【作者】 张祥坤；

【导师】 姜景山； 张云华；

【作者基本信息】 中国科学院研究生院（空间科学与应用研究中心） ， 计算机应用技术， 2007， 博士

【摘要】 合成孔径雷达(SAR)是一种可以工作在全天候、全天时条件下,能够进行目标区域高分辨率微波成像的雷达。高分辨率一直是SAR研究的重要目标。经过条带SAR和聚束SAR的发展,SAR成像分辨率不断提高,但由于这两种模式的雷达是在直线路径上观测目标区域,限制了空间频率范围的扩展,进而成像分辨率受到局限。圆迹SAR利用圆周路径突破直线路径观测对空间频率范围的限制,从而实现在条带SAR和聚束SAR这两种直线SAR模式基础上继续提高雷达成像分辨率的目标。本文重点研究了圆迹SAR高分辨率成像的相关理论和技术问题,并从空间分辨率和空间频率范围的对应关系入手,从信号处理的角度讨论SAR高分辨原理。以一维的利用雷达信号带宽的距离向成像和利用合成孔径的方位向成像的基本原理的讨论作为开始,进而针对条带SAR与聚束SAR的构成合成孔径的不同点,指出聚束SAR在观测过程中利用波束始终照射成像区域从而获得较长的合成孔径,进而使得方位向空间频率范围大于依靠波束宽度获得合成孔径的条带SAR的方位向空间频率范围,最终获得较条带SAR方位向分辨率高的成像结果。文中总结了直线SAR的信号带宽决定距离向分辨率,合成孔径长度决定方位向分辨率的规律。对比直线SAR成像,分析圆迹SAR在距离向和方位向都获得更高分辨率的原理,并针对圆迹SAR与计算机断层成像(CT)的异同,借鉴CT成像的滤波逆投影法展开对圆迹SAR成像机理及算法的研究。通过时域相关法、逆投影法以及分段直线近似圆周轨迹法的算法实现,发现圆迹SAR成像的特点:在圆迹SAR全视角(360o)观测的条件下,对于聚焦区域内的理想点目标,其距离向和方位向的分辨率基本相同;圆迹SAR分辨率取决于信号波长,波长越短分辨率越高;圆迹SAR在最大信号频率不变时,信号带宽增加反而降低分辨率,并且信号带宽的作用更大程度上表现在降低副瓣上。圆迹SAR的圆周合成孔径使其能够在信号为单频的情况下也能对目标区域高分辨率成像,这种能力是其他形式的SAR所不具备的。论文在对圆迹SAR单频成像进行理论分析的基础上,研究信号带宽对圆迹SAR成像的影响,从而从理论上证明圆迹SAR成像特点的正确性。圆周合成孔径还使得圆迹SAR具备了三维分辨能力,这也是普通合成孔径雷达所不具备的,文中分析了圆迹SAR的这种特殊能力。另外,文中还考虑了圆迹SAR的降低副瓣、运动补偿、动目标特征等与其性能相关的问题。最后给出圆迹SAR的地面实验方案,阐述了临近空间、星载平台应用的可行性。更多还原

【Abstract】 Synthetic Aperture Radar (SAR) is a kind of radar in microwave remote sensing which can obtain high-resolution image of target area with the capability of all-weather and all-time operations. High resolution imaging is one of the important goals during the development of SAR. With the advancement of Stripmap SAR and Spotlight SAR, the resolution of SAR imaging is continuously improved. However, because of the straight observing path of Stripmap SAR and Spotlight SAR, the extent of spatial spectrum is limited, and thus the resolutions of both range and azimuth are restricted. Circular SAR breaks up this restriction on the extent of spatial spectrum with circular observing path and realizes the objective of improving the resolution further.The dissertation focuses on theoretical and technical problems of high-resolution Circular SAR. According to the relationship between spatial resolution and the extent of spatial spectrum, the principle of high resolution imaging of SAR is discussed from the perspective of signal processing. The discussion about resolution in range and azimuth makes a beginning for understanding SAR. From the difference of constructing the synthetic aperture between Stripmap SAR and Spotlight SAR, the dissertation explains the reason why Spotlight SAR gets finer resolution in azimuth than that of Stripmap SAR. For Stripmap SAR and Spotlight SAR, wider azimuth spectrum leads to higher azimuth resolution while range resolution keeps almost invariant. The dissertation summarizes the conclusion that signal bandwidth determines the range resolution and length of synthetic aperture determines the azimuth resolution in linear SAR.Based on the analysis of linear SAR, the principle of Circular SAR getting higher resolution in both directions is examined. Using Computerized Tomography (CT) and its Filtered Backprojection algorithm as references, the imaging mechanism and algorithms of Circular SAR are studied. The procedures and results of algorithms of Time Domain Correlation, Backprojection and Circular path approximated by linear subpaths are demonstrated. The characteristics of Circular SAR imaging are observed through the study of imaging algorithms as follows:under condition of ideal point target in focusing plane with full aspect angle(360o) observation, the resolutions of range and azimuth obtained by Circular SAR are almost equal. The resolutions are determined by the wavelength of signal rather than bandwidth of signal. When the maximum frequency of signal is fixed, the increase of signal bandwidth decreases the resolution and the main effect of signal bandwidth is to reduce the sidelobe.Circular synthetic aperture makes Circular SAR with single frequency signal possess the ability to get high-resolution image of target area. Theoretical analysis of this ability provides a good basis for deriving the formulation of system response of Circular SAR with certain signal bandwidth. Therefore, the characteristics of Circular SAR imaging are verified from the aspect of mathematics. Another ability of Circular SAR is to distinguish targets with different heights. Compared with linear SAR, these particular features of Circular SAR are studied.The problems of sidelobe reduction, motion compensation and moving target imaging are also considered in the dissertation. In the end, the experiment schemes for testing the performances of Circular SAR are presented and the applications on near space platform and space platform are discussed.更多还原