Research Of Inverse Synthetic Aperture Radar Instantaneous Imaging Method

Imaging radar is of great significance for feature extraction and target recognition.Inverse Synthetic Aperture Radar(ISAR)can obtain high range resolution by transmitting a wideband signal,and high azimuth resolution is obtained through the target rotation relative to the direction of radar line of sight,thus a moving target image can be obtained.Because of little environmental influence,good real-time performance,long working distance and some other advantages,ISAR has important application value in the strategic defense,anti-missile,target recognition and so on.The irregular motion of most targets may lead to different levels of maneuver,in the process of ISAR observation,if a traditional Range Doppler(RD)algorithm is used to make the image blurry.However time-frequency analysis method is adopted instead of traditional Fourier analysis,the image quality will not only get a better improvement,but also increase signal-to-noise ratio(SNR).But single time-frequency analysis has its own disadvantage,so combined time-frequency analysis is introduced to make up for their respective shortcoming of single time-frequency analysis.Through the combined time-frequency analysis method,the better image quality can be acquired.Because the resolution of the target is higher,so the requirement for a wideband signal is higher,the increase of sampling data amount causes certain burden to the ability of radar system processing the data.ISAR imaging of multiple targets cannot guarantee sufficient observation time,which results in the azimuth data of radar echo incomplete and lower image quality.Compressed sensing(CS)can solve the problem of ISAR abundant data and the lack of the radar data and so on,which has caused great scholars' attention both at home and abroad.The time-frequency analysis is applied to the maneuvering target imaging,and CS can solve the problem of missing data.If echo data of the maneuvering target is missing,CS and time-frequency analysis can be combined to solve the problem.This paper firstly uses plane turntable model to illustrate the basic principle of ISAR imaging,and RD imaging process is introduced.Secondly basic motion compensation methods(envelope alignment and phase correction)are descripted,and the effectiveness of the basic method is verified by the measured data.Through the maneuvering target space model,the problems which the target maneuvering brings in are discussed.According to the demand of ISAR maneuvering target imaging,the necessity of time-frequency analysistechnology which applies in ISAR imaging is raised.At the same time,the concept and theory of time-frequency distribution is introduced.Time-frequency analysis can not only deal with the defocus of maneuvering target image,but also reduce noise.Then the combined time-frequency analysis method is obtained based on time-frequency analysis,which effectively avoids the limitations of a single method and integrates the advantages of every method to make the image quality much better.Finally combined with CS and time-frequency analysis,the combined method makes up for the fuzzy image which the target maneuvering causes and overcomes the drawback of lack of data.