Research On Imaging Algorithms For Forward-looking SAR

With the ability of all-time and all-weather,synthetic aperture radar(SAR)plays an important role in battlefield reconnaissance,environmental monitoring and weapon guidance.In order to improve the performance of SAR system,the new SAR technology has been studied by scholars at home and abroad and forward-looking SAR is a kind of radar system that appears in this process.Forward-looking SAR has developed rapidly in recent years due to its advantages of imaging the forward region.At present,forward-looking SAR is mainly divided into two modes: bistatic forward-looking SAR and linear array forward-looking SAR.These have a broad application prospects and deserve further studying.This dissertation focuses on topic of forward-looking SAR imaging the forward region and aims to improve the imaging performance of the front area by utilizing the system advantages of forward-looking SAR.Firstly,the research of the bistatic forward-looking SAR imaging algorithm is carried out,which can effectively improve the focusing ability of targets in large forward-looking angle and large imaging scene area.Then,the imaging technology of linear array forward-looking SAR is studied to improve the focusing ability of the target in the large imaging scene area in the case of high carrier motion speed.The main contents and innovations of this dissertation are as follows:(1)Focusing one-stationary bistatic forward looking synthetic aperture radar based on squint minimization and modified nonlinear chirp scaling algorithm.This algorithm firstly uses the squint minimization method to process the echo data,and transforms the echo data into the data under the low squint angle.Compared with the traditional algorithm,this method can effectively improve the orthogonality of range and azimuth.Then a modified azimuth NLCS algorithm is derived.Compared with the traditional NLCS algorithm,this algorithm can effectively improve the imaging quality of regional targets in large imaging scene.Finally,the theoretical analysis is validated by computer simulation experiments.The results show that the imaging quality of the targets in the scene area is still high in the large forward-looking view and large imaging scene.(2)Aiming at the two-dimensional space-varying problem of echo data in any bistatic forward-looking SAR mode,the 2rd-order keystone transform and modified NLCS algorithm based on improved elliptic model are proposed.The algorithm utilizes the characteristics of distance history in echo data.Firstly,the 2rd-order keystone transform is used to eliminate the coupling between range and azimuth.Compared with the traditional algorithm,this algorithm can effectively eliminate the high order coupling term of range and azimuth.Aiming at the problem that the traditional algorithm can not solve the problem of poor imaging quality of edge point targets caused by two-dimensional space-varying problem in large imaging scene,a modified NLCS algorithm based on improved elliptic model is proposed.The algorithm uses a modified perturbation function to makes the operation between the azimuth phases more precise,and improves the focus depth of edge target.The results show that the proposed algorithm can still achieve high quality imaging even if the two-dimensional spatial variation problem of echo data is serious in the case of large forward-looking views,large squint angle and large imaging scene.(3)Aiming at the problem of poor imaging quality caused by high carrier motion speed in linear array forward-looking SAR mode,an equivalent range model is proposed to compensate high carrier motion speed.Based on this,a NLCS imaging algorithm is proposed based on circular model.The traditional algorithm neglects the carier’s high speed,which leads to poor focusing imaging quality and even distortion.Due to the high speed movment of the platform,range cell migration(RCM)and 2-dimensional spatial variation are more serious.To overcome this problem,an equivalent range model is proposed,which takes the platform’s high speed into consideration.This model can convert the forward-looking mode of the linear array into the squint mode of mono/ bistatic station.Then a modified nonlinear chirp scaling(NLCS)algorithm based on a circle model is used to improve the focusing quality of edge targets.The results show that the imaging quality of the scene area is still high under the condition of high carrier motion speed.