Research On Low Frequency SAR Imaging And Moving Target Detection Techniques Based On Subbands And Subapertures

With great foliage penetration capability, low frequency Synthetic Aperture Radar (SAR) is a promising technique for detecting targets concealed in forest and under camouflage. As two of popular research topics in low frequency SAR, low frequency SAR/Ground Moving Target Indication (GMTI) and low frequency bistatic SAR with their key techniques-low frequency imaging and moving target detection are facing technical challenges from the properties of large beam angle and wide bandwidth in the system. In this thesis, subband and subaperture are employed as solutions to technical issues in low frequency monostatic/bistatic SAR/GMTI, including fast and high resolution imaging, moving target detection and parameter estimation in sparse array with wide bandwidth, coordination between SAR and GMTI processing and the general configuration bistatic SAR imaging.Firstly, a model of SAR echo and time-domain imaging is set. After the analysis on generation methods of subband and subaperture, a fast Back-Projection (BP) imaging algorithm based on subband and subaperture is proposed, laying a ground work for the theoretical research and practical application in the whole thesis. The subband division is processed in frequency domain of fast time, while the subaperture division in time domain of slow time. This strategy of division benefits a lot in reducing requirement of operation number and memory size in the information processing procedure and thus facilitates real-time imaging. As for GMTI, the division of subband and subaperture could handle a coordination of resolution between SAR imaging and moving target detection. In addition, indicating different central frequency and observation time, a series of different subband and subaperture images are combined to distinguish moving target from stationary ones and estimate its parameter.Subband and subaperture Along Track Interferency (ATI) method and multi-subband Space-Time Adaptive Processing (STAP) method are proposed. The former method overcomes the problems of blind velocity, radial velocity ambiguity and azimuth/range velocity coupling arising in traditional ATI. The latter method is realized by Extended Factorized Algorithm (EFA) for dimension reduction, thus obtaining an approximately optimal performance in clutter suppression. The method can not only compensate wideband effects, but also release the angular ambiguity caused by traditional STAP when used in wide bandwidth and sparse array. As for single-channel low frequency SAR system, a subband and subaperture change detection method is proposed. Compared with traditional method, it could be applied on single-channel and single-pass data and thus has the merit of saving hardware and flying cost.Based on the groundwork of the theory study in low frequency SAR imaging and moving target detection, real data processing technology in low frequency vehicle-borne SAR/GMTI system is studied. The division and generation of subband and subaperture is applied to coordinate SAR imaging and GMTI processing. A whole, high effective procedure for SAR/GMTI processing is constructed, as well as some non-ideal factors in the real system being estimated and compensated. Real data processed result is presented with analysis.A bistatic fast BP algorithm available for the general configuration is proposed-bistatic fast BP algorithm based on subaperture elliptical polar coordinates. Without a limitation in moving track of the receiver and transmitter, or an approximation from the polynomial expanding of the phase history, this algorithm holds a precise and direct signal processing procedure, thus taking an advantage in adaptability, accuracy and realization over the frequency domain algorithm. When the receiver and transmitter areare deployed on the same platform, the bistatic system degenerates to a monostatic one, and the proposed algorithm is degenerated to monostatic fast BP algorithm based on subaperture polar coordinates. Therefore, it could be considered as a unified fast BP algorithm suitable for monostatic/bistatic case. It is applied on Space-Surface Bistatic SAR (SS-BSAR) system, embodied in a whole real data processing procedure, which consists of acquision of satellite precise position, synchronization, and range compression. The processed result using real data is presented.