Research On RCS Predicting Technology Of Hypervelocity Target And Its Plasma Wake In Ballistic Range

Ballistic range is a facility to investigate the radar cross section(RCS)of hypervelocity target and its plasma wake.Several measuring problems occur in RCS measuring experiments,which include: white noise interference,near field measurement and dynamic measurement.This thesis concentrates on the research of near field measurement,accompanied with the research of other two problems,and investigates methods to predict the RCS of hypervelocity target and its plasma wake.The research consists of three parts:At the beginning,this thesis proposes a wavelet shrinkage(WS)method based on discrete wavelet transform(DWT),to restrain the white noise interference in ballistic range experiments.White noise is a significant source of noise in ballistic range experiments.The conventional WS denoising approaches have shown excellent white noise suppression for signals with high SNR but do not perform well for low-SNR signals.This thesis proposes an effective denoising algorithm to suppress the white noise for signals with low SNR.The concept of Local Sum Ratio(LSR)is introduced to determine whether this is a received signal or not within a given time window.The time-dependent threshold estimation method is implemented for the filtering of white noise.The structure similarity index measure(SSIM)and the mean-square error(MSE)are utilized to compare the performance of conventional and proposed denoising approaches.It is shown that the proposed approach demonstrates the best SNR improvement among all the methods involved.The advantage of the proposed approach is more significant when the signal SNR reduces from0 to-10 dB.The second part investigates RCS predicting methods.The existing methods mostly aims at the RCS measurement of static target,however,the measured target flies with a hypervelocity in ballistic range,that differs from conventional situations.This thesis refers to the time domain correlating method in inverse synthetic aperture radar(ISAR)imaging techniques,investigates an RCS predicting method called focusing function method that is suitable for the experiments.Meanwhile,this thesis based on near field to far field transform(NFFFT)method,develops a hypervelocity experiments suitable predicting method from the static one.In addition,the predicting method based on NFFFT is compared with two other methods,the focusing function method and the existing weighting function method,to predict the RCS of hypervelocity target.Comparing result shows that the RCS obtained from three methods agree with each other,and validates the proposed method.At last,the thesis presents a microwave method to measure the dynamics velocity distribution of plasma wake.Continuous wavelet transform(CWT)is utilized to obtain the Doppler frequency shift and the wake velocity is calculated based on the theory of Doppler effect.The validity of this experiment is confirmed by comparing the measured projectile velocity with that achieved by a laser-based velocity meter.As a contactless method,this approach does not interfere with the flow field of wake.Therefore,the velocity distribution achieved in this work exhibits better accuracy compared with that obtained from other contact approaches.Based on the measured velocity distribution of wake,this thesis develops the focusing function method to the time-varying velocity situation and the RCS distribution of wake is predicted.