Study On Airborne Multi-mode SAR Imaging Algorithm

At present,airborne SAR has several working modes: stripmap mode,spotlight mode,scanning mode and ground moving target detection.Now there are two main directions in the development of airborne SAR: large squint angle and high resolution.At this stage,the imaging algorithm of squint SAR is becoming more and more perfect,but the existing algorithms have some limitations for the process of squint angle,and the precision of the algorithm is not very high in the large squint angle imaging,however the study of spotlight SAR is an important part of the high resolution SAR.Therefore,it is necessary to study the imaging algorithm of large squint SAR and spotlight SAR.Based on the airborne SAR imaging geometry model and the derivation of echo signal in the time-frequency domain,this thesis mainly focuses on the SAR echo data simulation and the research of the stripmap SAR squint imaging and the spotlight SAR imaging.The main research contents are as follows:In order to simulate the actual SAR system and the subsequent study of the relationship between the imaging plane and the ground plane,this thesis presents a time domain simulation method for the echo data of the real scene.By modeling the stripmap SAR and the spotlight SAR imaging geometry,the calculation method of the key parameters of the stripmap SAR and the spotlight SAR echo simulation and time domain echo simulation method of dot matrix target are given,which provides data support for the research of stripemap SAR squint imaging algorithm and spotlight SAR imaging algorithm.In view of the characteristics of the squint SAR imaging,the Taylor expansion of the twodimensional coupling phase of the echo signal in the two-dimensional frequency domain is carried out,we concluded that secondary range compression and residual phase compensation are needed for the signal after the range compression.At the same time,the Doppler center should be aligned,because the Doppler center frequency of the squint SAR is not zero.Based on the traditional RD algorithm and the CS algorithm,the improved RD algorithm and the improved CS algorithm for the medium squint angle are proposed.At last,an airborne large squint SAR imaging algorithm based on range direction CS and azimuth direction extensional NCS is proposed for the large squint condition.The azimuth focus depth problem caused by distance walk correction is processed by introducing high order disturbance in azimuth direction.At the same time,the data processing flow of the algorithm and the calculation method of the reference function are given,and the validity of the three imaging algorithms is verified by simulation imaging of the simulated data.The characteristics of the spotlight SAR and the stripmap SAR are compared.In view of the geometric distortion of the PFA in the spotlight SAR imaging processing,a PFA for correcting the geometric distortion is studied,which needs to two-dimensional resample the PFA focused image in the image domain,the calculation process of the parameters required for the two dimensional resampling is given in this thesis.At last,a fast change PRF data acquisition method is proposed for spotlight SAR squint imaging,by controlling the time of transmitting pulse,the traditional PFA can process the echo data directly,which provides a new research angle for the spotlight SAR squint imaging algorithm.