The Application Of High Resolution Time-frequency Analysis In Radar Imaging

In inverse synthetic aperture radar(ISAR)imaging,the most basic range-doppler imaging method requires the Fourier transform of the echo signals in the pulse compression and motion compensation range units in turn to obtain the final target image.In order to avoid the blur of imaging results caused by time-varying doppler frequency,a complex motion compensation algorithm should be applied to face the non-cooperative targets.Moreover,even with ideal motion compensation,the micro-doppler effect generated by the moving part of the target will introduce frequency-modulated components into the echo signal,which is difficult to observe in the Fourier transform results.Therefore,time-frequency analysis,which describes the variation of signal frequency component with time by a joint function of time and frequency,is used to deal with the above situation in this paper.In addition,the synchrosqueezing transform post-processing method developed in the time-frequency domain can achieve the ideal resolution similar to the quadratic time-frequency distribution in the time-frequency domain,and retain the ability of reverse-transform back to the time-domain signal.Combining with the appropriate original method,it will be an ideal high-resolution time-frequency analysis method.This paper mainly studies the application of such kind of high-resolution time-frequency analysis in ISAR imaging.The main contents include:1.The synchrosqueezing method with high resolution is combined with S-transform,with the second order improvement for the strong frequency modulation signal,the Second-order synchrosqueezing S-Transform is proposed.S-transform can adaptively have better time resolution at high frequency region and better frequency resolution at low frequency region.In the ISAR echo signal,the former is used in the frequency modulation component generated by micro-doppler,which requires clear variation.The latter corresponds to the rigid body component at the low frequency and requires accurate positioning.Therefore,S-transform is suitable for the analysis of ISAR echo signal.At the same time,the second order synchrosqueezing S-Transform with frequency adjustment factor solves the problem of insufficient time resolution in the face of the stressed signal like micro-doppler component,and has better effect than the original synchrosqueezing transform.As a post-processing method,it also retains the advantage of S-Transform.2.Aiming at the problem of high-precision radar imaging,an ISAR imaging method based on high-aggregation time-frequency analysis is proposed.Time-frequency analysis is good at finding the characteristics of the state of frequency change just in time to obtain the doppler frequency transformation caused by the unexpected motion,so as to extract the instantaneous imaging results to obtain the changing state of the target.The micro-doppler effect generated by the moving part of the target will introduce frequency-modulated components to the echo signal,which will be easily found from the time-frequency plane and distinguished and extracted from the rigid-body part concerned.In addition,the analysis results of these micro-doppler components will be helpful to extract the parameters of the rotating part of the target,or to further assist in the identification of the target.In the above work,the performance of the selected time-frequency analysis method will play a decisive role in the imaging and analysis results.Compared with other time-frequency analysis methods,the Second-order synchrosqueezing S-Transform is suitable for the micro-doppler component in the echo signal with fast instantaneous frequency transformation.