Study On The Key Technique Of Multi-channel SAR Imaging

As the main part of the imaging radar, Synthetic aperture radar (SAR) is a powerful and well-established microwave remote sensing technique that enables two dimensional (2-D) high-resolution measurements of the Earth’s surface from long distance. SAR also is a powerful and effective tool for the retrieval of geographical information because of its capability to work under all weather conditions, day and night. SAR plays important roles in military and civil applications, especially in the fields of battlefield reconnaissance, resource exploration, disaster forecast, terrestrial surveillance and deforming observation of ground surface and so on. For applications with different requirements, many imaging modes have been developed in the literatures, such as, spotlight SAR,stripmap SAR, sliding spotlight SAR, Multiple Input and Multiple Output (MIMO) SAR, and Terrain Observation by Progressive Scans SAR (TOPS SAR). With superiority in high-resolution and wide-swath (HRWS) observation, ground moving target indication (GMTI). digital elevation model (DEM) generation, interference suppression etc.. multi-channel synthetic aperture radar (MC-SAR) system has attracted much attention in radar imaging and remote sensing domain, and becomes a research focus at home and abroad.In order to achieve a wide-swath imaging, the radar transmitter pulse repetition frequency (PRF) should be low enough to avoid the serious range ambiguity for the conventional single-channel SAR system. However, to obtain a high azimuth resolution, which indicates a wide Doppler bandwidth, the PRF should be high enough to ensure that the Doppler spectrum is ambiguity-free. This also means that in the single channel SAR system, high resolution and wide swath is a pair of contradictory performance index. The emergence of MC-SAR system can solve this problem well. Combinning with the digital beamforming (Digital Beam-Forming, DBF), it can be efficiently implemented with HRWS imaging of earth observation.Several key problems and difficulties for the MC-SAR imaging are researched in this dissertation. This dissertation studies new techniques for the squint mode MC-SAR imaging in some aspects, i.e. Doppler parameters parameters estimation, azimuth signal reconstruction and migration correction.Then, some useful imaging approaches are proposed.The main content of this dissertation is summarized as five parts, which are shown as follows:(1)Doppler centroid estimation for the MC-HRWS SAR imagingIn Chapter 3,the Doppler centroid estimation apptoaches are dicussed during the MC-HRWS SAR imaging processing. For the MC-HRWS SAR system, the azimuth Doppler spectrum of single echo is ambiguous. In this chapter, a novel Doppler centroid estimation approach is proposed for the MC-HRWS SAR system, where the channel mismatch is involved. To all appearances,the conventional apptoaches of the Doppler centroid estimation are not adapted for MC-HRWS SAR imaging. For the MC-HRWS SAR, a novel maximum-likehood based Doppler centroid estimation algorithm is developed Firstly, the phase for correlation function between neighboring channels is estimated.Then, a coefficient matrix that includes channel location information and Doppler centroid information is estimated by using the relationship between the slant-rangeand phase which estimated in above step.The matrix is used to extract the Doppler centroid. In addition, the experimental results of some real MC-SAR data also show that the proposed method works well.(2)Linear RCMC in the Doppler domain for squint MC-SAR ImagingIn Chapter 4, the echo model of squint mode MC-SAR system is introduced. The imaging algorithm for squint SAR is an important problem in microwave imaging domain. Linear RCMC in time domain will bring about space variability in the azimuth. In this Chapter, a squint SAR imaging algorithm whose linear Range cell migration is corrected in azimuth Doppler domain is proposed to solve the problem brought by the linear RCMC in time domain. For this new imaging algorithm, the slant-range formula is derived to three items, and then the two-dimensional spectrum expression is derived using series reversion. After that, the squint SAR imaging algorithm is proposed based on two-dimensional spectrum matched filter. This algorithm takes into account the problem of range-variance. At the first, the linear RCMC and the pre-filter is implemented in the two-dimensional frequency domain, and then the algorithm of linear chirp-scaling is operated in time domain in range and frequency domain in azimuth. Finally, a well focused SAR image can be get after pulse pressure in two-dimensional frequency. Comparing to the range migration algorithm (RMA), the proposed algorithm can effectively reduce the data, which will be processed during the imaging processing. By processing the simulation data, the effectiveness of this algorithm is proved.(3) Azimuth signal reconstruction algorithm for squint mode MC-HRWS SARIn Chapter 5. the squint mode MC-SAR with hybrid baseline is studied for HRWS imaging. During the imaging process, due to the cross-track baseline and fluctuant terrain, the azimuth signal reconstruction is a key problem for this imaging mode. To deal with this problem, in this paper a novel azimuth signal reconstruction approach is proposed, where terrain elevation of scene is considered. At first, the pre-processing of the linear RCMC and topography-independent phase compensation is implemented in the azimuth time domain. After that, combining the azimuth echo signal characteristics, the local polynomial Fourier transform (LPFT) is utilized to obtain the coarse-focused SAR image. Then, based on joint pixel pair vector and robust Capon beamforming (RCB). a Doppler ambiguity suppression approach is proposed to reconstruct the Doppler ambiguity-free azimuth signal in LPFT frequency domain, during which the influence of the cross-track baseline component and fluctuant terrain is eliminated using the coarse digital elevation model (DEM) for the imaging scene. At last, the existing imaging algorithm for the squint mode MC-HRWS SAR is utilized to focus the SAR image. The effectiveness of the proposed azimuth signal reconstruction approach is demonstrated via simulated and real measured squint mode MC-HRWS SAR data.