Research On Bistatic ISAR Imaging Of Ship Target With Complex Motion

In order to protect the territorial waters of our country from being violated and to safeguard maritime rights and interests,it is necessary to establish a comprehensive coastal surveillance system with both offshore defense and open-sea defense.Shorebase/GEO-shipborne bistatic inverse synthetic aperture radar(ISAR)has the ability of all day and all-weather and is able to realize long-distance continuous tracking,imaging and recognition of moving ship targets in offshore waters/open-sea.In addition,these two kinds of bistatic ISAR imaging systems have the advantages of high concealment,strong anti-strike ability,strong flexibility,multiple imaging modes and obtaining abundant target's information,which improves the reliability of the coastal surveillance system.Differing from stationary moving targets,the motions of ship targets are very complex.They have not only translational motions,but also nonstationary three-dimensional(3D)rotations induced by irregular ocean waves when the sea state is high.How to use their own 3D rotations for imaging has become a key issue.Besides,as the shipborne radar also has irregular 3D rotations,it is also an important issue to explore the effects of the shipborne radar's 3D rotations on ISAR imaging.Therefore,shorebase/GEOshipborne bistatic ISAR imaging of ship targets are studied in this dissertation.In order to study shorebase/GEO-shipborne bistatic ISAR imaging of ship targets,it is necessary to analyze the effects of the 3D rotations of shipborne radar on ISAR images,seperately.Hence,the received signal model of shipborne ISAR imaging of ship targets in the presence of a high sea state is proposed and established first.The effects of the distance approximate errors on ISAR imaging are analyzed and the constraint conditions of neglecting the approximate errors are given,i.e.electromagnetic plane wave assumption.Based on the received signal model,the complex ISAR image expression of the target and the point spread function(PSF)are derived.The effects of the radar ship's rotations on the ISAR image are analyzed theoretically.Then radar ship's rotational motion compensation algorithm and imaging algorithm for shipborne ISAR imaging of ship targets in the presence of a high sea state are proposed.The results of simulation and semi-real data verify the effects of radar ship's 3D rotations on imaging,and demonstrate the effectiveness of the rotational motion compensation and imaging algorithms.Secondly,according to the different motion characteristics of ship target and receiver ship in the presence of high/low sea state,shorebase-shipborne bistatic ISAR imaging of ship targets has been studied in two different cases,respectively.For shorebase-shipborne bistatic ISAR imaging of ship targets in the presence of low sea state,the received signal model is proposed and established and the complex ISAR image expression of the target and the PSF are derived.On the basis of the analysis,it has been found that by forcing the receiver ship and the target moving in the same or opposite direction,a desired ISAR image projection plane(IPP)is achieved in which the cross-range coordinate of a scatterer is proportional to the coordinate along its length direction in space.At such a case,the ISAR image can be rescaled and the target length can be estimated at the same time.Simulation results demonstrate the validity of received signal model,the effectiveness of the cross-range scaling and length estimation algorithms,and the effects of measurement errors on estimation accuracy.For shorebase-shipborne bistatic ISAR imaging of ship targets in the presence of high sea state,the received signal model when both the target and receiver ship have 3D nonstationary rotations is established and the complex ISAR image expression of the target and the PSF are derived.The effects of receiver ship's rotations and bistatic coefficient(bistatic angle)on the ISAR image are analyzed theoretically.After that,receiver ship's rotational motion compensation algorithm and imaging algorithm for shorebase-shipborne bistatic ISAR imaging of ship targets in the presence of a high sea state are proposed.Simulation results verify the effects of receiver ship's 3D rotations on imaging,and demonstrate the effectiveness of the rotational motion compensation and imaging algorithms.Afterwards,the effects of phase synchronization errors on the ISAR imaging results of shorebase-shipborne bistatic ISAR in the presence of high sea state are analyzed.On the basis of the received signal model when both the target and receiver ship have 3D nonstationary rotations,the received signal model in the presence of phase synchronization errors is established.After that the complex ISAR image expression of the target and the PSF are derived and the effects of phase synchronization errors on motion compensation are analyzed.Combined with the complex ISAR image and PSF,the effects of image distortions induced by the joint action of the synchronization errors,the rotations of the target and the rotations of the receiver ship on imaging results are analyzed quantitatively.Both theoretical analysis and simulation results prove that when the accuracy of the frequency oscillator meets some certain conditions,image distortions induced by the joint action of the synchronization errors,the rotations of the target and the rotations of the receiver ship on imaging results can be ignored.Finally,GEO-shipborne bistatic ISAR imaging of ship targets in the presence of high sea state is studied.According to the characteristics of GEO radar's orbital motion,target and receiver ship's 3D rotations,the received signal model is proposed and established,and the complex ISAR image expression of the target and the PSF are derived.The effects of GEO radar's orbital motion,receiver ship's rotations and bistatic coefficient(bistatic angle)on the ISAR image are analyzed theoretically.After that,GEO orbital motion compensation algorithm,receiver ship's rotational motion compensation algorithm and imaging algorithm for GEO-shipborne bistatic ISAR imaging of ship targets in the presence of a high sea state are proposed.Simulation results verify the effects of GEO radar's orbital motion and receiver ship's 3D rotations on imaging,and demonstrate the effectiveness of the GEO orbital motion compensation,receiver ship's rotational motion compensation and imaging algorithms.