| When the aircraft flies in the high-altitude atmosphere at hypersonic speed,the plasma sheath covering the aircraft will interfere with and shield the radar echo signal,resulting in distortion of signal amplitude and phase and abnormal broadening of Doppler spectrum,which increases the difficulty of radar detection.Aiming at this problem,this paper analyzes the critical angle of total reflection,damping effect and resonance surface shielding effect in the plasma sheath,improves the calculation method of electromagnetic wave reflection of equivalent transmission line,puts forward the calculation formula of radar echo signal considering fluid motion in the steady-state plasma sheath environment,and establishes the radar range profile model of the target covered by the steady-state plasma sheath,The radar high resolution range profile(HRRP)of steady-state plasma sheath coated target is simulated and analyzed.Aiming at the problem of poor sparsity and correlation of Radar HRRP in this environment,it is difficult to accumulate coherently,a Radar HRRP feature extraction method based on entropy is proposed,and the feature vector of HRRP is obtained by constructing element entropy.The main innovations and research contents of this paper are as follows:1.The calculation method of electromagnetic wave reflection of equivalent transmission line is improved.In view of the inaccuracy of the calculation results of the classical method of equivalent transmission line in the case of sparse plasma sheath data and grazing incidence of electromagnetic wave,this paper modifies the transmission path of electromagnetic wave in the classical algorithm of equivalent transmission line by introducing the critical angle of total reflection,damping effect and constructing resonance’shielding surface,and puts forward an improved calculation method of electromagnetic wave reflection.The simulation results show that this method can reflect the total reflection phenomenon of the outermost layer of the plasma sheath when the electromagnetic wave is grazing incidence in the case of sparse data,and can reflect the shielding effect of the resonance surface in the plasma sheath.In the case of sparse data and grazing incidence of electromagnetic wave,the calculation results of the improved method are more accurate than the classical method,and a lot of computing resources are saved.2.The radar echo signal calculation method of steady-state flowing plasma sheath is proposed.Aiming at the problem of radar echo signal calculation considering the fluid movement of plasma sheath,this paper adds the flow field velocity parameter to the layered medium model,uses the improved calculation method of electromagnetic wave reflection proposed in this paper,obtains the incident depth of electromagnetic wave and its corresponding reflection coefficient,and deduces the calculation formula of radar echo signal in this environment.The simulation results show that in the plasma sheath environment,the incident angle and electron density of electromagnetic wave will affect the incident depth of electromagnetic wave signal,resulting in different degrees of Doppler frequency offset and signal distortion of radar echo signal.Compared with the traditional radar echo signal calculation methods,the proposed calculation method can reflect the Doppler frequency offset of radar echo signal caused by plasma sheath environment,and can obtain more accurate radar echo signal calculation results in this environment.3.The radar range profile model of steady-state flow plasma sheath coated target is established.Aiming at the problem of abnormal broadening and reproduction of Radar HRRP Doppler spectrum of steady-state flow plasma sheath coated target,based on the traditional division of scattering center of rigid target,the remaining surface of the target is meshed,the steady-state flow plasma sheath coated target is divided into multiple scattering centers,and an independent layered medium model including flow field velocity is established in each scattering center,The calculation formula of the incident angle of the far-field plane wave at each scattering center is derived.The radar echo signal components of each scattering center are calculated by using the radar echo signal calculation method proposed in this paper.The radar range profile model of the target covered by the steady-state flowing plasma sheath is obtained by superimposing each signal component.The simulation results show that the plasma sheath environment will lead to different degrees of Doppler frequency offset of the radar echo signal component of each scattering center,resulting in Doppler spectrum broadening of the Radar HRRP in this environment.The radar range profile model established in this paper considers the fluid motion characteristics of the steady-state plasma sheath,and can reproduce the Radar HRRP Doppler spectrum broadening caused by the plasma sheath.4.A Radar HRRP feature extraction method based on entropy is proposed.Aiming at the problem of poor sparsity and correlation of HRRP of plasma sheath coated target radar,it is difficult to accumulate coherently,firstly,the real HRRP of multiple continuous pulses is extracted by hard threshold processing;Secondly,the sparsity is improved by calculating the power HRRP;Thirdly,using the element entropy proposed in this paper,the distance units with strong correlation and signal energy in the two power HRRP are highlighted;Finally,the feature vector of the target HRRP is obtained by accumulating multiple element entropy.The simulation results show that the element entropy can reflect the correlation and energy distribution characteristics of the target HRRP in the range dimension,and can stably obtain the feature vector of the target HRRP when the number of pulses is small.The feature vector curve of Radar HRRP in the plasma sheath environment is obviously different from that in the non sheath environment.The feature extraction method proposed in this paper can reflect the correlation and energy distribution of the target HRRP in the distance dimension,and can still obtain stable calculation results when the number of pulses is small,which can be used to identify the target covered by plasma sheath.In summary,the research results of this paper can provide radar echo data for the subsequent broadband radar echo signal processing for plasma sheath,provide new ideas for in-depth study of target recognition in plasma sheath environment,and have important significance for realizing the accurate attack of hypersonic targets. |
