Imaging Theories And Methods For Near-space Synthetic Aperture Radar

Near space refers to the airspace of 20 Km to 200 Km from the earth’s surface. Near space low-speed synthetic aperture radar(NL-SAR) has the advantages of long hang time and short revisit cycle, which is of great importance for many application areas such as reconnaissance of sensitive district and real-time monitoring of environment.The fundamental and essential problems in the imaging of NL-SAR include echo modeling, imaging mode, imaging algorithm, motion compensation and moving target detection, which are also the forefront of international research. The contents of the paper are as follows:(1) The echo model for NL-SAR is built and the range cell migration, Doppler, range-azimuth coupling and spatial variation characteristics are analyzed, which provide theoretical basis for the research of imaging modes and imaging algorithms.(2) The joint aperture imaging mode is proposed, which solves the problem of slow imaging for NL-SAR with long synthetic aperture time. This imaging mode is also a new way to achieve wide-swath imaging for NL-SAR.(3) The along-track motion compensation method based on NUFFT is proposed and the homogenization in frequency domain is realized by the scaling of the heterogeneous samples in time domain. The proposed method solves the problem of azimuth main-lobe widening and side-lobe rising induced by the along-track velocity variation in NL-SAR.(4) The imaging algorithm based on extended wave-number domain processing is proposed. The linearization in 2-D spatial domain of the 2-D point target reference spectrum with constant acceleration motion error and the spatial variation removal are inserted into the wavenumber processing. This method can effectively solve the problem of 2-D spatial variance under the condition of constant acceleration for NL-SAR.(5) The moving target detection method based on Doppler parameter equalization and FrFT is researched. The difference of Doppler FM rates between the moving target and the stationary targets after range cell migration are utilized to distinguish the moving target echo form the stationary target echo. Thus, the moving target imaging in single channel squint mode are realized for NL-SAR.The simulation results verify the effectiveness of the proposed theory and algorithms, which can be used in NL-SAR to achieve effective imaging with high precision.