Electromagnetic Backscattering Research Articles

Cloud Removal with SAR-Optical Data Fusion and Graph-Based Feature Aggregation Network

In observations of Earth, the existence of clouds affects the quality and usability of optical remote sensing images in practical applications. Many cloud removal methods have been proposed to solve this issue. Among these methods, synthetic aperture radar (SAR)-based methods have more potential than others because SAR imaging is hardly affected by clouds, and can reflect ground information differences and changes. While SAR images used as auxiliary information for cloud removal may be blurred and noisy, the similar non-local information of spectral and electromagnetic features cannot be effectively utilized by traditional cloud removal methods. To overcome these weaknesses, we propose a novel cloud removal method using SAR-optical data fusion and a graph-based feature aggregation network (G-FAN). First, cloudy optical images and contemporary SAR images are concatenated and transformed into hyper-feature maps by pre-convolution. Second, the hyper-feature maps are inputted into the G-FAN to reconstruct the missing data of the cloud-covered area by aggregating the electromagnetic backscattering information of the SAR image, and the spectral information of neighborhood and non-neighborhood pixels in the optical image. Finally, post-convolution and a long skip connection are adopted to reconstruct the final predicted cloud-free images. Both the qualitative and quantitative experimental results from the simulated data and real data experiments show that our proposed method outperforms traditional deep learning methods for cloud removal.

Simulation and Calculation of Sea State Bias Based on the Electromagnetic Backscattering from the Simulated Nonlinear Rough Sea Surface

Abstract Due to the improvement of satellite orbit determination technology, sea state bias(SSB) has become the main source of altimeter height measurement error. In this paper, the physical mechanism and causes of SSB are simulated and analyzed by electromagnetic scattering, and the SSB correction method and model are given. Firstly, this paper simulates and studies four kinds of nonlinear rough sea surface and their electromagnetic scattering characteristics for the first time, calculates SSB from the physical mechanism of SSB, and analyzes the physical mechanism and process causes of SSB from the electromagnetic scattering characteristics. Secondly, a fitting model to correct SSB is proposed in terms of the electromagnetic simulation. Finally, the paper compares the “true value of SSB” of HY-2B satellite radar altimeter in China’s offshore waters with SSB calculated by the simulating model. The comparison results show that they are consistent with a weak bias, indicating that it is an effective method to correct SSB by simulating nonlinear rough sea surface and its electromagnetic scattering. It is expected to improve the accuracy of SSB correction by the new way of simulating the physical mechanism and causes of SSB.

Asymptotic Modeling of Three-Dimensional Radar Backscattering from Oil Slicks on Sea Surfaces

This paper presents new results of a simulation of radar backscatter from oil slick areas on a real three-dimensional sea surface, based on a physical hydrodynamic model of surface wave damping in the presence of oil films, the local equilibrium model (MLB). To solve this problem, the modelling was carried out by using the first-order small-slope approximation (SSA1) model. It presents the advantage of having a very good compromise between rapidity and accuracy of the calculation. The choice of the model is justified by solving the two-dimensional problem with several asymptotic methods and further comparing the results with a rigorous numerical method, based on the Method of Moments (MoM). Two approaches called “thin-layer” (TL) and “classical” were used to deal with the double layer (air/oil/sea) problem. The TL approach assumes that this double-layer problem can be seen locally as a Fabry–Pérot interferometer, which implies that the Kirchhoff-tangent plane approximation (KA) is valid. The classical approach consists in neglecting the presence of the oil layer for dealing with electromagnetic backscattering, which is valid for very thin oil films compared to the electromagnetic (EM) wavelength. It is shown that these two approaches have rather complementary validity domains: The TL approach is always valid for small observation angles, which makes it suitable for near nadir sensors such as altimeters, whereas the classical approach is valid for moderate observation angles, which makes it suitable for most satellite applications. The 3D modelling results are compared with C-band and X-band measured data (CSK experiment and OOW NOFO experiment) in VV polarization. The calculation takes into account that the oil film on the sea surface is mainly in an emulsion state. The results highlighted the relevance of the MLB hydrodynamic model, as well as the SSA1 EM model combined wit the classical approach, for quantifying NRCS in seas contaminated with marine oil or surfactants. The agreement is indeed very good in the X-band range.

Electromagnetic Backscattering Mechanisms of Frequency Selective Surfaces for Effective Analysis of Radar Cross Section by the First-Order Approximation of Floquet Modes

Full-wave analysis of finite but electrically large frequency selective surfaces (FSSs) substrates for the radar cross section (RCS) estimation is too time-consuming and unbearable for practical applications. Such FSS substrates can be either absorbing or reflective types and are widely used as radomes of antennas for stealth applications, where the electromagnetic (EM) backscattering is of much concern. This large EM problem is simplified by applying Floquet mode decomposition on the scattering fields, where the Kirchhoff approximation is employed to find the backscattering fields for RCS estimation. The resulted formula allows one to explore the scattering mechanism for RCS enhancement or reduction. Besides, the approximation is used to resemble the FSS radome by equivalent substrates. Thus, the transmission and reflection coefficients of the equivalent dielectric substrates are similar to those from FSS elements under an infinite array assumption. Afterward, the resulting EM problems of antenna radiations and scattering can be easily analyzed using the equivalent substrates of EM materials. Numerical examples of planar FSSs to form radomes demonstrate the feasibility of numerical simulations’ proposed work. The presented work can also be directly applied for fast analysis of EM shielding effectiveness by FSS substrates.

STLS-LADMM-Net: A Deep Network for SAR Autofocus Imaging

Synthetic aperture radar (SAR) can provide high-resolution electromagnetic backscattering images of the illuminated area, playing a significant role in various applications. However, achieving focused SAR images is challenging under sparse sampling and phase error conditions. By exploiting the sparsity or compressibility priors, the state-of-the-art sparsity-driven SAR imaging methods can reconstruct images under the condition of sparse sampling. However, the handcrafted priors used in these methods limit the imaging performance, and the iterative solution schemes reduce the computational efficiency. Besides, the measurement inaccuracy introduced by the phase error also degrades the reconstruction performance of the sparsity-driven imaging methods. To address these issues, a deep network for SAR autofocus imaging is proposed, which alternately performs image reconstruction and phase error estimation. When performing image reconstruction, the sparsity-cognizant total least-square (S-TLS) model is introduced to handle the problem of measurement inaccuracy, contributing to robust reconstruction performance under the condition of phase error. During the implementation of the deep network, a feature transform operator is used to realize data-driven prior knowledge learning and overcome the limitations of handcrafted priors. Moreover, the deep network approach can significantly improve computational efficiency. Experiments on simulated and real data verify the effectiveness and efficiency of the proposed method.

GEO-RADAR LNAPL MODEL BY ARTIFICIAL INTELLIGENT

Ground Penetrating Radar (GPR) as non-destructive measurement of full-wave electromagnetic(EM) backscatter completely relies on dielectric permittivity. Interpreting GPR reflection configuration is a complex qualitative with positioning and depth determination would be misleading due to severe polarization and velocity mismatch in travelling-wave. As a result of these studies, a GPR signal segmentation algorithm model was developed to map and identify light non-aqueous liquid (LNAPL) contaminated in laterite soil utilizing dielectric permittivity prediction. Simultaneous registration of a Global Positioning System (GPS) signal was performed while acquiring georeferenced GPR data sets to pinpoint the appropriate location of the soil layers. In this way, georeferenced GPR dispersion was assessed and dielectric permittivity was retrieved by velocity extraction. Empirical model relationship was established by the higher-order regression. Calibration function used verification measurement with root mean square error (RMSE) and calibrated Performance Network Analyzer (PNA). Segmentation and classification using Support Vector Machine (SVM) classifier as Artificial Intelligence (AI) was executed using predicted dielectric permittivity to construct the GPR automated recognition model. The model was compared with actual data and logistic regression classification. The result shows both classification techniques have provided good quality with root mean square errors (RMSE), which were 0.1391 and 0, respectively. The classification produces correct instances classified above 98% for SVM and 100% for logistic regression.

Monostatic radar sensing of partially nonreciprocal mediums backscatter

This paper presents the results of theoretical and experimental studies of electromagnetic waves backscattering by partially non-reciprocal media. It is proposed to implement a controlled reflector that allows to simulate the polarization properties of an arbitrary partially non-reciprocal medium.

Effects of symmetry-breaking on electromagnetic backscattering

Systems with a discrete rotational symmetry 2pi /n where nge 3 that also have electromagnetic duality symmetry exhibit zero backscattering. The impact of breaking one of the two symmetries on the emerging backscattering has not yet been systematically studied. Here, we investigate the effect that perturbatively breaking each of the two symmetries has on the backscattering off individual objects and 2D arrays. We find that the backscattering off electromagnetically-small prisms increases with the parameters that determine the symmetry breaking, and that the increase of the backscattering due to the progressive breaking of one of the symmetries can be related to the other symmetry. Further exploration of the interplay between the two symmetries reveals that, in systems lacking enough rotational symmetry, the backscattering can be almost-entirely suppressed for a given linear polarization by deliberately breaking the duality symmetry. This duality breaking can be interpreted as an effective increase of the electromagnetic degree of rotational symmetry for that linear polarization.

Data Encoding Improvement and Size Reduction of Chipless RFID Tag Using Short-circuit Line Effect and Angle-based Divided Loops

This paper focuses on a short circuit and concentric loop effects to improve the structure dimension as well as enhance the ability to encode electromagnetic wave data for chipless RFID tags. The tag is composed of star-shaped rings that are concentrically nested together. The following factors help reduce the tag size: using the phenomenon of electromagnetic backscattering, assessment by the Radar Cross Section (RCS), the antenna-free tag only includes the multi-frequency resonators. By inserting short circuit lines combined with concentric loops, the number of resonant peaks increases exponentially without increasing the tag overall dimension. In addition, by inserting the short circuit lines at different angles between the two consecutive loops, we can adjust the resonant frequency in the frequency bands, thereby changing the value of the bit by shifting the frequency position. The overall dimension of the resulted tag is 13mmx13 mm and the tag is aimed to be a printable tag on the flexible substrate to minimize the fabrication cost. With the 18-vertex star-shaped tag with five concentric loops, this chipless RFID tag can encode up to 16 bits with high resolution and large data density.

Enhancing Electromagnetic Backscattering Responses for Target Detection in the Near Zone of Near-Field-Focused Phased Array Antennas

The analysis of electromagnetic (EM) scattering from a target located in the near zone of an electrically large near-field-focused (NFF) array of antennas is presented. The analysis first examined the EM scattering from a metal sphere, attempting to create a reference for comparison with other targets in a fashion analogous to conventional far-field analysis of radar cross section (RCS). Realistic targets were afterward examined and analyzed to exhibit their RCS characteristics. A numerical simulation-based maximization of EM scattering fields for effective target detection in the near zone of antenna was developed, which allows one to obtain a set of optimum element excitation weightings by applying the steepest ascent method (SAM). RCS phenomena were validated by full-wave simulations and experimental measurements to justify the feasibility of the proposed concept.

Analysis of the Electromagnetic Scattering Characteristic and Identification of Kelvin Wake on 2-D Sea Surfaces by Feature Selective Validation

Bi, N. and Jiang, T., 2020. Analysis of the electromagnetic scattering characteristic and identification of Kelvin wake on 2-D sea surfaces by feature selective validation. In: Bai, X. and Zhou, H. (eds.), Advances in Water Resources, Environmental Protection, and Sustainable Development. Journal of Coastal Research, Special Issue No. 115, pp. 622-626. Coconut Creek (Florida), ISSN 0749-0208.The electromagnetic scattering characteristics and the methods of identifying Kelvin wakes from a two-dimensional sea surface were studied. The two-scale method is used to calculate the electromagnetic scattering coefficients before and after superimposing the Kelvin wakes on rough sea surface. Then, by using the feature selective validation method, the feature differences between the two sets of scattering coefficients are extracted and compared, the global difference measures are obtained before and after the superimposed wake on the sea surface and its corresponding evaluation level. The results showed that under the appropriate conditions of sea surface wind speed and ship sailing speed, the evaluation level after superimposing the Kelvin wake on the sea surface could reach level three or higher. This shows that this kind of method can quantify the influence of a Kelvin wake on the electromagnetic backscattering characteristic of rough sea surface intuitively and accurately and realize the recognition of the Kelvin wakes.

A Modified Model for Electromagnetic Scattering of Sea Surface Covered with Crest Foam and Static Foam

With the increase of sea surface wind speed, whitecaps will appear on the sea surface. Generally, for Electromagnetic (EM) scattering of the foam-covered sea surface, medium-scale waves are used to replace the breaking waves of the real sea surface. Another treatment in computation is to adopt one of the whitecap coverages and fixed foam layer thickness. In fact, the evolution process of a breaking wave goes through two stages: stage A (crest foam) and stage B (static foam). In this paper, a geometric model of the sea surface covered with crest foam and static foam is established. The coverage ratio of stage A and stage B is proposed for the first time for a given sea state. In addition, different foam layer thickness distributions in each foam for various wind speeds are also considered. Based on the facet scattering theory of sea surface, this paper adopts the modified facet-based scattering model to deal with the scattering contribution of the sea surface and the effect of foam. Finally, in order to verify the accuracy of the geometric modeling and the scattering model of the sea surface, the EM backscattering of sea surface under different sea states are calculated. Simulation results show that the results of the proposed model are more consistent with the measured data than the results of the sea surface covered with individual crest foam or the sea surface covered with individual static foam.

Modification of the Extended Advanced IEM for Scattering From Randomly Rough Surfaces

In this letter, we modify the extended advanced integral equation model (EAIEM) for electromagnetic backscattering and bistatic scattering from rough surfaces with small to moderate heights. We extend the first-order approximation of the error function as introduced in the EAIEM model to the second order, in a hope to be more suitable for large roughness and high frequency. In addition, a new transition model for the reflection coefficient is proposed to make the dependences explicit on the mean surface curvature, frequency, and dielectric constant, whereas making no use of the complementary term, the effect of inadequate evaluation of this term is mitigated. Comparison with POLARSCAT data for backscattering and with European Microwave Signature Laboratory (EMSL) measurements for bistatic scattering demonstrates the validity of the updated model.

SPRITE: 3-D SParse Radar Imaging TEchnique

An original 3-D high resolution radar imaging approach, called SPRITE for “SParse Radar Imaging TEchnique,” is presented. It incorporates in an original way the available prior knowledge about the electromagnetic backscattering, extending the commonly used sparse point-wise scatterers to sparse facet-wise scatterers. It is based on a regularization scheme that accounts for information of sparsity and support. The radar map formation is performed efficiently based on a penalized and constrained criterion, and an Alternating Direction Method of Multipliers algorithm, largely used in image restoration and machine learning. It is customized in a such way that, at each iteration, the map update is fast in the frequency domain by 3-D FFT and IFFT while the updates of the auxiliary variables are direct and separable. SPRITE is both evaluated on synthetic and real measurement data from a spherical measurement setup. In comparison to the conventional method, called Polar Format Algorithm, the resolution is drastically enhanced. The main scatterers are recovered with increased accuracy, leading to a deeper understanding of the scattering behavior. Furthermore, compared to recent $\ell _1$ based methods that are limited to point-wise scatterers, SPRITE and its facet-wise scatterers provide an impressive and improved spatial backscattering representation.

C-Band Simulations of Melting Icebergs Using GRECOSAR and an EM Model: Varying Wind Conditions at Lower Beam Mode

This article presents an electromagnetic backscatter model of iceberg and compares the modeled scattering behavior with C-band RADARSAT-2 synthetic aperture radar (SAR) images. It also explores iceberg SAR signature variability over various ocean parameters. Three-dimensional (3-D) profiles of icebergs were captured in a field study off the coast of Bonavista, NL, Canada, in June 2017 at the time of an SAR satellite overpass. The 3-D profiles were captured from a vessel, using a LiDAR and multibeam sonar. The SAR image and 3-D profiles were captured within hours of one another. Simulated SAR images of the icebergs were generated in a simulator called GRECOSAR with the satellite, target orientation, and ocean parameters that closely mimic the real SAR scene. A new ocean model was introduced to model an ocean backscatter at satellites' lower incidence angle beam mode. Comparison between real and simulated SAR images of the icebergs shows good agreement in terms of SAR signature, total radar cross section, and polarimetric decomposition. Wind direction was varied over 90° extent to observe icebergs' backscatter variability in the simulator. Furthermore, simulated SAR images were generated for low and high wind conditions. Our study finds that the macrostructure of the melt iceberg dominates its polarimetric behavior of its backscatter. Large variability of iceberg SAR signature over varying ocean parameters was also observed. A mathematical model that considers the melting condition of iceberg suggested that significant backscattering can reflect from top surface when the melt water layer could be as little as 0.1 mm.

Space-Filling Electromagnetic Skins for the Wireless Monitoring of Surface Defects

Polymer-based objects (cable harness, gaskets, tires) are exposed, during their lifetime, to mechanical and chemical stress that often generates surface defects like crack and scratches. Early detection of signs of aging may enable a Predictive Maintenance to extend the life of the object and avoid severe failures. For this purpose, Space Filling Curves (SFC) are here proposed as an artificial electric skin, suitable to envelope a surface to wireless detect the presence of small aging signs by resorting to an electromagnetic backscattering platform. Size and resolution of the skin can be controlled by just two parameters and multiple skin cells can be arranged together to tessellate a large surface in order to even identify the position of the defect. By following a theoretical analysis of the sensor-oriented properties of SFCs, and in particular of the Gosper-Fukuda family, the feasibility of the idea is demonstrated by the way of preliminary experiments with a Radio Frequency Identification (RFID) IC, providing a 1-bit anti-tamper port.

An Improved Two-Scale Model for Electromagnetic Backscattering From Sea Surface

Based on composite rough surface and stochastic multiscale models, an improved two-scale model (TSM) is proposed. Different from the classical TSM, the proposed method adopts the integral equation method (IEM) to replace the small perturbation method (SPM) for calculating the contribution from small-scale roughness. Kirchhoff approximation (KA) is still kept in the model to calculate the contribution from large-scale roughness. Because the IEM has wider application range than the SPM, the proposed model exhibits a higher accuracy and has a wider application range in terms of sea surface roughness compared with the classical TSM. Usually, the classical TSM is sensitive to the cutoff wavenumber. The proposed model adopts an adaptive cutoff wavenumber instead of choosing a cutoff wavenumber in the classical TSM. The effectiveness of the proposed model is demonstrated through comparisons with the simulation results of multilevel fast multipole algorithm (MLFMA) and the measured results. Different roughness at different frequencies is analyzed as well.

Assessing the Usefulness of Iceberg Electromagnetic Backscatter Modeling Using a C-Band SAR Classifier

This letter presents the validation of an electromagnetic (EM) backscatter model of icebergs at C-band by comparing the performances of target classifiers trained with both modeled and real synthetic aperture radar (SAR) data. Simulated SAR data were obtained in a combination of imaging beam modes and scene parameters to produce 216 simulated Sentinel-1 C-band SAR images. Parameters consisted of Sentinel-1 IW1 (33.1°) and IW3 (43.1°) beam modes with varying wind speed (5 and 10 m/s), wind direction (0°, 45°, and 90°), and target orientation (0°, 45°, and 90°). Simulations were created from an EM SAR simulator called GRECOSAR, which took 3-D profiles of iceberg and ship targets and parameters necessary to closely mimic the real scenes. 3-D models of three icebergs were captured in a field study off the coast of Bonavista, Newfoundland, and Labrador, Canada in June 2017. Three generic ship models were sourced from an online inventory and scaled to a size equivalent to that of the iceberg targets. Real SAR image data were drawn from in-house data set collected from a complementary research program. Classifiers including support vector machine (SVM), Random Forest (RanFor), k-nearest neighbor (kNN), and neural network (NN) were trained with targets from modeled SAR data and then gradually mixed with real SAR data. Target classifier performance from the modeled target data was shown to be similar to classifiers trained entirely from real SAR data. The similarity in accuracy provides an indication of the validity of the modeled SAR data for this specific application.

Computation and Experiment on Linearly and Circularly Polarized Electromagnetic Wave Backscattering by Corner Reflectors in an Anechoic Chamber

Electromagnetic wave backscattering by corner reflectors in an anechoic chamber is studied using our developed computational tool. The tool applies the Finite-Difference Time-Domain (FDTD) method to simulate the propagation of the wave’s electric and magnetic fields. Experimental measurement in an anechoic chamber is also carried out as a comparison. The two results show agreement, including the finding that the backscatter intensity variation amongst the four circularly polarized modes is significantly smaller than the variation amongst the four linearly polarization modes.

Anisotropy of cold-worked Type-304 austenitic stainless steel: Focus on the hydrogen diffusivity

Anisotropic nature of effective hydrogen diffusivity was investigated on a cold-worked (CW) Type-304 stainless steel. The material was characterized by using disk-shaped specimens sampled from two directions of steel plates with various rolling ratio. The thickness direction of the disks was parallel to the rolling direction for SL specimens and perpendicular for LT ones. Electromagnetic induction (EMI) and electron backscatter diffraction (EBSD) clarified the content and distribution of strain-induced martensite (SIM). The effective diffusivities and solubilities were jointly determined by desorption method and thermal desorption analysis (TDA) in H-charged specimens with high-pressure gas. The increase of SIM with CW ratio and the differences of SIM distribution observed between LT and SL specimens could justify the anisotropic effective diffusivities. Finite element method (FEM) was used to simulate permeation tests based on multiple EBSD maps. Simulations supported the experimental findings: at the CW ratio of 60%, the CW process increased the diffusivity by twenty and the diffusivity was five time greater in the SL specimen than the LT one. The inhomogeneous SIM distribution justified the modifications of diffusion properties by CW in both specimens.