Research On Electromagnetic Scattering Characteristics And Feature Recognition Of Hypersonic Vehicles

In the near space,the plasma sheath that covers the surface of hypersonic vehicle can have complex interactions with the electromagnetic waves of detecting radars,resulting in significant attenuation or enhancement of the scattered radar echo signals in the direction of the radar antenna,leading to a serious decline in the accuracy of radar detection.This makes it difficult to quickly and accurately identify,track,and predict the type,flight status,and even flight trajectory of hypersonic targets.This dissertation aims to address the urgent need and existing bottlenecks for the identification capability of hypersonic targets on space-based platforms,and to obtain the electromagnetic scattering characteristics of large-scale,non-uniform plasma sheath-covered targets in complex three-dimensional(3D)environments.It also aims to establish a comprehensive database of hypersonic target identification samples and to use intelligent algorithms to quickly and accurately classify and identify hypersonic targets.Based on the high-frequency Physical Optics(PO)approximation algorithm,this dissertation employs key techniques and schemes such as the flow field and electromagnetic coupling grid matching model,dynamic electromagnetic scattering echo feature extraction technology,high-resolution multi-view microwave imaging methods,and popular artificial intelligence network structures to calculate and analyze the electromagnetic scattering characteristics and laws of hypersonic vehicle covered with plasma sheaths,as well as to build the theoretical foundation for related classification and identification problems.The main achievements and contributions of this dissertation are as follows:1.Starting from the differential form of Maxwell’s equations,the harmonic wave equation for introducing the electromagnetic field is derived.By solving the d’Alembert equation for the vector potential A and the scalar potentialφin the Lorenz gauge,the expression for the Green’s function ψ in free space is obtained.On the other hand,using the divergence theorem,the vector Green’s second identity is derived.By substituting the electromagnetic E or magnetic field H and Green’s function ψ parameters into the equation,and using a series of differential identities in vector operations,the Stratton-Chu integral equation expression is established.Combining the PO basic assumption,the Kottler line integral term correction for the illumination-shadow boundary field,and the far-field approximation conditions,the general calculation formula for the PO approximation in the scattering field in the far zone is obtained.By simultaneously satisfying the boundary conditions for metal-free space and metal-lossy dielectric-free space,the analytical expression for the frequency domain PO far-field integral is obtained,realizing rapid calculation of the electromagnetic scattering characteristics of coated plasma sheath hypersonic targets.2.Starting from the motion equations of plasma fluid,a detailed derivation of the formulae for plasma angular frequency ω_p and relative dielectric constant ε_r of non-magnetized plasma sheath was conducted under the Lorentz-Drude model.By applying the Huygens equivalence principle and the boundary conditions of tangential continuity of electric and magnetic fields in the plasma medium,continued fraction expressions for the reflection coefficients R_⊥and R_∥of TE and TM waves in multilayer plasma sheaths were systematically derived.Based on this,numerical results of electromagnetic scattering from hypersonic targets were obtained using the Multilayer Lossy Media Model(MLMM),and the influence of plasma turbulence on the backscattering characteristics of hypersonic targets was studied.3.Building upon the high-frequency approximation algorithm for solving electromagnetic scattering problems of composite targets,a Inhomogeneity Zonal Medium Model(IZMM)was proposed that can fully fit and characterize the complex 3D flow field of the outer surface of hypersonic vehicles.The model divides the plasma sheath flow field into non-uniform computational subregions with different spacing sizes,and their shapes are modeled by fitting parabolic surfaces and circular platforms with different radii.By collecting electromagnetic data(including electron density,temperature,and pressure distribution)at different positions in the sheath subregions,the segmented PO method was used to compute and analyze the effects of incident electromagnetic wave frequency(L-X band),flight altitude,and speed on the backscattering characteristics of RAM C-Ⅱ and HTV-2 targets.4.Based on space-borne radar(SBR)detection platform,an angle-time conversion algorithm was proposed for the horizontal flight path of hypersonic targets.The dynamic electromagnetic scattering characteristics of hypersonic targets were computed and analyzed in the microwave frequency range under different flight states and computational parameters.Using the Continuous Wavelet Transform(CWT)technique,ten effective mathematical statistical characteristic values were extracted from the time-scale matrix of the target’s dynamic scattering echo sequence and composed into a feature vector.Finally,a feature recognition database for hypersonic targets was constructed.5.A high-speed hypersonic target electromagnetic imaging model combining the Inhomogeneity Zonal Medium Model(IZMM)and the Scattering Center Model(SCM)was proposed based on the signal detection system of a distributed space-borne array radar.By calculating and analyzing the distribution of high-resolution one-dimensional range images in the ultra-wideband microwave frequency range,and applying the polar coordinate format algorithm and motion compensation theory,multi-angle ISAR imaging of hypersonic moving targets enveloped by plasma sheaths was achieved.The influence of the flight state of hypersonic targets on ISAR imaging characteristics under different environmental background noise conditions was analyzed and discussed in detail.This research can provide a large amount of data support and technical guarantee for establishing an ISAR ATR database.6.By combining the feature vector database of hypersonic targets and the ISAR ATR database,we systematically studied and analyzed the classification models and network architectures of hypersonic targets covered by plasma sheaths using popular intelligent algorithms,including SVM,MLP,and CNN.In addition,we proposed a classification training model based on the U-WGAN-GP framework to study the generalization of the VGG-16 network for high/low-resolution images.The results show that the proposed model can significantly improve the classification performance of high/low-resolution ISAR image datasets of hypersonic targets.