Peng Peng

The calculation of electromagnetic scattering from large and deep cavities remains a significant challenge. This paper proposes an enhanced domain decomposition method (DDM) based on integral equations for analyzing scattering from cavities with zero or thin thickness. The key features of this work include: (1) introducing a virtual interface to partition the cavity into interior and exterior regions; (2) improving Robin transmission conditions (RT) by incorporating variable coefficients to ensure the continuity of electric and magnetic currents across the aperture surface and adjust computation convergence; (3) adopting a more flexible expansion form of magnetic currents; (4) implementing non-conformal meshing to improve discretization adaptability for different parts; (5) applying the discontinuous Galerkin (DG) technique to maintain continuity of currents between non-conformal regions. By integrating DDM, variable-coefficient RT (VaRT), and DG, we establish EnCAV-DDM, which represents an enhanced version of CAV-DDM. Numerical examples demonstrate the improved computational efficiency and accuracy of the proposed EnCAV-DDM approach.

This paper proposes an Synthetic Aperture Radar (SAR) imaging and detection model of multiple targets at the maritime scene. The sea surface sample is generated according to the composite rough surface theory. The SAR imaging model is constructed based on a hybrid EM calculation approach with the fast ray tracing strategy and the modified facet Small Slope Approximation (SSA) solution. Numerical simulations calculate the EM scattering and the SAR imaging of the multiple cone targets above the sea surface, with the scattering mechanisms analyzed and discussed. The SAR imaging datasets are then set up by the SAR image simulations. A modified YOLOv7 neural network with the Spatial Pyramid Pooling Fast Connected Spatial Pyramid Convolution (SPPFCSPC) module, Convolutional Block Attention Module (CBAM), modified Feature Pyramid Network (FPN) structure and extra detection head is developed. In the training process on our constructed SAR datasets, the precision rate, recall rate, mAP@0.5 and mAP@0.5:0.95 are 97.46%, 90.08%, 92.91% and 91.98%, respectively, after 300 rounds of training. The detection results show that the modified YOLOv7 has a good performance in selecting the targets out of the complex sea surface and multipath interference background.

An iterative hybrid method is proposed for analyzing the electromagnetic (EM) scattering from complex target above rough surface. Firstly, the electric/magnetic current combined field integral equation (JMCFIE) is formulated to calculate the EM scattering from the complex target combining conductor and multiple dielectric objects. Then, the single integral equation method for multiple dielectric objects in the complex target (MSIE) is applied to reduce one half of the unknowns in dielectric regions, and the hybrid JMCFIE-MSIE method is formulated. Afterwards, physical optics (PO) approximation is employed for computing the EM scattering on the rough surface, and the iterative hybrid based on the combination of JMCFIE-MSIE and PO is derived for composite scattering from complex target above rough surface. Furthermore, the multilevel fast multipole algorithm (MLFMA) is utilized to accelerate the matrix-vector multiplications in the computation process. The equivalent edge currents (EEC) method is applied to meliorate the result. The validation and applicability of the proposed iterative solution are demonstrated by several numerical examples in the last part.

The electromagnetic (EM) scattering characteristics of target above coastal environment are complicated due to the simultaneous existence of ground region and sea region. Nevertheless, the actual target possesses both conducting part and dielectric part. Such a composite scattering problem involves multiple media parameters and large amount of unknowns. Hence, aiming at accurately and efficiently calculating the EM scattering from combined conducting and dielectric target above coastal environment, the hybrid SIE-KA method is introduced in which single integral equation (SIE) method is utilized to reduce the number of combined target’s unknowns and Kirchhoff approximation (KA) is applied to obtain induced electric and magnetic current on the surface of coastal environment. Moreover, multilevel fast multipole algorithm (MLFMA) is adopted to significantly reduce the computational cost and memory requirement. Several numerical examples demonstrate the accuracy and efficiency of the hybrid method. Finally, the EM scattering characteristics of combined conducting and dielectric target above coastal environment have been studied. The radar cross section (RCS) curves with different category of environment, polarization mode, working frequency, statistic parameters of coastal environment, permittivity of target’s dielectric part, target’s height and target category have been analyzed in detail to provide some useful conclusions. The approach and results can be broadly applied in remote sensing simulation of coastal environment with combined target.

The composite electromagnetic (EM) scattering characteristics of low-altitude target above valley composite rough surface are studied. The composite rough surface modeling theory is adopted to simulate the natural cavity structure of valley environment. Aiming at efficiently and accurately calculate the composite EM scattering from low-altitude target above valley environment, a hybrid method combining shooting and bouncing ray (SBR) technique and equivalent edge currents (EEC) method is proposed, in which SBR method based on the ray-density normalization (RDN) is applied to consider the multiple interactions between target and environment, and EEC method meliorates target's edge diffraction. Numerical examples illustrate the efficiency and validation of proposed hybrid method. Finally, based on the hybrid SBR-EEC method, the EM scattering characteristics of low-altitude target above valley environment are further investigated to show scattering mechanisms of this composite model and provide some useful conclusions through physical phenomena.

In the actual scenario, the target is often located in a certain environment and the influence of the environment scattering and the scattering from the multipath interactions between the target and background environment can make the synthetic aperture radar (SAR) characteristics quite different from those of the targets in the free space. The SAR imaging of the low‐altitude target in the complex environment is not well studied yet. This study has developed an electromagnetic scattering model to illustrate the SAR characteristics of a low‐flying airplane target at the maritime scene. The results can reflect the influence of sea surface scattering and the multipath scattering effect very clear. The results can also help understanding the SAR characteristics of the low‐flying target at the practical maritime scene.

An efficient hybrid SIE-KA-FMM method is presented to analyze the electromagnetic scattering from combined conducting and dielectric objects above the rough surface. In this approach, the induced electric current on the rough surface is obtained by the Kirchhoff approximation (KA). The single integral equation (SIE) method is utilized to efficiently compute scattering from the composite target with markedly reducing the numbers of unknowns compared with the conventional coupled integral equation method. The fast multi-pole method (FMM) is incorporated for faster operation and less memory requirement. Several examples verify the validation of the hybrid SIE-KA-FMM method. The composite scattering characteristics of diversiform targets above the rough surface are obtained. Moreover, the numerical results with the different permittivity of dielectric part, polarization mode, working frequency, root mean square (RMS) and correlation length of rough surface, incident pitching/azimuth angle, and target height and target category have been analyzed to provide some useful conclusions.

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 novel electromagnetic scattering model is presented for studying coupling field of super-low altitude target under diverse terrestrial environments in semi-active radar system. The PO+MEC hybrid method is applied to obtain the electromagnetic scattering of target. The Physical Optics (PO) is introduced to calculate the scattering from facets of target. The Method of Equivalent Currents (MEC) is introduced to calculate the scattering from edges of target. Antenna radiation pattern of seeker is introduced to modify traditional Four Path Method (FPM) in semi-active radar system. Based on the modified FPM, the characteristics of coupling field from super-low altitude target under diverse terrestrial environments in semi-active radar system are studied. Simulation results can use for reference in the fields of remote sensing and detection for super-low altitude target.

The present paper deals with a new electromagnetic model in evaluating the radar multipath scattering and its influence on radar High Resolution Range Profile (HRRP) characteristics of a low altitude target above the composite scale rough surface. The model is underpinned by the theory of facet-based rough surface description at high radar frequencies. It involves the coupling mechanisms by employing an extended image method on locally mirror-like facets with two-scale formulas. It has the advantage to reflect both the specular and diffuse features in description of the multipath scattering mechanisms in the real scene. Meanwhile, it can accomplish electromagnetic simulations over the electrically large scale and complex environments effectively and efficiently. The model is validated by the exact computational electromagnetic numerical method. Numerical simulations investigate the Normalized Radar Cross Section (NRCS) of a missile-like target above a soil land area at different heights and observe the multipath impacts on the HRRP characteristics with different simulating parameters. The conclusions of this work are valuable for the radar detection and remote sensing.