| A reentry aircraft entering the Earth's atmosphere can generate serious friction,cause the air ionized,and then resulting in the so-called plasma sheath which covers the aircraft.This nonlinear time-varying plasma sheath reflects,scatters and absorbs the electromagnetic waves,which leads to the amplitude and phase's nonlinear time-varying of radar echo.As a result,the radar detection of reentry spacecraft is seriously influenced,making detection results distorted,deflected and even lost.In this thesis,the influence rule of plasma on radar echo is found and a method for reducing the plasma's impact on radar echo is proposed.A time-varying plasma model is constructed by adding aircraft surface medium model and the time-varying property of plasma sheath to SMM.Based on the constructed plasma model,the influence of the plasma's main parameters on its reflectivity is analyzed.Single frequency pulse and linear frequency modulation pulse are constructed as transmitted wave.Radar echo from plasma sheath is received.Then,the variation of the ambiguity function of echo along with the change of plasma density and that of thickness is studied,and the influence rule of plasma on radar echo's amplitude and phase is found.The results show that plasma's influence on radar echo is in connection with the relative relations of reflection coefficients to radar echo's bandwidth and center frequency.In a certain electron density and thickness,it is extremely serious that plasma's impact on radar echo.For linear frequency modulated pulse,doppler shift can reach 1/5 times the bandwidth and time delay can reach 1/5 times the radar pulse width.A method for echo information decoupling is analyzed,and then composed linear frequency modulation and random coded signal is proposed as radar transmitting wave to reduce the impact of plasma on radar echo.The feasibility of this method is verified by simulation,and the advantages and disadvantages of two kinds of waveforms are analyzed. |
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