The Study Of Real-time Forecast On Plasma Sheath Parameter And Channel Characteristics

The hypersonic vehicle flying in atmosphere or space shuttle's reentry contain the high temperature of plasma shock layer around it,namely "plasma sheath".Plasma,a kind of dispersive lossy medium,causes and the mismatching of antenna impedance and the decrease of radiation efficient.In the meantime,plasma reflects and attenuates telemetry signals.As a result,the influences on antenna feed system and communication signals lead to the well known "radio blackout" problem.Both the changes on communication signals and the study on overcoming blackout problem are related to the characterization of channel.In order to avoid the measurement difficulties on plasma sheath channel,two estimation methods are proposed to study the parameter and characteristics on channel from the perspective of antenna parameter and reflected signal.Based on the steady state plasma,the first way aims to obtain the main attenuation of plasma sheath channel from the voltage standing wave ratio(VSWR)of antenna.By using theory of equivalent transmission lines,the influence mechanism of VSWR based on plasma sheath is given.And then,a model of GPS antenna is set up to have a study on VSWR during a reentry process.Simulation is carried out based on a stratified model of plasma sheath.The results demonstrate that VSWR is largely influenced with the electron density and the collision frequency.There is a distortion when plasma cut-off frequency closes to the wave frequency.And the intensity of distortion mainly depends on the degree of the collision frequency.In addition,the relationship between VSWR and transmission loss can support the idea that channel performance can be predicted by detecting the change of VSWR without requiring an additional feedback channel.In the meanwhile,the mean electron density can be calculated through numerical solution.And the inversion result is verified by experiment of VSWR measurement.In the second place,this paper study the reflection signal with the dynamic plasma sheath.To begin with,the amplitude and phase are analyzed when the plasma electron density varies as a sine wave.Then,the correlated characteristic between the reflection signal and transmission signal after time-varying plasma is briefly introduced.We obtain that the amplitude of two signals have a strong negative correlation,while the relationship of two phases are positive.So using the strong correlation above,the characterization of channel can be estimated through reflection signal.After extracting the modulation information from the reflection signal using the partial time-domain coherentalgorithm and the homomorphic transform,some examples are simulated under different plasma parameters.Finally,we get the statistical property including the amplitude probability density function,the phase probability density function,power spectrum density and coherence time.The results demonstrate the small-scale fading of time-varying plasma channel.The study of reflection signal provides basic theoretical reference for the further research of "blackout" problem.