The study of surface-layer evaporation ducts (EDs) maintains persistent scientific significance due to their critical role in modulating tropospheric propagation. While multiple ED quantification methodologies exist, the inherent complexity of these atmospheric structures radically constrains the observational completeness achievable through singular technologies. To address this limitation, we develop a novel Bayesian fusion framework that systematically integrates multi-source knowledge through probabilistic reasoning to estimate ED from point-to-point propagation loss (PL) observations. This approach reformulates the inverse problem as a maximum likelihood estimation challenge, systematically synthesizing prior ED distributions (derived from diverse knowledge sources) with PL-constrained likelihood functions through a rigorous Bayesian approach. This probabilistic integration enables robust determination of posterior ED distributions while inherently quantifying retrieval uncertainties. Validation experiments using a dedicated dataset confirms the framework’s viability, demonstrating sufficient accuracy to support real-world marine applications.

This study introduces RFCFormer, an innovative framework for retrieving evaporation duct refractivity from radar sea clutter, which synergistically integrates a dual-stream Transformer architecture with Gramian Angular Field (GAF) methodology. The framework first converts the input sea clutter into two distinct GAF matrices, effectively transforming the raw clutter into more manageable sparse matrix representations. This transformation enhances the model’s ability to identify underlying patterns within the sea clutter, improving the accuracy of the mapping between clutter characteristics and duct parameters. The dual-stream Transformer architecture then utilizes parallel processing pathways, where coordinated self-attention mechanisms process the GAF matrices in depth, facilitating the integration of multi-scale clutter features through cross-stream feature fusion. Experimental evaluations show that RFCFormer surpasses existing approaches in both inversion accuracy and generalization capability. Furthermore, the model achieves simultaneous retrieval of four duct parameters while maintaining adaptability to diverse sea states, which conclusively demonstrates its practical utility in real-world marine environments.

In this paper, we propose an effective composite scattering model to deal with the scattering problem of a target on snow ground. It is a hybrid model based on the four-path model (FPM), the Kirchhoff approximation (KA) and vector radiative transfer (VRT) theory, and the shooting and bouncing ray (SBR) method. In our work, we focus on the active remote sensing of snow ground. The scattering of snow ground mainly consists five components, which are ground scattering, ice crystal scattering, ice crystal− ground, and ground− ice crystal− ground scattering. Firstly, we introduce KA based on 3D geometric model to calculate the electromagnetic (EM) scattering of rough ground. And the other scattering contributions of snow ground are solved by VRT. Then, the random phase correction model is adopted to restore the scattering field of snow ground, the scattering field of target is obtained by the SBR method, and the composite scattering from target and snow ground is solved by FPM. In the numerical results, the scattering behavior of dry and wet snow is validated using the KAVRT model through comparisons with measured data. The EM scattering model of target on snow ground is also analyzed by FPM-KAVRT-SBR method. Moreover, the synthetic aperture radar (SAR) of target on snow ground is investigated in this paper. The combination of range frequency-domain pulse coherence (RFPC) with the frequency response function is proposed to simulate the SAR echo. RFPC and frequency response function can be effectively integrated in the frequency domain, extending the application of the FPM-KAVRT-SBR method in SAR echo simulation.

To address the prevalent overfitting issue in traditional machine learning methods when predicting the cement ground electromagnetic (EM) scattering coefficient under small sample conditions, this paper introduces a prediction model that integrates a multi-strategy optimized particle swarm optimization (PSO) algorithm with least squares support vector regression (LSSVR). Incorporating the coefficient of determination as the fitness function enhances the model's learning performance on data features. Furthermore, the introduction of a linearly decreasing inertia weight (LDIW) adjustment strategy and a rebound method boosts particle diversity and search capacity, thereby facilitating optimal parameter selection in the LSSVR model and maximizing its efficacy for small sample problems. The model is finally tested on a simulated dataset based on VRT theory, featuring cement ground EM scattering coefficients. The results demonstrate that, in comparison with the traditional PSO-LSSVR method, the enhanced model significantly improves prediction accuracy and generalization performance under limited sample conditions, achieving over a 90% improvement in prediction accuracy.

This paper focuses on the Synthetic Aperture Radar (SAR) image simulation of the vehicle target on the real complex ground covered with different types of medium. Firstly, rough ground surfaces with different roughness are combined together using a weighted arctangent function to create the geometric model of the complex ground. On this basis, the composite scattering model is established. Here, the facet-based modified two-scale model (FMTSM), which combines the Kirchhoff Approximate (KA) and Integral Equation Model (IEM), is used to calculate the electromagnetic (EM) scattering of random rough surface medium. While the hybrid algorithm based on the FMTSM and the Vector Radiative Transfer (VRT) theory is utilized to model the EM scattering from the ground covered the vegetation, which is regarded as the random discrete medium. In addition, the vehicle target scattering and the coupled scattering between the ground and the target are calculated using the Shooting and Bouncing Ray (SBR) method. Finally, the Range Frequency-domain Pulse Coherent (RFPC) based on the composite EM scattering calculation is employed for SAR echoes simulation. And the SAR images is achieved through the Range Doppler (RD) imaging method. Our work provides a theoretical foundation and data support for radar imaging, vehicle target recognition in real ground environment.

Traditional Inverse Synthetic Aperture Radar (ISAR) image simulation methods based on electromagnetic (EM) scattering calculation often face the challenge of high time costs, making it difficult to generate high-resolution ISAR image samples of the target in real-time. To address the low efficiency in constructing a dataset of image samples for complex targets, this paper proposes a machine learning-based rapid prediction model for ISAR image. This model utilizes a small amount of ISAR echo data by EM calculation of the complex target as input. Through feature engineering to expand the feature information of the echo data, it further employs ensemble techniques, combining five base models including Multilayer Perceptron (MLP), decision trees, k-Nearest Neighbor algorithm (KNN), Support Vector Machine (SVM) and gradient boosting. This ensemble approach enables quick predictions of the ISAR echo data, reducing the number of EM simulation calculations and significantly improving the efficiency of sample generation. Experimental results demonstrate that the proposed model can accurately predict the entire data required to generate images using only 10% of the echo data, achieving an overall efficiency improvement of approximately 90%. Especially, as the complexity and resolution of the target increase, the time required for image generation using simulation calculations will significantly rise. In such cases, our proposed model demonstrates even greater advantages.

To accurately cognize the long-term variations in evaporation duct height (EDH), this letter develops a novel environmental knowledge-driven prediction model based on the Informer for long-term EDH (EKDInformer-LTEDH). The model utilizes the Informer’s powerful time series (TS) processing abilities to capture long-term dependencies in EDH. Considering the influence of the marine environment on EDH, this letter adopts a knowledge-driven method, which incorporates multiple MEPs as prior knowledge inputs into the model. The results of testing the performance show that the EKDInformer-LTEDH model has significant advantages over other models in long-term EDH prediction. Additionally, integrating MEPs into the model as environmental priori knowledge reduces prediction errors and significantly improves its long-term EDH prediction performance.

For fast solving the electromagnetic scattering of multiple moving targets above or on a rough surface, the multi-dynamic-octrees-based SBR algorithm is proposed in the paper. Firstly, a local coordinate system (LCS) for each object is established that moves with targets in the geodetic coordinate system (GCS). The dynamic octree structure based on LCS is applied to keep the affiliation between facets and leaf nodes unchanged during target motion, which omits the reconstruction of the octree. Secondly, the emission plane determined by the incident direction is used for object projection. The scan line algorithm is improved and utilized to accelerate the elimination of shadows. Moreover, the ray-marching technique is employed to reduce the traversal of octree nodes for the high-order ray tracing on each object. For the ray tracing between different objects, the ray information is converted by the transformation matrix. Thirdly, the scattering fields from targets and the surface (including the couple scattering between each other) are computed in LCS according to the illuminated facets. Then the scattering fields are transformed to GCS and the phase change caused by target movement is compensated. Finally, the total scattering of the composite model is acquired. Numerical results show the scattering of stationary and moving targets above/on the surface. The accuracy and efficiency of the proposed method are verified by comparing it with FEKO-MLFMM and the traditional SBR method. Moreover, the electromagnetic scattering of multiple moving targets on the rough surface under different parameters is analyzed.

In this article, a hypersonic target electromagnetic (EM) scattering echo model combined with the inhomogeneous zonal medium model (IZMM) and the classical scattering center model (SCM) is proposed with a distributed satelliteborne array radar as the detection platform. A parallel physical optics (PO) method is used for multiview inverse synthetic aperture radar (ISAR) imaging of a moving hypersonic target covered with plasma sheath based on the analysis of high-resolution range profile in the S-X ultrawideband range, reconstructing 2-D EM scattering echo data (the target) and motion compensation. The results show that the surface of the inhomogeneous plasma sheath flow field is an excitation layer with random and irregular fluctuation characteristics, which increases the false scattering centroid of the 1-D range profile of the hypersonic target and can interfere with and disrupt the radar localization of the target along the radial direction. In addition, shallow scattering of EM waves occurs in the plasma sheath, and the average signal intensity of the target can gradually reduce from 0.5$\times \,\,10^{-5}$at 60 km and 20 Ma to 0.1$\times \,\,10^{-5}$at 30 km and 20 Ma, with a fivefold weakening of the overall scattered echo signal. In particular, the faster the hypersonic target travels at 30-km altitude, the weaker the imaged scattered echo signal becomes, with the average intensity of the imaged signal weakening by approximately threefold from 15 to 25 Ma. This study provides considerable technical support and data assurance to establish a synthetic aperture radar (SAR) automatic target recognition (ATR) database, and the findings of this study can be used as a reference for fine-structure feature analysis of hypersonic targets for feature extraction and the classification and identification of targets.

Optical intelligent reflecting surface (IRS) is an emerging and low-cost technology that can establish a stable communication route in free space optical (FSO) transmission environment with obstacles. In this work, optical IRS-aided dual-hop mixed FSO and RF system is first proposed for cloud radio access network (C-RAN). Specifically, polar codes are introduced to combat turbulence and building sway induced fading in FSO link, and transmit beamforming (TBF) technique is designed to achieve optimal data-rate for RF link. Supposing that the IRS-aided FSO link is subject to exponentiated Weibull distribution with geometric and misalignment loss, whereas the RF link experiences the Fisher-Snedecor$\mathcal {F}$composite fading, the analytical closed-form outage probability expression is derived in terms of Meijer G and Lauricella multivariate hypergeometric functions with decode-and-forward strategy. Exact closed-form average bit error rate expressions are obtained when the relay of C-RAN adopts single antenna and multiantenna beamforming techniques. On the basis of moment generating function of end-to-end signal-to-noise ratio, the ergodic capacity is obtained in terms of univariate and multivariate Fox H-functions over independent but not identically distributed Fisher-Snedecor$\mathcal {F}$fading channels. The correctness of the analytical results is verified through Monte-Carlo simulations. We further provide an asymptotic analysis and discuss the achievable diversity orders.

The perfect optical vortex (POV) beam with the characteristic of the topological charge (TC)- independence radial profile is first considered to improve the spatial-mode-based underwater wireless optical communication (UWOC) performance. The evolution model of POV beams is proposed on the basis of Rytov approximation and the mutual coherence function to explore the impact of wave parameters on the orbital angular momentum (OAM) detection probability spectrum. The instantaneous crosstalk coefficients describing the POV-based UWOC systems are simulated by the multiple phase screen method with the signal coefficient modeled by a mixture Gamma-Gamma and normal distribution. A discrete and memoryless channel and a set of noncollaborative channels are modeled to obtain the capacity performances of the spatial-mode modulating and multiplexing systems, respectively. Results show the transmission quality of POV beams in oceanic turbulence is most related to the beam radius, while it is nearly free from the wavelength, TC, and radius-thickness ratio. Enjoying the advantage of the TC-independence beam radius, POV beams-based UWOC systems could achieve better bit error rate and capacity performances than the widely used Laguerre-Gauss beams when the higher-order TCs are adopted to modulate and multiplex information, which also confirms the superiority of POV beams in spatial-mode-based UWOC systems.

In this article, a theoretical study is carried out to seek an approach to remotely measure the thickness of the oil film. It is known that it is difficult to probe the oil thickness using a microwave radar due to the fact the dielectric constant of oil is very small compared with seawater for microwave frequencies. While in the range of terahertz (THz) electromagnetic (EM) wave frequency, the dielectric constants of oil and seawater are comparable, resulting in the THz EM fields scattered from the air-oil interface and those from the oil-water interface can produce constructive and destructive interference with the variation in the oil film thickness. Based on this principle, the sensitivity of the THz EM wave to the oil thickness is investigated in this work. First, the oil-seawater two-layer medium is equivalent to a single-layer medium by an equivalent dielectric constant model. Then, the THz EM scattering from the oil-covered sea surface is simulated using the first-order small slope approximation method (SSA-1) and the obtained equivalent dielectric constant. Meanwhile, the influences caused by the damping effect of the oil film, the reduction for friction velocity, and the change in equivalent permittivity on the normalized radar cross section (NRCS) are analyzed. The numerical results show that the NRCSs of THz frequencies are more sensitive to the change in oil thickness than that of microwave frequencies. This property makes the THz EM wave have the potential to probe the thickness of the oil film.

In our paper, an approximation extended Kirchhoff approximation method is used for predicting the Doppler spectrum of the sea rough surface covered by an oil film. Here, the thin oil film and the sea surface interfaces obey the same statistics. They have the same surface height PDF and spectrum, in fact, they can even be considered identical and parallel. Furthermore, the capillary waves of the sea surface will be weaken when the sea surface is covered by insoluble oil material. So the extended Kirchhoff approximation method is fit for the simulation of the Doppler spectrum of the sea surface covered by oil film. The two interfaces (air/oil and oil/sea surface) can be substituted with one interface by replacing the Fresnel reflection coefficients to equivalent reflection coefficients to get the efficiency improved. Obviously, the method is valid for thin oil film and low incidence angles.

A bi-iterative physical optics (PO) method is presented in this letter to investigate the composite scattering from a perfectly electric conducting object partially buried in the dielectric rough sea surface. In the present method, both the scatterings of a partially buried object and the underlying sea surface are calculated by the PO method. And a bi-iterative strategy is considered, including the mutual interaction among points on the object in direct scattering and the mutual interaction between the object and the sea surface in coupling scattering. In addition, the coupling interaction between the partially buried object and the sea surface contains two parts: 1) the upper surface of the sea surface and the upper part of the object and 2) the lower surface of sea surface and the lower part of the object. In numerical simulations, the bistatic normalized radar cross sections of the composite model are computed by the bi-iterative PO method and are compared with those by the conventional method of moments for different object types and polarizations. The results show that the proposed method has a good accuracy and can greatly reduce the computational time and memory requirement.

A hybrid facet-based electromagnetic scheme is developed in this letter to investigate the multipath effect on the electromagnetic scattering and the Doppler spectrum of a low-flying target at maritime scene. In the scheme, the target and sea surface are both facetized. The facet-based small slope approximation method is developed to calculate the scattering upon the sea facet, which involves the composite scattering mechanisms upon both the long wave and short wave structures. The geometrical optics and physical optics method are combined with the fast ray-tracing technique in consideration of the multipath scattering. The equivalent edge current method is used to consider the scattering from the edged structures on the target. The tapered wave is used in the calculation process to eliminate the nonphysical edge diffractions. Numerical simulations present the multipath effect on the radar scattering characteristics of a low flying missile-like target above the sea surface. The Doppler spectra of the multipath scattering from the target flying at the time-evolving maritime scene are eventually simulated. The multipath effect is also investigated in the Doppler domain.

A multihybrid method combining numerical vector finite-element method/boundary integral method (FEM/BIM) and analytic Kirchhoff approximation (KA) is developed to study electromagnetic scattering from a 3-D coated target above 2-D conductive randomly rough surface. The hybrid vector FEM/BIM is applied to model scattering from the coated target, while the scattering from the rough surface is studied by KA, which can greatly reduce the computation time and memory requirement. An iterative approach is adopted to take into account the interaction between the target and underlying rough surface. The electric fields integral equations of the induced difference current on the rough surface and the induced electric and magnetic currents on the surface of the target are derived based on green's function and the boundary conditions. Our numerical code is validated by the commercial software. The convergence and the iteration error of the multihybrid method are discussed. Using the Monte Carlo method to generate 2-D Gaussian rough surface, bistatic difference field radar cross section of a 3-D coated sphere above a 2-D conductive rough surface is numerically simulated.

This letter focuses on scattering from rough sea surface covered by insoluble oil. Oil film damps the capillary wave of the rough sea surface, which leads to a smooth profile of the contaminated sea. We investigate this phenomenon using a highly efficient iterative model based on physical optics (PO) and Huygens' equivalence principle. PO is used to calculate the scattering from the oil film and underlying rough sea surface. The Huygens equivalence principle and iterative strategy are employed to evaluate the coupled scattering between the oil film and rough sea surface. The scattering of contaminated rough sea surface based on the iterative model is compared with those using the method of moments as well as computational time and memory requirements. Then, the influence of some important parameters, such as wind speed, oil thickness, incident wave frequency, and oil coverage, on scattering is investigated and discussed in detail.

A hybrid approach combining the edge-based vector finite-element method (FEM) and the method of moments (MoM) is first extended to study 3-D scattering from a dielectric object above a conductive rough surface. The hybrid FEM/MoM technique is based on the electric field vector Helmholtz's equation. To reduce the calculational domain, the solution region is divided into the FEM region containing the object and the MoM region containing the rough surface. In our hybrid method, tetrahedral elements are applied to handle the object region based on vector basis functions, whereas triangular elements are used to deal with the rough surface based on Rao-Wilton-Glisson basis functions. The electromagnetic coupling between the object and the rough surface is considered by the surface integral equation with the Green's function. The hybrid technique presented here combines advantages between FEM and MoM, which is highly efficient in terms of computing memory, time consumed, and versatility.

An efficient multiregion hybrid method of the finite element method (FEM) combined with the boundary integral method (FEM-BIM) is first proposed for the fast simulation of 2-D scattering from an arbitrary dielectric target above a “Pierson- Moskowitz” rough sea surface. In the implementation of the multiregion hybrid method, the entire model is divided into multiple computational regions depending on energy distribution of an incident wave. With mutual coupling between the target and the dominant region of the sea considered, FEM and BIM are respectively applied to study the target and the dominant region to exactly obtain electromagnetic fields and currents, which have powerful illuminated field and strong interaction with each other. Mutual coupling between different subordinate regions is approximately considered by field integral equations based on Kirchhoff approximation by which the approximate currents in subordinate regions can be obtained. Because FEM and BIM are only performed on the target and the dominant region of the sea, the number of unknowns and the time required in the new method are dramatically reduced than those in the traditional FEM-BIM, making the hybrid method more efficient in the simulation of the composite problem.

Scattering problem of a dielectric target above a dielectric rough surface is studied by a hybrid finite element-boundary integral method (FE-BI). In the interior region containing the target, the finite element method (FEM) is used to study the scattering problem, whereas the truncated boundary of the interior region can be obtained by the boundary integral method (BIM). Conversely, the dielectric rough surface can be treated as two half-open homogeneous region that can be exactly simulated by BIM. The interaction between the rough surface and the target can also considered by BIM. The hybrid techniques presented here are efficient and versatile in the simulations of the scattering from the model of a dielectric target above the dielectric rough surface, and numerical results are presented to discuss the scattering properties of a dielectric airfoil above the dielectric rough surface under different conditions.

Wide band electromagnetic (EM) scattering characteristics of randomly rough surface in the terahertz (THz)frequency range are investigated through the use of finite difference time domain(FDTD) algorithm. Then the scattering characteristics of double negative materials(DNM) randomly rough surface are investigated further. It's worth nothing that in order to avoid introducing numerical divergence, setting the negative permittivity and negative permeability of the negative material region indirectly is necessary. By using the constitutive relation and the transform relation of time domain and frequency domain, the two dimensional (2-D) differential equations in FDTD are obtained.

Two-dimensional scattering from an arbitrary dielectric target above a rough sea surface is studied by an efficient hybrid method of finite element method combined with boundary integral method (FEM-BIM). The composite model is divided into multiple computational regions depending on energy distribution of an incident wave. With mutual coupling between the target and the dominant region of the sea considered, the finite element method (FEM) and the boundary integral method (BIM) are respectively applied to study the target and the dominant region. Mutual couplings between different subordinate regions are considered approximately by field integral equations based on Kirchhoff approximation (KA). Because FEM and BIM are only performed on the target and the dominant region of the sea, number of unknowns and time required of the new method are reduced dramatically than those of traditional FEM-BIM, making the hybrid method more efficient in the simulation of the composite problem.

The purpose of this study is to describe a new hybrid method based on the reciprocity theorem, the physical optics (PO) and the Kirchhoff approximation (KA) for calculating the composite electromagnetic scattering from a target above a one-dimensional rough interface. The KA method is used to investigate characteristics of electromagnetic scattering from the rough interface (including the equivalent electric current densities and the scattered field from the rough interface). The scattered field from the isolated target was simulated by the PO method. Based on the reciprocity theorem, the multiple scattering up to 3rdorder by the target and the underlying randomly rough interface was considered. The validity of our methods is shown by comparing our results with that of Method of Moments (MoM). It is found that our methods are in good agreement with MoM and has a higher computational efficiency.

This new semi-deterministic facet model is aimed at providing a more accurate result in evaluating the radar cross section (RCS) from large-scope 2-D ocean like surface. The modified equivalent current approximation (MECA) method is blended with Bragg components in calculating the elementary radar returns from each facet. Compared with the Kirchhoff Approximation (KA) method which have used in the published relevant papers, the MECA can take the scattering from each facet into consideration. The result obtained by this new facet model would be more accurate than the conventional one in small and moderate scattering angles.

In this letter, the composite scattering of a plasma-coated target above a randomly rough sea surface is investigated by the finite-difference time-domain (FDTD) method with pulsed wave excitation. Both sea water and plasma layer are regarded as isotropic and dispersive media, and they satisfy the Debye model and the Drude model, respectively. The auxiliary differential equation (ADE) technique is incorporated into the FDTD scheme to deal with the dispersive media. To ensure the feasibility of our present method, the scattering of a perfect electrically conducting target without coating above dispersive sea surface by the ADE-FDTD method is compared with the result obtained by the parallel method of moments, which requires an individual run for every frequency of interest. Finally, the normalized radar cross section of a plasma-coated target above rough sea surface is calculated and analyzed for different parameters in detail.