Radar Imaging Study For Space Targets Based On Space-Borne Platform

With the increasing activity in aerospace in powerful nations,there exist more space targets than before.The security of orbital spacecraft is threatened and air-defense of the homeland is challenged.Countries like America and Canada have already made the project about surveillance system for space targets.To develop our own system is extremely urgent.At present,the spatial target monitoring is mainly dependent on the ground monitoring system,to achieve more than 90% of the space target catalog.While the monitoring system based on space-based platform is still in the theoretical research and prediction stage.Due to the long distance of observation and the presence of atmospheric attenuation during the propagation of electromagnetic waves in some frequency bands,the low energy of low orbit small size targets and medium and high orbit targets are weak,which limits the ability of the ground monitoring system to monitor long distance space targets.The space-based surveillance system based on satellite platform can observe space targets without boundaries,and realize the high-precision monitoring of the remote and weak targets in the ground monitoring system,which can effectively compensate the shortcomings of land/sea-based surveillance system.The system is highly valued by military power.The radar imaging system faces serious challenges due to the high-order relative high-speed motion between space targets and space-borne surveillance platforms.In addition,the platform and the target are running in some specific orbit,subject to observation geometric conditions,the radar observation time may be shorter.It is difficult to achieve high resolution imaging and recognition in short data conditions.The space target(especially the debris)due to quality,shape and other factors exist high-speed spin phenomenon,at the same time,due to spacecraft surface material off,stripping and other reasons there are similar fragments of the orbital state,in the radar beam irradiation range,there will be a large number of group targets.In this paper,we study the imaging theory in broadband and narrowband systems for high-order and high-speed motion targets,high-spinning targets and group targets under space-borne platform.The research has important theoretical significance for the acquisition of spatial situation information,and has a wide application prospect.The main contents of the paper include:The first chapter gives an overview on the history,the current status of radar imaging in our country,and the research focus in the future.The state of the art development oversea in radar imaging is also introduced briefly.With the development of space resources,the major countries have carried out space surveillance system research.Space target imaging based on space-borne platform has put forward new challenges.The second chapter studies the basic theory in radar imaging.Combing with the pulse compression technology,high range resolution is achieved by transmitting wideband signal.After translational motion compensation,high cross-range resolution is obtained by exploiting the diversities of viewing angles of the target.The measurement model of space targets with space-borne platform is also analyzed and the imaging conditions of the following work are all within the same orbital inclination.Finally,emphasis is placed on the sparse sample and reconstruction principle of compressed sensing theory.Chapter three focuses on the range echo modulation for the space target imaging on the space-borne radar.As the space target relative to the space platform radar,there is ultra-high-speed relative motion.The range echo after de-chirp is still a linear LFM signal with unknown chirp-rate.A range compression algorithm of high precision was proposed through analysis on the relative movement between the high-speed stationary target and space-borne radar,which uses the FrFT and polynomial fitting.FrFT is applied to implement the range compression of some de-chirped echo and polynomial fitting is utilized to acquire the radial velocity varying with the slow-time.Accordingly we get the precise rotation angle for range compression.Compared with the traditional inverse synthetic aperture radar(ISAR)range compression method,the proposed algorithm accomplishes the range compression with high precision,and improves the quality of imaging.The numeric simulations testify the feasibility of the algorithm.Chapter four presents the research on space group targets imaging.Space debris often appears in the form of groups,and the radar echoes overlap each other along the range direction.Utilizing the block structure,a high resolution space debris imaging method on the space-borne radar is proposed based on the block-sparse compressed sensing.Considering the characteristics of space debris,this method can get high resolution 1-D range profile of each debris based on the block-sparse compressed sensing(CS),and get the ISAR image combined with the translation motion compensation and RD algorithm.The simulation results illustrate that the proposed method can achieve high resolution ISAR image with less reconstruction error and iterative number compared to the non-structure CS method under limited measurements.Chapter five focuses on the space debris imaging in narrowband radar system.Space targets,especially space debris,usually rotate at a high speed for the gravitation.Short data and large Doppler bandwidth in low pulse repetition frequency(PRF)system is a challenging problem in such circumstances.Meanwhile,the echo suffers from the shadow effect during the observation for the rotation.To solve the problem,we propose a novel high resolution imaging algorithm based on spase-resample.Based on the fact that space debris usually rotates several periods during the observation and the scattering field presents strong spatial sparsity,the proposed method can efficiently improve the imaging quality by constructing the measurement matrix to utilize the support data in multiple cycles.Theoretical analysis and simulation results confirm that the methodology can obtain well focused image.As space debris often appears in group,the aforementioned method was improved to solve this problem.Based on the diversity of mass,density and other factors,space debris usually rotates at different speed along its main axis.The angular velocity can be obtained through combining the auto-correlation characteristic and peak match process due to the diversity of targets' echo.We can increase the equivalent sample rate by utilizing the corresponding measurement matrix to extract the data of different debris and reconstruct the image of each debris with the rearrangement.Furthermore,we can solve the Doppler ambiguity with the measurement matrix in low PRF system and suppress energy of other debris somewhat.Theoretical analysis confirms the validity of the methodology.Simulation results illustrate that the proposed method can achieve high resolution ISAR image of space group debris separately in low PRF system.