Study On Azimuth Beam Steering SAR Imaging And New System Design

As an all-day all-weather and high resolution wide swath radar system,synthetic aperture radar(SAR)is widely used in military and civil fields,such as ocean observation,topography mapping,target recognition,precision guidance and so on.In recent years,with the development of technology and the application requirements,SAR is developing towards the goal of more flexible,higher resolution and wider swath.In order to achieve the above goal,by beam control,SAR is developed with the beam steering SAR(BS-SAR),such as terrain observation by progressive scans SAR(TOPS SAR),spotlight SAR and its multi-azimuth channel mode.However,the diversity of working mode bring more challenge of the imaging algorithm and prompt the presentation of reflector-array antenna based SAR system.Thus,in order to improve the processing efficiency and the processing flow of BS-SAR imaging and complete the parameters design of the reflector-array antenna based SAR system,the main work of this paper is as follows(1)Research on the back projection algorithm of high resolution spotlight SAR imagingThe second chapter focuses on the problems of back projection algorithm(BPA)of high resolution spotlight SAR imaging,and the main research contents are as followsAs an important parameter of SAR system,the analysis of the influence of time bandwidth product(TBP)of BPA has not been systematic analyzed.To fulfill this blank,the influence of TBP on the BPA imaging is first analyzed is this chapter.It is found that small TBP will lead to point spread function distortion and spectrum ambiguity of the traditional BPA imaging results.To solve the above problems,the improved algorithm is proposed.In this algorithm,the effective integral range of each target is optimized first to corrects the point spread function deformation,and then based on the image azimuth ambiguity to signal ratio,the image grid is replot to avoid spectrum ambiguity.The processing results of the real data show that the proposed algorithm can effectively complete the small TBP SAR imaging.The defocus problem of BPA imaging of high resolution spotlight SAR is discussed.The research results indicate that large measured error of trajectory and system calibration will cause cross-range defocus problem of BPA imaging of high resolution spotlight SAR and lead to the invalidation of existing autofocus algorithm for time domain imaging.Thus,combined with fast factorized BPA(FFBP),an modified factorize geometric autofocus algorithm for spotlight SAR is proposed.Firstly,the raw data is segmented and focused on the virtual polar coordinates by FFBP and phase gradient autofocus algorithm(PGA).Then,based on the hypothesis of the geometric error between sub-images,the fusion problem is decomposed into image deformation of image domain,the Doppler frequency migration of Doppler frequency domain and the phase error of complex domain and the corresponding parameters are estimated and calibrated in each signal domain independently.Finally,the calibrated sub-images are coherently added to obtain the full resolution imaging.Compared with the traditional factorize geometric autofocus algorithm and the maximum contrast based autofocus algorithm,the proposed algorithm can avoid the across-range defocus problem with higher data processing efficiency.The experimental results based on the real data verify the effectiveness of the algorithm.(2)Research on imaging algorithm of BS-SAR with attitude angle errorAttitude angle is the key parameter that affects the performance of SAR system,so it is necessary to study the influence and the imaging algorithm of attitude angle of BS-SAR system.In this regard,the third chapter of this paper mainly carried out the following research workFirstly,the influence of attitude angle on the signal character of BS SAR is analyzed.The analysis results show that the attitude angle error will lead to a range varied Doppler frequency and beam rotation speed of BS-SAR signal.Therefore,an azimuth spectrum analysis based attitude angle estimation algorithm(AAE)of BS SAR is proposed.Based on the relationship between Doppler spectrum and attitude angle,AAE constructs an optimization equation of attitude angle,and uses nonlinear least square method and minimum Doppler spectral entropy criterion to estimate the attitude angle.The processing results of read data show that AAE can effectively estimate the attitude angle.A full data imaging algorithm(FDA)is proposed to finish the TOPS SAR imaging with the attitude angle error.Firstly,a range varied azimuth preprocessing algorithm is proposed to obtain unambiguous spectrum.Secondly,the chirp scaling algorithm is used to complete range cell migration correction and secondary range compression.After that,a frequency chirp scaling algorithm is proposed to solve the problem of the azimuth varied imaging parameters and finish azimuth compress.Finally,the image is geometrically corrected to obtain the final results.Compared with the traditional TOPS SAR imaging algorithm,the advantage of FDA is that the range varied Doppler spectrum is fully considered and the processing flow is concise without complex data segmentation and sub-image fusion.The effectiveness of FDA is verified by the processing results of real data.(3)Parameter estimation technique for azimuth multi-channel BS SARMulti-channel parameter estimation is a key technology in azimuth multi-channel BS SAR imaging.Different from traditional azimuth multi-channel strip SAR parameter estimation,the Doppler parameter of multi-channel BS SAR is azimuth varied and affected by the beam rotation speed.The conventional multi-channel parameter estimation algorithm of strip SAR is no longer suitable.Therefore,in order to solve the problem of azimuth multi-channel BS SAR parameter estimation and data processing,the following research work is carried out in fourth chapterAn azimuth multi-channel BS SAR parameter estimation algorithm based on along track interferometry(ATI)is proposed.Firstly,the phase of the ATI result between adjacent channels is analyzed.It is found out that the phase of the ATI is related to multiple parameters.Using this characteristic,this algorithm first obtains the reference channel self-ATI phase based on space-time cross-correlation algorithm,and then the phase unwrapping and least square method is used on self-ATI phase to estimates the beam rotation speed and Doppler center frequency.Then,combined with the above estimation results and the adjacent channels ATI phase,the phase unwrapping and least square method are performed to estimate the azimuth baseline error and the phase error between adjacent channels.Compared with the traditional multi-channel parameter estimation algorithm,this algorithm can estimate multiple multi-channel BS SAR parameters at the same time.Based on the above parameter estimation algorithm,an azimuth multi-channel TOPS SAR data processing flow is given.The processing flow fully considers the particularity of SAR processing,and gives an effective motion compensation,error estimation and imaging strategy.The experimental results show that the method can effectively process the azimuth multi-channel TOPS SAR data.(4)Research on pulse repeat frequence design of spaceborne SAR system with reflector array antennaIn the fifth chapter,a spaceborne SAR system based on reflector array antenna is introduced,and some key technologies in its high resolution and width swath mode are studied.Firstly,the wide space beam generation method of reflector array antenna is studied.Secondly,according to the beam design results,the receiving performance of the system's wide swath data by scan on receive mode is analyzed.Finally,the pulse repeat frequence(PRF)design strategy of the azimuth multi-channel system with Stagger SAR mode is studied,and the corresponding designed PRF sequence are given.