Multiple Apertures Research Articles

Potential of model-based polarimetric decomposition extended with multi-frequency and multi-incidence PolSAR observations

ABSTRACT This letter attempts to extend the model-based polarimetric decomposition (PD) theorems with multiple polarimetric synthetic aperture radar (PolSAR) observations. To consider both the surface and dihedral scattering depolarization effects, the X-Bragg model, the extended double Fresnel scattering model, and the step-wise volume scattering model are introduced in the model-based PD. The proposed methodology is explored by exploiting Airborne Synthetic Aperture Radar (AIRSAR) C-band and L-band data from southern Oklahoma, United States of America, and L-band Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) polarimetric data from Winnipeg, Canada, with two flight lines of 31605-03 and 31606-03 in different incidence angles. The potential and superiority of the proposed decomposition method are examined by illustrating the false color composition images, and the power ratio statistics of surface, dihedral, and volume scattering components over selected bare soil, forest, grass, and urban patches.

Super-Resolution Processing for Multiple Aperture Antenna to Suppress Multipath

Angle estimation for low-altitude targets above the sea surface is a challenging problem due to multipath interference from surface reflection signals, and various approaches have been proposed. This paper proposes a matrix pencil method with multiple apertures. The matrix pencil method effectively responds to dynamic scenarios because it performs better when using a single snapshot than other methods. Also, employing multiple apertures is more economical than using one large aperture. Therefore, we propose a computationally efficient approach using this method and structures. The proposed two-stage MP method incrementally improves the resolution in two stages: in stage 1, we extract the denoised signals at each aperture level, and in stage 2, we further improve the resolution with those signals. In comparison with the angular resolution defined by the half-power beamwidth (HPBW) of a uniform linear array (ULA) antenna with an equivalent number of arrays, the proposed method demonstrated a superior resolution of less than 0.087 of the HPBW at a high signal-to-noise ratio (SNR) of 40 dB, and less than 0.31 of it even at a relatively low SNR of 15 dB, based on 90% of the resolving probability. For the multipath problem, the proposed scheme has the advantage of not requiring prior geometric information, and its performance is demonstrated through simulations to be better than the adaptive beamforming method and the composite monopulse method.

Atmosphere turbulence effects on a multiple apertures free space optical communication systems

Abstract Wireless communication signals experienced fluctuations in intensity due to atmospheric turbulence. The presented work uses a multi-input-multi-output (MIMO) system to study the performance free-space optical (FSO) communication over a diverse atmospheric turbulence condition channel. The findings produced demonstrate how various channel models perform in FSO system at varying levels of channel turbulence. From 7.9 for the Weak turbulence to 45.3 for the strong turbulence, the turbulence attenuation rises linearly. The data could be transmitted via a link length of up to 1 km with strong turbulence. Strong turbulence could have higher Q-factor, yet is constrained by short distance (1 km) in comparison with weak turbulence, which has greater distance (3.8 km), therefore communication quality is lower. For moderate and weak turbulence, the maximum link is around 1.5 km and 3.8 km, respectively. This study is useful for designing a stable FSO system for regional environments suffer from frequent fluctuated weather turbulence such as the Middle East and South Asia.

Performance evaluation of different types of indirect evaporative coolers: A CFD-based comparative study

Indirect evaporative cooling is an environmentally friendly alternative to the vapor compression refrigeration cycle. Numerical modeling and comparison of different types of indirect evaporative coolers are presented in this study. The developed models are categorized according to the process and working channel arrangement. The categorization comprises Type 0, denoting conventional configuration, alongside Type 1 and Type 3, characterized by singular aperture air passages, and Types 2 and 4, characterized by multiple aperture air passages. The study employed CFD models to investigate the impact of different operational parameters such as air temperature, humidity, velocity and the ratio of working to intake air on various performance parameters. Dew-point effectiveness, cooling capacity per unit area, water consumption per cooling capacity and COP were determined as performance parameters. The study reveals that the performance characteristics of different cooler types vary under diverse conditions. In most examined cases, Type 0 demonstrates superior cooling capacity per unit area, whereas Type 3 exhibits higher dew-point effectiveness. Furthermore, Type 2, with its favorable water consumption per cooling capacity and COP, emerges as the optimal choice in many scenarios, also demonstrating superior dew-point effectiveness at high working air ratios.

A Multi-Aperture Technique for Longitudinal Miniaturization of UWB 3 dB Dual-Layer SIW Coupler.

Microwave couplers are used in large numbers in beamforming networks, and their miniaturization can lead to a significant size reduction in the overall phased array. While the miniaturization of 3 dB couplers in the transverse direction (width) has been given considerable attention in the literature, there is minimal to no information on reducing coupler length. This is because of the trade-off between aperture length, bandwidth and coupling strength. The Bethe-Hole theory requires adding multiple apertures in the longitudinal direction for wide bandwidth, thus increasing the device length. Another factor is the aperture size, which determines the coupling strength and puts additional strain on the compactness of a 3 dB coupler. Contrariwise, this paper proposes to merge two weak (and hence compact) coupling mechanisms to design a wideband 3 dB coupler. This is achieved by using a longitudinal rectangular slot and three cross-slots in the transverse direction. Because of weak coupling, the slot sizes are smaller than a conventional 3 dB coupler, hence yielding a device whose length is less than one guided wavelength (λg) without compromising the bandwidth. The presented coupler is 0.63 λg in length, which is smaller than the state-of-the-art while maintaining a fractional bandwidth of 37% that is comparable to half-mode substrate integrated waveguide (HMSIW) couplers.

All-in-Focus Three-Dimensional Reconstruction Based on Edge Matching for Artificial Compound Eye

An artificial compound eye consists of multiple apertures that allow for a large field of view (FOV) while maintaining a small size. Each aperture captures a sub-image, and multiple sub-images are needed to reconstruct the full FOV. The reconstruction process is depth-related due to the parallax between adjacent apertures. This paper presents an all-in-focus 3D reconstruction method for a specific type of artificial compound eye called the electronic cluster eye (eCley). The proposed method uses edge matching to address the edge blur and large textureless areas existing in the sub-images. First, edges are extracted from each sub-image, and then a matching operator is applied to match the edges based on their shape context and intensity. This produces a sparse matching result that is then propagated to the whole image. Next, a depth consistency check and refinement method is performed to refine the depth of all sub-images. Finally, the sub-images and depth maps are merged to produce the final all-in-focus image and depth map. The experimental results and comparative analysis demonstrate the effectiveness of the proposed method.

Experimental demonstration of scene-based cophasing in optical synthetic aperture imaging using the SPGD algorithm

Large-aperture telescopes based on optical synthetic aperture imaging are investigated for recent high-resolution spaceborne observations. An enabling technique of aperture synthesis is a cophasing method to suppress a piston-tip-tilt error between sub-apertures. This paper proposes a scene-based cophasing technique using the stochastic parallel gradient descent (SPGD) algorithm, assuming application to high-resolution Earth observation. A significant advantage of the SPGD algorithm is a model-less cophasing capability based on extended scenes, but the simultaneous scene-based piston-tip-tilt correction between multiple apertures has not been demonstrated. In this paper, we developed a tabletop synthetic aperture imaging system with 37 sub-apertures and demonstrated extended-scene-based piston-tip-tilt control by optimizing applied voltages to 111 actuators simultaneously. The demonstration experiments used not only static scenes but also a time-varying dynamic scene for observation targets. In every measurement, the proposed scene-based approach reduced the initially defined piston-tip-tilt errors, and the image sharpness significantly improved, although the correction rate in the dynamic scene observation was slower. Finally, this paper discusses the influence of scene dynamics on image-based cophasing.

A Segmentation-Based Optimal Seamline Generation Method for SAR Image Mosaic

In the mosaic creation of multiple high-resolution synthetic aperture radar (SAR) images, achieving an optimal seamline in overlapping areas is crucial for seamless and visually satisfactory results. Many existing seamline generation methods are designed primarily for optical remote sensing images, but due to the differing characteristics of SAR images and optical images, applying these methods directly to SAR images poses challenges in finding the optimal seamline. In response, this paper proposes a segmentation-based optimal seamline generation (SOSG) method for SAR image mosaics. The SOSG method involves a multi-step process. First, SAR image joint segmentation is performed within the overlapping areas. Subsequently, homogeneous areas are identified based on the segmentation results. Following this, a pixel cost matrix is constructed, incorporating homogeneous areas and intensity differences. Finally, the minimum path cost from the starting pixel to the end pixel is computed using the Dijkstra algorithm to determine the optimal path. To assess the feasibility and effectiveness of the proposed method, experiments are conducted using multiple SAR images from the Chinese Gaofen-3 01 satellite as datasets. The experimental results demonstrate that the proposed method yields seamless mosaic images when compared to other methods, while delivering satisfactory outcomes. This indicates the potential of the proposed method in addressing the unique challenges posed by SAR images and enhancing the quality of SAR image mosaics.

Derivation of 3D Coseismic Displacement Field from Integrated Azimuth and LOS Displacements for the 2018 Hualien Earthquake

A 3D surface deformation field for an earthquake can aid in understanding fault behaviors and earthquake mechanisms. However, SAR-based 3D surface deformation estimates are often limited by insufficient observations and hampered by various error sources. In this study, we demonstrate the derivation of a 3D coseismic displacement field from different InSAR processing algorithms. The azimuth displacements from Multiple Aperture Interferometry (MAI) and Pixel Offset Tracking (POT) were integrated to ensure reliable displacements at low coherent areas. The 3D displacement field was inverted pixel-by-pixel by Line-of-Sight (LOS) displacement and integrated azimuth displacement. The results showed that MAI and POT could compensate for the weaknesses of each algorithm. Also, pixels with less than three sets of observations showed higher noise levels. Such noisy pixels were removed by a denoising criterion proposed herein. For the vertical direction, the proportion of pixels inverted with two sets of azimuth and one set of LOS displacements was 26.1%. After denoising, the proportion dropped to 2.4% due to the insufficiency of LOS displacements. This shows that the viewing angle influences the overall performance of 3D surface displacement inversion. Implementing various displacement vectors should reduce such limitations.

Laser-Based Command Injection Attacks on Voice-Controlled Microphone Arrays

Voice-controlled (VC) systems, such as mobile phones and smart speakers, enable users to operate smart devices through voice commands. Previous works (e.g., LightCommands) show that attackers can trigger VC systems to respond to various audio commands by injecting light signals. However, LightCommands only discusses attacks on devices with a single microphone, while new devices typically use microphone arrays with sensor fusion technology for better capturing sound from different distances. By replicating LightCommands’s experiments on the new devices, we find that simply extending the light scope (just as they do) to overlap multiple microphone apertures is inadequate to wake up the device with sensor fusion. Adapting LightCommands’s approach to microphone arrays is challenging due to their requirement for multiple sound amplifiers, and each amplifier requires an independent power driver with unique settings. The number of additional devices increases with the microphone aperture count, significantly increasing the complexity of implementing and deploying the attack equipment. With a growing number of devices adopting sensor fusion to distinguish the sound location, it is essential to propose new approaches to adapting the light injection attacks to these new devices. To address these problems, we propose a lightweight microphone array laser injection solution called LCMA (Laser Commands for Microphone Array), which can use a single laser controller to manipulate multiple laser points and simultaneously target all the apertures of a microphone array and input light waves at different frequencies. Our key design is to propose a new PWM (Pulse Width Modulation) based control signal algorithm that can be implemented on a single MCU and directly control multiple lasers via different PWM output channels. Moreover, LCMA can be remotely configured via BLE (Bluetooth Low Energy). These features allow our solution to be deployed on a drone to covertly attack the targets hidden inside the building. Using LCMA, we successfully attack 29 devices. The experiment results show that LCMA is robust on the newest devices such as the iPhone 15, and the control panel of the Tesla Model Y.

Polarimetric Measures in Biomass Change Prediction Using ALOS-2 PALSAR-2 Data

The use of multiple synthetic aperture radar polarizations can improve biomass estimations compared to using a single polarization. In this study, we compared predictions of aboveground biomass change from ALOS-2 PALSAR-2 backscatter using linear regression based on (1) the cross-polarization channels, (2) the co- and cross-polarizations from fully polarimetric SAR, (3) Freeman–Durden polarimetric decomposition, and (4) the polarimetric radar vegetation index (RVI). Additionally, the impact of forest structure on the sensitivity of the polarimetric backscatter to AGB and AGB change was assessed. The biomass consisted of mainly coniferous trees at the hemi-boreal test site Remningstorp, located in southern Sweden. We found some improvements in the predictions when quad-polarized data (RMSE = 79.4 tons/ha) were used instead of solely cross-polarized data (RMSE = 84.9 tons/ha). However, when using Freeman–Durden decomposition, the prediction accuracy improved further (RMSE = 69.7 tons/ha), and the highest accuracy was obtained with the radar vegetation index (RMSE = 50.4 tons/ha). The corresponding R2 values ranged from 0.45 to 0.82. The bias was less than 1 t/ha for all models. An analysis of forest variables showed that the sensitivity to AGB was reduced for high values of basal-area-weighted mean height, basal area, and stem density when predicting absolute AGB, but the best change prediction model was sensitive to changes larger than the apparent saturation point for AGB state estimates. We conclude that by using fully polarimetric SAR images, forest biomass changes can be estimated more accurately compared to using single- or dual-polarization images. The results were improved the most (in terms of RMSE and R2) by using the Freeman–Durden decomposition model or the RVI, which captured especially the large changes better.

Lensless imaging in one shot using the complex degree of coherence obtained by multiaperture interferences.

The van Cittert-Zernike theorem states that the Fourier transform of the intensity distribution function of a distant, incoherent source is equal to the complex degree of coherence. In this Letter, we present a method for measuring the complex degree of coherence in one shot by recording the interference patterns produced by multiple aperture pairs. The intensity of the sample is obtained by Fourier transforming the complex degree of coherence. The experimental verification by using a simple object is presented together with a discussion on how the method could be improved for imaging more complex samples.

Information Extraction and Three-Dimensional Contour Reconstruction of Vehicle Target Based on Multiple Different Pitch-Angle Observation Circular Synthetic Aperture Radar Data

The circular synthetic aperture radar (CSAR) has the ability of all-round continuous observation and high-resolution imaging detection, and can obtain all-round scattering information and higher-resolution images of the observation scene, so as to realize the target information extraction and three-dimensional (3D) contour reconstruction of the observation targets. However, the existing methods are not accurate enough to extract the information of vehicle targets. Through the analysis of the vehicle target scattering model and CSAR image characteristics, this paper proposes a vehicle target information extraction and 3D contour reconstruction method based on multiple different pitch-angle observation CSAR data. The proposed method creatively utilizes the projection relationship of the vehicle in 2D CSAR imaging to reconstruct the 3D contour of the vehicle, without prior information. Firstly, the CSAR data obtained from multiple different pitch-angle observations are fully utilized, and the scattering points of odd-bounce reflection and even-bounce reflection echoes are extracted from the two-dimensional (2D) coherent CSAR images of the vehicle target. Secondly, the basic contour of the vehicle body is extracted from the scattering points of the even-bounce reflected echoes. Then, the geometric projection relationship of the “top–bottom shifting” effect of odd-bounce reflection is used to calculate the height and position information of the scattering points of odd-bounce reflection, so as to extract the multi-layer 3D contour of the vehicle target. Finally, the basic contour and the multi-layer 3D contour of the vehicle are fused to realize high-precision 3D contour reconstruction of the vehicle target. The correctness and effectiveness of the proposed method are verified by using the CVDomes simulation dataset of the American Air Force Research Laboratory (AFRL), and the experimental results show that the proposed method can achieve high-precision information extraction and realize distinct 3D contour reconstruction of the vehicle target.

Detection and analysis of potential landslides based on SBAS-InSAR technology in alpine canyon region.

The Lancang River flows through the alpine canyon region of southwest China, an area that has experienced frequent geological disasters over the years. Early monitoring of geological hazards is essential for disaster prevention and mitigation. However, traditional ground monitoring techniques are limited by the complex terrain conditions in high-altitude valley regions. In contrast, interferometric synthetic aperture radar (InSAR) technology can provide a high-precision, wide-range monitoring of slow rock-slope deformation, making it an effective tool for studying geological hazards. Within the study area, multiple synthetic aperture radar (SAR) images from the Sentinel-1A satellite were collected, and surface deformation was obtained using the small baseline subset InSAR (SBAS-InSAR). The results demonstrate that combining ascending and descending orbit images can be successfully applied to landslide monitoring in complex mountainous areas. Over 30 potential landslides were identified by combining InSAR results with optical images. The Line-Of-Sight (LOS) direction deformation features and their relationship with precipitation were analyzed based on two typical landslides, and two-dimensional/three-dimensional (2D/3D) deformation decomposition was carried out to reveal its motion characteristics. It was found that the cumulative deformation fluctuation amplitude was higher during the rainy season, and the main movement direction of the landslide was east-west. In addition, based on the spatial distribution and statistical analysis of deformation points along with meteorological data, geological elements, human activities, and topographic conditions, it is inferred that factors such as low vegetation coverage, tectonic movements, human activities, and high-altitude glacier thawing may contribute to the occurrence of disasters. And it was found that areas with high vegetation cover, high rainfall, and snow cover exhibit lower coherence coefficients. This study offers valuable insights for investigating large-scale geological in alpine canyon regions.

Design of a bulk-lead zirconate titanate ultrasonic transducer array addressing specific excitation modes

Conventional ultrasonic transducer arrays generally use simultaneous or delayed excitation methods to generate various types of wave fronts in ultrasonic testing. Such array excitation modes are relatively simple and unable to maximize the functions of the array elements or element apertures. Accordingly, this study developed a novel excitation ultrasonic transducer array with flexible excitation aperture. A minimum row-column-addressing planar array structure for ultrasonic transducers based on bulk lead zirconate titanate (PZT) was designed for industrial ultrasonic testing. In addition, a transducer structure design scheme and truth table for array addressing with excitation logic were provided. By performing an acoustic field simulation, this study analyzed the relationship between the longitudinal amplitude and resonant frequency of the array elements and the impedance distribution. The transducer was fabricated and packaged using a circuit board and bulk PZT array elements. Testing revealed that the resonant frequency of the array was 1.07 MHz, the effective electromechanical coupling coefficient was 0.40, and the fractional bandwidth of the pulse echo response and spectrum was 38.46%. Using an aluminum block, performance in single-element, row-, column-, and full-array excitation was tested, and echo curves were obtained for multiple apertures. Using a single-circuit module, the transducer realized flexible gating and excitation of the array elements. The research provided a new method for industrial ultrasonic transducer array design.

FMMAS-OCT: Aberration correction for high-resolution cellular imaging

In optical coherence tomography (OCT), spatial resolution and signal strength often suffer due to aberrations from both the imaging system and the sample. In this work, we introduce a novel method, FMMAS (Forward-Model Multiple Aperture Synthesis), which effectively compensates for these aberrations. It combines forward-model (FM) based method for transverse resolution restoration and multiple aperture synthesis (MAS) to mitigate depth domain aberrations. Our proof-of-concept experiment demonstrates a significant improvement in the mean transverse full width at half maximum (FWHM) of microparticles by 46.1 %. Compared to original OCT and FM method, FMMAS produces sharper, 'pencil beam'-like imaging, resolving cell nuclei in swine esophagus ex vivo and human blood cells in vivo, suggesting a potential for significant enhancements in OCT image quality.

Ultra-low SLL slotted waveguide antenna array using Chebyshev aperture distribution function for MM-wave tracking radar applications

A non-uniform aperture distribution slotted waveguide antenna is presented. The slotted waveguide antenna array is chosen for its ease for aperture distribution control and the flexibility of gain adjustment. Multiple aperture distribution functions, including uniform, binominal, Chebyshev, and Blackman, have been extensively investigated to achieve the best possible side lobe suppression. These distribution functions have been converted to slot displacement from the center line of the antenna. Number of slots in the proposed array is chosen to be eight to satisfy the required gain for mm-wave radar tracking application. The proposed antenna is simulated and optimized using the full wave simulator package (CSTMWS). Rounded edge slots are adapted instead of the sharp ones, as they are more applicable for the CNC fabrication process. An eight-slot waveguide antenna is fabricated and gold-plated, considering the Chebyshev distribution approach. The proposed design achieves very stable radiation characteristics and performance over the operating bandwidth, which extends from 33.6 GHz up to 35.9 GHz. At 35 GHz, the total realized gain of the overall antenna is found to be 13.4 dBi, with an overall radiation efficiency of 95.21 %. An obvious low SLL of −30 dB is obtained at the frequency of 35 GHz. The low side lobe level parameter is very crucial for radar applications as it enhances the overall target SNR and reduces false target observations. The pre-stated antenna characteristics make it a good candidate for mm-wave radar applications and point-to-point communications (P2P).

Alarming a tailings dam failure with a joint analysis of InSAR-derived surface deformation and SAR-derived moisture content

A tailings dam failure can be catastrophic. One constantly dumps or removes ore-related materials behind or within a dam before its closure, creating substantial surface change or deformation. The deformation can be assessed with multiple temporal synthetic aperture radar (SAR) observations and interferometric SAR (InSAR) techniques. The surface deformation is one prerequisite causing the failure. The water content increase is another primary factor that can reduce the surface friction among piled materials and induce further surface deformation. Intuitively, large deformation and high water content behind the tailings dam are concerns of a possible dam failure. Thus, we propose a new joint analysis of the InSAR-derived surface deformation and SAR-derived moisture content to achieve the objectives, delineating the stable/unstable status of a tailings dam and capturing a possible pre-failure signal of the dam. We study three types of tailing dams to verify the proposed analysis. The first is the Majiatian tailings dam in China, closed safely at the end of 2020. Our analysis determines that the dam status is stable. The second is the collapsed Córrego de Feijão Tailings Dam I in Brazil. The analysis identified a turning point from stable to unstable near December 2018, and the dam failure occurred in January 2019. In case three, an unstable status first at the Maanshan tailings dam was delineated. The findings were conveyed to parties interested in dam safety, and they carried out mitigation activities afterward. The analysis continued until February 2023, indicating a stable status of the tailings dam. The analysis is also robust to slight variations of moving window size around the proposed value. Thus, a valid joint analysis has been studied and is transferable to locations since the analysis and multitemporal SAR observations are available.

A cross‐cross‐correlation based method for joint coregistration of rotated multitemporal synthetic aperture radar images

AbstractCoregistration is among the most important and challenging tasks when dealing with multiple synthetic aperture radar (SAR) images, especially when they are acquired at different time instants and characterised by low signal to noise power ratio (SNR) that contributes to their coherence reduction. However, even if some technological expedients could be implemented to maintain the same trajectory and to compensate for these inaccuracies during the acquisition campaign, multitemporal SAR images always need additional registration refinements after compression. Usually, to coregister a series of multitemporal SAR images, one of them is selected as the master, and the remainders are separately registered to it. Differently, in this study, a new strategy is developed to jointly coregister a stack of multitemporal SAR images. It is based on the exploitation of the cross‐correlations in turn computed from each couple of cross‐correlations (a.k.a. cross‐cross‐correlations) of the extracted patches. By doing so, the method is capable of exploiting also the respective misregistration information between the slaves during the estimation process. In this respect, this methodology is applied to enhance the registration capabilities of the constrained Least Squares (CLS) optimisation method, which instead does not account for the reciprocal information related to the slaves. Several tests are performed on multitemporal airborne‐measured SAR data. Obtained results show the effectiveness of the proposed algorithm in terms of achieved root mean square error for images affected by respective rotations also in comparison with the CLS counterpart.

Multi-Aperture Coherent Digital Combining Based on Complex-Valued MIMO 2N × 2 Adaptive Equalizer for FSO Communication

Collecting light using multiple small apertures offers a lot of advantages over doing so using a single large aperture, including the effective mitigation of scintillation under strong turbulence, low implementation cost, and ease of scalability. However, the tricky of a multi-aperture system is to combine the multiple branches signals with static skew mismatch and with time-varying characteristics of gain, phase and state of polarization, especially for coherent digital combining of polarization multiplexing (PM) signals. In this paper, a complex-valued multiple-input multiple-output (MIMO) 2N×2 adaptive equalizer is designed to combine multi-aperture signals, and the tap coefficient of each branch is calculated based on the constant modulus algorithm (CMA). This technique relaxes hardware constraints on the time alignment of multi-aperture branch since the skew can be compensated electronically thanks to the proposed blind digital combining algorithm. In addition, laser linewidth within evaluation range smaller than 10 MHz has no effect on the performance of 2N×2 adaptive equalizer. Moreover, the feasibility of the proposed adaptive equalizer for moderate and strong turbulence suppression is verified by a 10-Gbps data rate PM-QPSK modulation offline two- and four-aperture simulated turbulence experiment.