Research On Shipborne Monostatic/Bistatic ISAR Imaging For Aircrafts

Inverse synthetic aperture radar(ISAR)is an all-weather,all-time,beyond the visual range detection equipment,and has the ability of imaging the moving target.Hence,ISAR has been widely applied in target tracking,target recognition and airport monitoring.Compared with conventional radar,the high range resolution is achieved by transmitting the wideband signal,while the high azimuth resolution is obtained from the Doppler frequency caused by the target's movement.Since the shipborne radar does a 3-D rotational movement with the sea wave,the ISAR echo in azimuth has higher order phase terms,and the conventional R-D algorithm suffers from defocusing problem.Hence,a better higher order phase signal processing algorithm is the premise for improving the quality of the shipborne ISAR image.On this basis,eliminating the distortion problem cause by the time-varying bistatic angle for shorebased-shipborne bistatic ISAR,and overcoming the shortage of hardware complexity for shipborne 3-D ISAR are another two topics.In this thesis,we researches on the topic of shipborne ISAR as discussed above,and the following achievements are obtained.1.The research model for shipborne ISAR,bistatic ISAR and interferometric ISAR is established and the feature of the echo is analyzed.For shipborne ISAR,we analyze the defocusing problem,and explain the limitation of the existing shipborne ISAR imaging methods based on the simulation.For bistatic ISAR,we analyze the defocusing problem and the distortion problem.By comparing the defocusing term,distortion term and the resolution cell,we obtain the conclusion that the defocusing problem can be neglected in most cases,while the distortion problem must be considered and eliminated.For interferometric ISAR,we analyze the impact factor of the precision in height direction and the non-aliasing region.Simulation results show that the interferometric ISAR has the disadvantage of hardware complex.2.To estimate and eliminate the higher order phase terms in shipborne ISAR,a novel signal parameter estimation algorithm(named RCCD)is proposed in this thesis.By improving the CPF core function and introducing the Radon transform,the proposed algorithm can estimate the chirp rate and the cubic chirp rate in one step,and is released from error accumulation.The proposed algorithm applies the amplitude modulated-cubic phase signal(AM-CPS),which is closer to real shipborne ISAR echo.Hence,the reconstructed precision is increased.Computational costs analysis shows that the algorithm has a moderate computational burden.Discuss on the cross term shows that the algorithm is not influenced by the cross-terms.By comparing with other algorithms,experiments validate the effectiveness of the method.3.The defocusing problem and the distortion problem are analyzed.A novel defocusing elimination method based on RCCD algorithm and a novel distortion compensation based on linked scatterers are proposed in this thesis.Shorebased-shipborne bistatic ISAR combines the features of bistatic ISAR and shipborne ISAR,and the Shorebased-shipborne bistatic ISAR ISAR image suffers from both defocusing problem and distortion problem.In this thesis,we establish the research model for bistatic shipborne ISAR,derive the echo,and provide the expression of defocusing term and distortion term.Our analysis shows that the defocusing problem is caused by the shipborne radar's 3-D rotations,and the distortion problem results from the time-varying bistatic angle.As for the defocusing problem,we apply RCCD algorithm to estimate and eliminate the defocusing term.As for the distortion problem,a novel distortion compensation method based on linked scatterers is proposed.By linking the scatterers in bistatic ISAR image and monostatic ISAR image from the transmitter and solving the coordinate equations for the linked scatterers,the method can estimate and compensate the defocusing term.Several groups of the simulation results validate the effectiveness of the method.4.3-D shipborne ISAR imaging is researched.Utilizing the higher order phase terms caused by shipborne radar,a novel 3-D shipborne ISAR imaging method based on single transmitter-single receiver is proposed in this thesis.For the proposed method,the range resolution,azimuth resolution,and the height resolution are obtained from the information in range,Doppler frequency in azimuth,and chirp rate in azimuth,respectively.The core of the method is to establish the relationship between signal's chirp rate and the scatterer's height coordinate.After that,the scatterer's height coordinate can be obtained by estimating the signal's chirp rate using RCCD algorithm.Compare with interferometric technique,whose height resolution depends on a large number of receivers,our method needs only one transmitter and one receiver,therefore,the hardware complexity is reduced.We also analysis the impact factor of height resolution and reconstructed precision,and result shows that there is a contraction between high height resolution and high reconstructed precision.Based on the result,an imaging interval selection method is presented.The method considers the request on height resolution,azimuth resolution and reconstructed precision,and can fit different requests by conveniently changing the weighting coefficient.Besides,we analyze the error induced from low-precision geometric data or noise,and the results show that the error is within our toleration.Finally,simulation and semi-real experiment proves the correctness of the analysis and the effectiveness of the method.