Research On Hypersonic Vehicle-Borne Radar Target Detection Under Plasma Sheath

The characteristics of "long time,high maneuverability,high survival rate,and high supersonic speed" of high-speed aircraft in near space make it a suitable radar-borne platform.However,the hypersonic vehicle rubs violently with the air during the flight,forming a plasma sheath.When a hypersonic aircraft-borne radar is applied,especially for target detection,the influence of a plasma sheath on the target detection of a hypersonic aircraftborne radar must be considered.Plasma sheath have effects including amplitude attenuation,phase modulation,and dispersion on radio signals.However,in the current research,the propagation characteristics of radar signals in plasma sheaths have not been studied,and the ground transmission experiments of radar signals in plasma have not yet been carried out.Therefore,the following researches will be carried out in this paper in order to solve the above problems.First,a calculation method of radar signals in steady-state plasma and timevarying plasma is proposed,and ground experiments are performed to verify it.Secondly,a hypersonic vehicle model covered by a plasma sheath is established,the influence of the plasma sheath on the DOA estimation is analyzed in depth,and corresponding solutions to DOA estimation error are proposed.Then a radar echo model under steady-state and timevarying plasma is established,and the pulse compression and coherent accumulation under the plasma sheath are studied in depth.In addition,a radar ranging model for hypersonic aircraft based on plasma sheath and intra-pulse motion is established,and a method for ranging error compensation is proposed.The main contents and contributions of this article are as follows: 1.Calculation methods of radar signal propagation in steady-state and time-varying plasma are proposed,and the intra-pulse distortion and spectrum broadening caused by steady-state and time-varying plasma are studied.A calculation method based on FDTDIFFT is proposed to calculate the LFM(linear frequency modulation)pulse propagation in the steady-state plasma,and experiment that LFM pulse passing through plasma is conducted.The study finds that when the carrier frequency and the plasma electron density are close,the intra-pulse distortion is the largest,and when the collision frequency is small,the pulse amplitude shows a rising trend,and the pulse amplitude changes from a rising trend to an decreasing trend as the collision frequency increases.A method based on instantaneous frequency and transmission line method is proposed to calculate the radar signal propagation in time-varying plasma,and a ground experiment of LFM signal propagation in time-varying plasma is carried out.Through different experimental studies,it is found that the frequency interval of different frequency bands is consistent with the oscillation frequency of the electron density,and the bandwidth of the newly generated signal is consistent with the original signal bandwidth.2.A new cross-domain simulation calculation method from electromagnetic waves to signals is proposed for the estimation of the incoming wave direction of a hypersonic aircraft-borne array antenna covered by a plasma sheath and a method for compensating the array antenna DOA error under a plasma sheath based on electromagnetic simulation calculation is proposed.The finite time domain integration method is used to calculate the amplitude and phase of the array antenna element on the aircraft covered with the plasma sheath.The direction of the incoming wave under the plasma sheath is obtained by the spatial spectrum estimation algorithm,and the DOA estimation error is calculated.The study finds that when the peak value of the spatial spectrum is greatly reduced under the plasma sheath,the peak of the spatial spectrum becomes flat and not sharp,so DOA estimation error occurs.When the electron density is very high,multi-peak phenomenon will occur,which will cause the DOA estimation to fail.Subsequently,this paper proposes a method for compensating the amplitude and phase error of the plasma sheath based on electromagnetic calculation.First,calculating the amplitude and phase of the signal received by each array element at a specific incident angle when the environment is free-space,and then the amplitude and phase of the signal received by each element when a plasma sheath covers the vehicle is calculated.Then amplitude and phase error caused by plasma sheath are obtained.The amplitude and phase errors at different incident angles are calculated,and then different amplitude errors are brought into the correction formula.The incident direction when the spectral peak is the largest is the incoming direction.It is found that the method can still compensate the DOA estimation error at lower SNR by experiments.3.Radar ranging model for a hypersonic aircraft under a plasma sheath is established and a method of radar ranging error compensation for a hypersonic aircraft is proposed.A pulse compression model under a steady-state plasma sheath is established,and the influence of different parameters on radar ranging is analyzed.As a result,it is found that the lower collision frequency and the larger the incident angle,the greater the range measuring error.A coherent accumulation model under time-varying plasma sheath is established.Then,a coherent accumulation model under the time-varying plasma sheath is established,and it is found that the time-varying plasma sheath would cause the amplitude and phase of the radar echo change with time and the amplitude and phase changing rules between different pulses are inconsistent,resulting in inter-pulse coherence is seriously affected,resulting in the phenomenon of multiple false targets in coherent accumulation result.In the case of a small collision frequency and a large incident angle or when the radar signal carrier frequency is close to the cut-off frequency of the plasma sheath,the phenomenon of multiple false targets in coherent accumulation results is the most serious.A radar ranging model under the combination of plasma sheath and intrapulse motion is established,and the ranging errors under different factors are analyzed by combining using two typical hypersonic aircraft flight conditions.To solve the hypersonic aircraft-borne radar ranging error during flight,a joint correction method using FIR to correct the phase distortion caused by plasma combined with MPD matched filter to correct intra-pulse motion is presented in this paper.It has a good correction effect on the ranging error.In summary,this article mainly explores and researches on radar target detection of hypersonic aircraft under plasma sheath.The calculation method of radar signal propagation in steady state and time varying plasma is proposed.Intra-pulse distortion and spectral broadening phenomena are analyzed.This lays the foundation for the research of hypersonic aircraft-borne radar under plasma sheath.A new cross-domain calculation method from electromagnetic waves to signals is proposed for the estimation of the incoming wave direction of a hypersonic aircraft-borne array antenna covered by a plasma sheath.It provides a new idea for the DOA estimation under the plasma sheath.Radar ranging model for a hypersonic vehicle under a plasma sheath is established,which organically combine the effects of the plasma sheath and the high-speed flight of the aircraft.This makes the research on ranging of hypersonic aircraft radar closer to the real environment.This article can provide certain theoretical basis and experimental data for subsequent research,and provide theory reference for the application of radar on board a hypersonic vehicle.