Study On Very High-resolution Wide-swath Spaceborne SAR Imaging

High-resolution wide-swath spaceborne syntheti aperture radar(SAR) has significant value and wide application foreground. The feature extraction ability and accurate characterization ability of radar can be greatly promoted by the enhanced resolution, thus the value of SAR image in each domain can be greatly promoted too. The efficiency of remote surveying and mapping to large scene can be greatly promoted by the enhanced swath width. So many researchers have joined in the research of high-resolution wide-swath spaceborne SAR, and many products have been proposed. But there are still some questions need to investigate and solve. For example, in spaceborne SAR, the satellite moves around the earth, while the earth is rotating itself form west to east. When the satellite moves to different latitude, the ground tangential velocity changes accordingly. This is a complex geometry. When the resolution is low, the spaceborne curved orbit can be approximated by the straight line model. But when the resolution is high enough, the spaceborne curved orbit cannot be approximated by the straight line model anymore. So the traditional hyperbolic range model(HRM) and the imaging algorithms based on HRM are not valid any more. For this problem, aiming to satisfy the needs of high resolution and wide swath, this dissertation proposes two range models, i.e., the equivalent acceleration range model(EARM) and the squint equivalent acceleration range model(SEARM), which improve the HRM model to meet the need of very high resolution. Based on these two range models, the velocity scaling algorithm(VSA) for very high resolution spaceborne sliding spotlight SAR and the full-aperture imaging algorithm for very high resolution spaceborne squint sliding spotlight SAR are proposed respectively. Then together with the milti-input multi-output technique, the very high resolution and wide swath spaceborne MIMO SAR imaging is studied.Specifically, the main content of the dissertation is summarized as follows.1. The basic theory of spaceborne SAR imaging is firstly introduced, which includes the geometry relationship of spaceborne SAR, some basic special terms, the conception and expression of two dimensional resolution, and the principle of minimum antenna area. Then the sliding spotlight mode, which is frequently discussed in this dissertation, is briefly introduced. The geometry and time-frequency relationship of sliding spotlight mode are analyzed. Two kinds of imaging algorithm for sliding spotlight mode, i.e., the imaging algorithm by sub-aperture and that based on deramping, are discussed.2. In spaceborne sliding spotlight SAR, the satellite moves around the earth, while the earth is rotating itself form west to east. When the satellite moves to different latitude, the ground tangential velocity changes accordingly. When the resolution is low, the spaceborne curved orbit can be approximated by the straight line model, and the imaging algorithms based on HRM are valid. But when the resolution is high enough, the spaceborne curved orbit cannot be approximated by the straight line model anymore. So the traditional hyperbolic range model(HRM) and the imaging algorithms based on HRM are not valid any more. For this problem, this dissertation proposes a equivalent acceleration range model(EARM), which can precisely take into account the spaceborne curved orbit. Then based on this new range model, the velocity scaling algorithm(VSA) is proposed, which make a azimuth time scaling according to the moving rule of the satellite, increase the azimuth time interval when the satellite is faster and decrease the azimuth time interval when the satellite is slower, then the satellite can be thought moving with a constant equivalent velocity in the new azimuth time domain. In other words, the azimuth variance of equivalent velocity is eliminated. The VSA algorithm can meet the need of very high resolution spaceborne SAR. At last, the results of the simulation validate the effectiveness of new range model and imaging algorithm.3. Differing from the usual sliding spotlight mode, in the squint sliding spotlight mode, the raised squit angle brings about new problems. For example, the azimuth variance of the range history is larger, and the coupling of range dimension and azimuth dimension is more serious. For this problem, the EARM model proposed before is extended to the squint equivalent acceleration range model(SEARM), which can precisely take into account the squint spaceborne curved orbit. Then based on this range model, a full-aperture squint imaging algorithm is proposed, which combines the ideas of two-step method and the range migration algorithm(RMA), and is improved according to the expression of SEARM. The proposed algorithm can handle the azimuth variation of the equivalent velocity and the range variation of the 2-D frequency spectrum. At last, the results of the simulation validate the effectiveness of new range model and imaging algorithm.4. Combining the velocity scaling algorithm for very high resolution spaceborne sliding spotlight SAR and the multi-input multi-output technique, high resolution and wide swath can be realized. Firstly, the signal model of the very high resolution and wide swath spaceborne MIMO SAR is established. Then the whole imaging flowchart is discussed, which includes the ambiguity suppression in azimuth dimension, the and bandwidth synthesis in range dimension, and the subsequent imaging processing. In the procedure of bandwidth synthesis, high level virtual image appears due to the frequency aliasing of adjacent sub-bands. For this problem, this paper proposes an optimized linear FM transmit waveform, which can suppress the virtual image level greatly. Besides, to provide reference for engineering realization, three typical high-resolution wide-swath structures are analyzed contrastively from different angles.