repeated, and distorted. Second, to refocus the images, the proper Green function is calculated. Basically, the 2-D scattering from a cinder block wall is addressed, analytically and asymptotically. The imaging formulation is modified, using the structure Green function phase. To calculate the Geen function, wall parameters should be evaluated beforehand. Third, to estimate the wall parameters, we proposed the estimation of those using measured wall reflections. Using analytical formulations, direct wall reflections are estimated theoretically and compared with those of measured ones. Using genetic algorithm (GA), wall parameters are extracted, correctly. Having the optimum parameters, the direct wall reflection is effectively subtracted, the Green function is calculated, and the target image is refocused successfully. Finally, using the experimental data, the effectiveness of our proposed approach is demonstrated. [ABSTRACT FROM AUTHOR]

Electromagnetic waves carrying orbital angular momentum (OAM) have drawn much attention in the fields of radar detection and wireless communication. In such applications, vortex beams with multiple OAM modes need to be transmitted and received at the same time. This communication introduces the shared-aperture concept into metasurfaces for generating OAM vortex beams with different modes. Two independent geometry phase profiles on the same aperture are utilized to modulate the phase of two orthogonal linear-polarization waves. The alliance of polarization and orbit provides a routine to generate two different OAM modes simultaneously and achieves higher aperture efficiency than the conventional shared-aperture schemes. In this manner, different OAM modes can be separated easily by hardware. The simulated results validate the proposed method and structure. [ABSTRACT FROM AUTHOR]

A passive compressive device (PCD), which works at millimeter waves, is proposed in this paper. In a passive manner, the device allows to compress the signals from multiple separate channels into a single or a reduced number of channels. This permits a simplification of the transmitter or receiver chains for radar imaging applications. The PCD is a 3-D cavity that ensures each channel to be decorrelated with each other. If the transfer functions are known, the compressed signals are decompressed in postprocessing. This paper proposes a suitable method to estimate simultaneously the transfer functions of the PCD. This is validated by measurements in a multiple-input-multiple-output (MIMO) configuration to produce a complex synthetic aperture radar image. Measurements revealed that the point spread function generated by the MIMO configuration with the use of the PCD leads to almost similar results, in terms of spatial resolution and sidelobe level ratio, as those of a single-input-multiple-output configuration (uniform amplitude), with a fewer receiving elements. [ABSTRACT FROM AUTHOR]

In this paper, we formulate a recently proposed qualitative imaging approach named near-field orthogonality sampling method (NOSM) for a general measurement configuration. In the very first version, the NOSM was basically a backpropagation method, which analytically transforms the scattered fields, which is obtained with some specific multiview–multistatic measurement configurations (circular or linear), to reduced scattered electric fields on the sampling domain, which encapsulates the targets. The contributions of this paper are: 1) physical meaning of the reduced scattered field of the NOSM is explained by showing how it approximates the contrast source values in the sampling domain for different generic and limited aperture configurations; 2) the application of the NOSM is generalized to a generic configuration and some comments are made how to reduce the computational complexity and source requirements; 3) the reciprocity of the data is accounted for to improve the performance for limited aperture measurement scenarios; and 4) finally, a useful modified indicator is obtained by relying on the physical meaning of the indicator. The accuracy and stability of the proposed formulations are corroborated with numerical examples for which the excitation-measurement configuration is of limited aperture. [ABSTRACT FROM AUTHOR]

The technology of “through-the-wall radar imaging” enables seeing through walls by reconstructing the image based on backscattered waves passing through an optically opaque dielectric wall. However, when the electromagnetic waves cannot penetrate the shielding walls (such as metallic walls), a problem called “hard-wall radar imaging” (HWRI) must be investigated. As the objects are located in the shadow region, where only the diffracted field can be reflected back via edges, conventional imaging techniques are not applicable. The HWRI problem is an imaging scenario with an intrinsic multipath of signal propagation. Therefore, it has been studied using a model-based time-reversal (TR) decomposition of TR operator imaging method. In this paper, an imaging technique with a distributed transmitting and receiving array is proposed as a robust way resolving the problem with a low signal-to-clutter ratio in TR imaging. In this method, the interelement response matrix is collected for the construction of the covariance matrix, from which the geometry of the hard wall is determined from the direction of arrival by the multiple signal classification techniques. Next, the adaptive interference cancellation technique is applied to suppress the interference terms in the image function due to the multipath effects. The experimental results were obtained from a distributed multiple-input multiple-output radar system and show that this method is useful not only for the detection of a weak object echo but also for the localization of multiple targets hidden behind the wall. [ABSTRACT FROM AUTHOR]

Target probing based on wideband pulse radar is a challenging task for radio frequency simulation (RFS) in anechoic chamber. Due to the long time duration of pulse radar signal and the limited space of anechoic chamber, the reflected signal returns before the pulse radar signal is fully transmitted, which makes it difficult for the signal reception. As a consequence, the target high-resolution range profile (HRRP) cannot be obtained for target imaging and recognition. In this paper, we propose a novel strategy that contains two steps to solve this problem and obtain the target HRRP. First, the randomly interrupted transmitting and receiving (R-ITR) method is proposed to receive the pulse radar signal in RFS. Second, the target HRRP is reconstructed by compressive sensing (CS) since the R-ITR echo is piecewise sparse. The protocol to choose R-ITR parameters and performance of HRRP reconstruction are provided. Simulations and experiments with different R-ITR parameters are also conducted to verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

In this paper, two kinds of cascaded metasurfaces are proposed for the simultaneous control of the reflected and transmitted wavefronts in the terahertz (THz) frequency band. First, an array of arrow-shaped elements backed by a strip grating is investigated. When the polarization of the incidence field is parallel to the orientation of the strip grating, the reflected and transmitted waves with approximately equal amplitude will be generated and there is a constant phase difference between them when the element dimensions are varied. Proved by the simulation and measured results, the metasurface based on the proposed element can simultaneously deflect the transmitted and reflected wavefronts but only by the same angle. Second, independent control of both amplitudes and phases of the transmitting and reflecting waves is achieved by cascading four metallic layers in order, i.e., a y-oriented strip layer, a y-directed grating layer, the arrow-shaped element, and an x-directed grating, with respect to different polarizations of the incident field. The simulation and measured results show that the y-polarized incident waves can be reflected by the four-layered metasurface into a designed reflection angle, while the x-polarized incident waves can be focused on the other side of the metasurface in a transmitting manner. Compared with the reported THz metasurfaces, the proposed elements can provide more advanced wavefront-control ability and they also feature the advantages of compact size, low cost, and lightweight. Therefore, the proposed metasurfaces can be applied as beam splitters for low-cost and compact THz imaging and detection systems. [ABSTRACT FROM AUTHOR]

Real noise environment may not be Gaussian due to the existence of man-made interferences, natural bursts, and so on. In this paper, a new multiple radar subbands fusion algorithm in complex noise environment is proposed. The considered complex noise includes impulsive noise and the mixture of impulsive and Gaussian noises. The proposed subband fusion algorithm consists of the following steps. First, the incoherent factors are estimated and the multiple subbands are compensated to be mutually coherent. Then, coherent subbands are fit by the geometrical theory of diffraction (GTD) model, where model parameters are estimated in the frame of sparse reconstruction. Different probability distribution functions are used to describe different noise environments, and the maximum a posteriori probability (MAP) estimate of scattering model parameters is achieved. It is shown that the MAP estimate can be obtained by iteratively solving a support vector regression problem. Finally, the fused full band is obtained by extrapolating the measured subband data based on the estimated GTD model. Validity and performance of the proposed algorithm are investigated by analytical data, simulated data, and static-range data. [ABSTRACT FROM AUTHOR]

We investigate the use of a dynamic metasurface as the transmitting antenna for a synthetic aperture radar (SAR) imaging system. The dynamic metasurface consists of a 1-D microstrip waveguide with complementary electric resonator (cELC) elements patterned into the upper conductor. Integrated into each of the cELCs are two diodes that can be used to shift each cELC resonance out of band with an applied voltage. The aperture is designed to operate at K-band frequencies (17.5-20.3 GHz). We experimentally demonstrate imaging with a fabricated metasurface aperture using well-known SAR modalities, showing image quality comparable to traditional antennas. The agility of this aperture allows it to operate in spotlight and stripmap SAR modes, as well as in a third modality inspired by computational imaging strategies. We describe the dynamic metasurface antenna's operation in detail, demonstrate highquality imaging in both 2-D and 3-D, and examine various tradeoffs governing the integration of dynamic metasurfaces in the future SAR imaging platforms. [ABSTRACT FROM PUBLISHER]

Traditional sparse representation-(SR) based inverse synthetic aperture radar (ISAR) imaging schemes can achieve significant performance, but they suffer from high costs of memory and computational complexity, because the SR of a 2-D image is converted into that of a 1-D vector. Instead of memory consuming vector operations, we propose a fast ISAR imaging algorithm, where the decomposition and reconstruction of a 2-D scene is implemented using matrix operations directly. Besides the spatial sparsity of a scene, its structural sparsity is presented using the range profile of the scene, where both can be used to enhance sparsity exploitation during image reconstruction. Also, benefitting from the structural sparsity of the range profile, the target energy can be accumulated during the process, which further improves performance. Compared to available SR-based ISAR imaging algorithms, the proposed algorithm reduces both memory costs and computational complexity significantly, which is proven using simulated and real data. [ABSTRACT FROM PUBLISHER]

A comprehensive set of simulations is performed to analyze the accuracy of an analytical solution for characterizing the backscattering responses of circular cylindrical tree trunks located above a dielectric ground. The formulation of interest here is from the volumetric current integration method, combined with a ray-based approach for the treatment of ground effects. Through comparisons with a reference solution provided by a full-wave solver, the region of validity of the closed-form approximate solution as a function of trunk length, trunk radius, incidence angle, and polarization is derived. Trunks with and without tapering along their lengths are considered. It is noted that, in general, the error behavior of the vv solution exhibits more complexity than that of the hh solution, due to the Brewster angle effects of the trunk and the ground; consequently, the hh solution is valid over a wider range of length and radius values. The backscattering signatures of the trunks are also examined in the imaging domain. It is found that the analytical solution can correctly predict the single ground-bounce returns but may not consistently capture other less prominent effects, such as scattering from the top of the trunk and higher order interactions. [ABSTRACT FROM PUBLISHER]

Radar-based microwave imaging requires the effective dielectric properties of the imaged object as a priori information. A technique to determine the effective complex permittivity seen by each imaging antenna across the used frequency band of a multistatic imaging domain is presented. The method uses spatial statistical techniques to model the complex permittivity of the imaging domain as a function of scattering parameters. The proposed method does not require any predefined gap between the antennas and the imaged object, nor does it need the imaged object to be centered within the imaging domain. Also, the method does not need to know boundaries of the imaged object. The proposed method is tested via simulations and experiments using a multistatic human torso imaging system. The collected data across the band 0.65–1.75 GHz using twelve antennas around the human torso are processed to generate accurate images. The results demonstrate significant improvements in image quality and detection accuracy compared to conventional average-permittivity methods. The efficacy of the proposed method is verified experimentally in detecting an early case of lung cancer in a human torso phantom. [ABSTRACT FROM AUTHOR]

In this paper, the superluminal behavior of a radiated pulse in the near-field of an ideal dipole and an ultrawideband antenna is explained and demonstrated through simulations and measurements. The deformation of the radiated pulse in the near-field is illustrated using the ideal dipole antenna radiation as well as by a Vivaldi antenna through full-wave simulations and measurements. The apparent superluminal pulse velocity is due to the pulse reshaping of the radiated pulse in the near-field of the antenna. The pulse velocity is measured by tracking pulse peak, pulse envelope peak, and also calculating the pulse centrovelocity. The superluminal pulse peak velocity of the radiated pulse from a miniaturized Vivaldi antenna is confirmed through measurements in face-to-face and quasi-monostatic radar configurations. The effect of different values of pulse velocities on the quality of reconstructed images using synthetic aperture radar processing is demonstrated as well. [ABSTRACT FROM PUBLISHER]

A novel approach to full-polarimetric short-range imaging with rotating arrays is proposed. Instead of taking advantage of all four possible combinations of the polarizations of transmit and receive antennas as in the typical full-polarimetric imaging systems, the suggested antenna array acquires three different copolarized measurements at each spatial sampling point with its rotation. A simple algebraic operation is derived to accurately retrieve the full-polarimetric signals in the H/V polarization basis from the copolarized measurements. After the retrieval of the full-polarimetric signals, the traditional imaging algorithms can be applied to reconstruct the polarimetric images. The effectiveness and accuracy of the suggested approach to full-polarimetric imaging are validated on the signal and image levels through both numerical simulations and experiments. The results show that the proposed approach can accurately retrieve the full-polarimetric signals and provides an alternative method to signal acquisition for full-polarimetric imaging. [ABSTRACT FROM PUBLISHER]