Study On Ultra-high-resolution Space Target ISAR Imaging

Inverse Synthetic Aperture Radar(ISAR)can observe non-cooperative targets with high resolution imaging in all weather and round the clock effectively,which is important to obtain space target information and has been widely used in military and civil fields.In this thesis,three key problems in ultra-high-resolution space targets ISAR imaging are studied from the perspective of engineering application,including the intra-pulse error caused by high speed target movement,the phase error caused by atmospheric interference and the space-variant second-order range migration correction.The space target has a higher motion speed,and the pulse width of the ultra-high-resolution radar system is usually wider.The pulse modulation characteristics caused by the target motion make the echo signal received by the system cannot match the transmitted signal completely,which leads to the intra-pulse error.The intra-pulse errors not only cause the compression defocus of the range pulse,but also make the pulse offset along the range direction,which further affects the azimuth focusing quality of the image.In this thesis,an accurate ISAR imaging model is established to deal with the influence of the intra-pulse errors caused by the high-speed motion of space target.The ISAR echo model of high-speed target was transformed into ISAR echo model of turntable target after five steps of intrapulse error compensation,RVP correction,residual phase approximation,velocity approximation and dynamic compensation,which was used in the research of ultra-highresolution imaging algorithm.In the detection and imaging of space target,radar waves need to pass through the whole atmosphere,which is inevitably affected by atmospheric interference.Troposphere and ionosphere have the greatest interference to radio wave transmission because of their characteristics.Their influence on electromagnetic wave is mainly manifested as path bending caused by refractive index change and time delay error caused by propagation velocity decrease.To analyze the influence of tropospheric interference on ISAR imaging of ultra-high-resolution space targets,this thesis establishes the corresponding relationship between the path refractive index change and the initial elevation angle of signal transmission and the delay phase error based on the geometric model of tropospheric radar signal transmission.The simulation results show that the delay phase error caused by the tropospheric interference causes the image to be offset along the range and defocus in the azimuth.In view of the influence of ionospheric interference on ultra-high resolution ISAR imaging of space targets,this thesis presents a phase error model of ionospheric interference and analyzes the influence of different total electron concentrations on the error.It can be seen from the error model that the ionospheric interference error is inversely proportional to the radar signal frequency,while the signal frequency of ultra-high-resolution radar is generally very large.The simulation results show that the impact of ionospheric interference on the imaging of ultra-high-resolution space targets can be ignored.In ISAR imaging,the target motion is usually decomposed into translational component and rotational component.The rotational component can provide effective imaging information.Therefore,the imaging of rotational component has always been the focus of ISAR imaging,in which the correction of range migration is an indispensable step.In the traditional radar imaging,only the enveloped cross-cell phenomenon caused by first-order RCM is generally considered.At present,the resolution of ultra-high resolution ISAR system can reach the centimeter level,and the influence of high-order RCM is obvious.In this thesis,the method combining Keystone and CS algorithm is used to correct RCM.After the first-order RCM correction by Keystone algorithm,the linear migration of the signal across the range unit is corrected,and the envelopes of scattered points of the same distance cell are superimposed.However,due to the existence of high-order RCM,the envelope is still curved.CS algorithm is used to change the scale of the signal,and then the bending of the envelope is corrected,and the RCM is removed.In the process of RCM correction,it is a key step to estimate the effective rotation velocity of the target.The minimum entropy of envelopment is used as fitness criterion for optimization search to obtain accurate effective rotation velocity of target.Finally,the simulation results show that the proposed algorithm can effectively compensate the RCM of ultra-high-resolution ISAR.