Research On Key Techniques Of Moving Target Imaging With Distributed SAR

Earth observation with spaceborne synthetic aperture radar(SAR) is an important approach to accomplishing resource exploration, environment protection, disaster warning, territorial defense, etc. Recently, as the demand on development of spaceborne earth observation grows from mono-task to multi-task, the small satellite technology based distributed SAR has been proposed. Distributed SAR is a multi-antenna radar system with transmit- and receive-antennas located on different satellites which fly in a certain formation configuration and cooperatively work. This system can survey the ground in full-time and all-weather, as well as penetrate the ground with some bands. Furthermore, it can accomplish multiple tasks with the complementary information obtained from multi-baseline, multi-perspective, multiband, and multi-polarization. Among the tasks, moving target location, speed measurement and high resolution imaging are significant for civil and military use including emergency rescue and disaster relief, traffic monitoring, battlefield reconnaissance. Continuous observation of ground moving target with distributed SAR is conductive to improve the motion information acquirement and imaging resolution. However, it faces the problems including spatial synchronization arising from satellite attitude jitter, low accuracy of parameter estimation for moving target, the contradiction between imaging resolution improvement and sidelobe increase. These problems can be abstracted as parameter estimation and beamforming issues for array signal, by establishing the array signal model of the echoes received by distributed SAR regarded as a sort of antenna array. In this dissertation, the research on key techniques of moving target imaging with distributed SAR based on polynomial phase array signal(PPAS) will be achieved, with the main contents and contribution listed as follows:(1) We establish the array signal model of echo received by distributed SAR, and design the formation configuration. We fit the echo of distributed SAR with second-order PPAS, named bistatic extended space-time model(BESTIM), and then do the optimal design of array formation according to minimum model error criterion, to lay the foundation of the following methods.(2) We solve the spatial synchronization problem by calibrating satellite attitude jitter based on PPAS. We first analyze the phase error arising from the attitude jitter, and then model the error using high-order PPAS based on BESTIM, next propose a method of high-order instantaneous moment-multiple signal classification(HIM-MUSIC) to calibrate phase error, finally demonstrate the proposed method overweighs conventional non-parametric auto-focus methods, with imaging results.(3) We propose two joint space-time-frequency methods of parameter estimation for moving target with distributed SAR based on BESTIM. First, we propose fractional Fourier transform- space-time-frequency distribution(Fr FT-STFD) to estimate the range velocity and initial azimuth, then we propose fractional Fourier transform-cross Spectrum multiple signal classification(Fr FT-CSMUSIC) to estimate the cross-range velocity. The proposed methods can solve the multi-channel signal processing problem for non-linear configuration distributed SAR, and have better performance than conventional velocity synthetic aperture radar(VSAR), mono-static Fr FT, etc.(4) We propose a super-resolution imaging method named fractional range Doppler algorithm spectrum synthesis(Fr RDASS) for distributed SAR based on BESTIM, to improve the image resolution and simultaneously suppress the sidelobe; and then we simulate the raw data of distributed SAR, with which we demonstrate that the proposed method can obtain better image performance than conventional range Doppler algorithm(RDA).This dissertation aims at accomplishing moving target imaging to improve the image performance with the current measurement and control precision of distributed SAR system, by proposing novel methods of satellite attitude jitter calibration, moving target parameter estimation and moving target imaging, under the PPAS framework. This research is not only helpful to exploit enhance the system function, but also useful to other application fields such as synthetic aperture sonar(SAS), ultrasonic sensor array, towed linear sonar, etc.