The Mechanism And Law Of The Influence Of Plasma Sheath On Various Polarimetric Radar Waves

When the near space hypersonic vehicle is flying in the atmosphere at Mach 10-25,the air around the vehicle has a violent friction with the surface of the vehicle,resulting in a plasma sheath covering the surface of the vehicle.There are many free electrons in the plasma sheath.When the radar wave enters the plasma sheath,it will have different amplitude and phase effects on the radar wave of different frequencies and cause the loss of polarization information of radar waves,the reversal of polarization direction,and the polarization mismatch of the radar receiving antenna of the ground station,it greatly interferes with the effectiveness and accuracy of hypersonic vehicle radar detection.Due to the relative position of the aircraft with the ground station and other reasons,there is an angle between the radar wave and the target to be detected in most cases.Therefore,this paper mainly studies the reflection and absorption characteristics of the plasma sheath to different polarized electromagnetic waves in the case of oblique incidence,on this basis,the influence of sheath on time delay(range detection)and Doppler(velocity detection)of radar wave is analyzed.In this paper,the layered calculation model of plasma sheath clad metal plate is established.Based on the ISMM(improved scattering matrix)method,the oblique incidence ISMM method is derived.The transmission coefficient of the oblique incidence ISMM method is modified and verified by ray tracing method.On this basis,a dynamic plasma with sinusoidal fluctuation of electron density with time is constructed by combining the distribution model of electron density.Based on the model,the influence of the parameters of static sheath and dynamic sheath on the reflection and absorption characteristics of electromagnetic wave is simulated and analyzed respectively.On this basis,the LFM signal radar wave is constructed,the radar wave is applied to the established model,and the influence law of plasma sheath on the time delay and Doppler frequency shift of radar wave is obtained by combining the fuzzy function.It is found that there are similarities and differences in the effects of plasma sheath on radar waves of different polarizations.In the reflection characteristics,there is a concave area,the reflection wave is greatly attenuated,In the concave area,and the phase changes violently,resulting in a high group delay.In the case of circular polarization,there is also a phenomenon of polarization reversal.In the absorption characteristic,there is a convex area,at this time,the radar wave is absorbed by the sheath.With the sinusoidal wave of electron density,there is a corresponding sinusoidal wave in the positions of the concave and convex areas.At the same time,it is found that under some parameters,there is a best matching value,at this time,the attenuation of the reflected wave is very large.It is also found that the incidence angle,electromagnetic wave frequency and electron density have a great influence on the location of the concave area,however,the impact of collision frequency and sheath thickness on the position of the concave area is not obvious.According to the characteristics and mechanism of the concave area,combined with the total reflection phenomenon,the total reflection angle is used to calibrate the position of the concave area and the position of polarization reversal under circular polarization.When the frequency of the radar wave is in the concave area,the time delay and Doppler fluctuations caused are large,and the radar waves with different polarizations have different amplitudes.In addition,the larger the pulse width,the higher the delay,but the corresponding Doppler frequency shift is smaller,and the larger the bandwidth,the lower the corresponding delay,but the wider the frequency band causing high delay,Doppler Larger frequency shift.