Echo Characteristics And Calculation Methods Of Plasma-sheath-covered Re-entry Targets

When the reentry target is flying at a speed of 10～25 Mach,a strong compression wave will be generated at the stagnation point,resulting in a complex chemical reaction between the air molecules near the stagnation point and the exothermic material,and further molecular ionization,forming a layer of "plasma sheath" covering the surface of the reentry target.The plasma sheath will produce a series of electromagnetic effects such as scattering,refraction,attenuation,and cross depolarization of transient electromagnetic waves,resulting in abnormal fluctuation of radar cross section and polarization mismatch between transmitting and receiving antennas of reentry target.At the same time,in the actual reentry process,the pulse width of radar waves usually reaches the order of tens or even hundreds of microseconds.The hypersonic characteristics of the reentry target lead to the fact that the plasma sheath can no longer be regarded as an instantaneous state,and the electromagnetic effect of the moving plasma sheath is more diverse than that of the transient field,resulting in parasitic modulation of the radar signal in multiple domains such as time,frequency and distance,which seriously affects the detection and recognition of the reentry target.Therefore,a finite difference Time domain(FDTD)method of moving plasma sheath based on inter-grid parameter transfer is proposed in this paper to realize the echo calculation of moving rigid objects covered by fluid.The plasma-sheath-covered blunt cone’s echo amplitude and polarization characteristics are studied under different incidence angles,frequencies,and polarization modes.The characteristics of echo time,frequency,and the one-dimension range profile of plasma-sheath-covered blunt cones at different flight altitudes and velocities are studied.The main contributions of this paper are:(1)A finite difference time domain(FDTD)method for fluid-coated rigid moving objects is proposed.According to the spatial distribution characteristics of the velocity field of the plasma sheath,an electromagnetic parameter transfer method between grids is proposed.The motion velocity of each grid is simulated by controlling the transmission velocity of the electron density of the plasma sheath on each grid to the surrounding grid.The flight speed of the target body is simulated by the conformal mesh method of the moving target surface.Eventually,the far-field echo signal is calculated in case the velocity distribution is different at each target body position and plasma sheath.The time-frequency analysis of the echo signal shows that the abnormal phenomena of the amplitude-frequency and polarization characteristics of the target covered by the plasma sheath are mainly affected by resonance absorption,collision absorption,electromagnetic shielding and cross depolarization.The abnormal phenomenon of waveform distortion and spectrum expansion of plasma-sheathcovered target echo in the time domain is caused by the parasitic modulation effect in the frequency domain caused by multiple Doppler frequency components generated by the different velocities of the target and plasma sheath.(2)The amplitude-frequency and polarization characteristics of plasma-sheath-covered target echo are studied.The electromagnetic calculation model of a non-uniform plasmacovered metal plate and the electromagnetic calculation model of the plasma-sheath-covered blunt cone were established under typical parameters.The influence of electromagnetic wave incidence Angle polarization mode and plasma characteristic parameters on echo amplitude and polarization characteristics were analyzed by polarization isolation degree.The results show that the amplitude-frequency characteristics of the reentry target are mainly affected by the incident depth of electromagnetic waves in the plasma sheath.Under the same electron density,the larger the incident depth is,the stronger the resonance absorption effect of electromagnetic waves is.The polarization characteristics are mainly affected by the polarization mode of the incident electromagnetic wave.When the electric field polarization vector of the electromagnetic wave is at an Angle with the plasma surface,the electromagnetic wave is subjected to cross depolarization effect,and when the electric field polarization vector is parallel with the plasma surface,the electromagnetic wave is subjected to polarization isolation effect.(3)Echo parallel calculation method and time-frequency distance domain characteristics of a fluid-coated rigid moving target.Usually,the radar echo is in the order of microseconds,and it takes millions of iterations of electromagnetic field value to complete the calculation of a plasma-sheath-covered target echo,which is a very high time cost.Therefore,a parallel computation method for fluid-covered rigid body moving target echoes is proposed based on a graphics processor,which parallelizes moving mesh parameter update,field value calculation,and boundary conditions in a fine-grained way,and realizes a complete microsecond linear frequency modulated radar echo efficient computation,and the computation time of a radar echo is reduced from one week to about 6 hours.The timefrequency range characteristics of the plasma-sheath-covered blunt cone are studied by timefrequency analysis.The results show that there is a velocity difference between the plasma sheath and the target body,which leads to multiple frequency components in the echo,causing the abnormal phenomena of radar signal distortion in the time domain,frequency domain extension,and multiple targets in the distance domain.In this paper,a finite-difference time-domain(FDTD)method for plasma-sheath-covered moving targets is proposed.The algorithm is parallelized by a graphics processor with parallel computing architecture.Compared with serial computing,the speed of the algorithm can reach 20 times.The research results can guide the selection of antenna polarization mode and radar station layout of the reentry vehicle radar system,reduce the additional loss caused by polarization mismatch,and improve the detection probability of the reentry target.At the same time,it can also provide an auxiliary means of radar signal simulation to study the radar signal processing method of the reentry target.