【作者】 杨永红；

【导师】 皮亦鸣；

【作者基本信息】 电子科技大学 ， 信号与信息处理， 2008， 博士

【摘要】 与单基地SAR相比,双基地SAR可以获取目标的丰富信息,提高系统的抗摧毁性,抗隐身性等优点。在地形测绘、战场监视、地面运动目标侦查等领域具有广阔的应用前景。双基地SAR的收发装置位于不同的载体上;例如,飞机、卫星以及固定的高塔。星载-机载混合BiSAR(SA-BiSAR)是由卫星作为发射机,飞机作为接收机所组成的系统。卫星按照用途可分为通信、气象、侦察、导航、测地、地球资源和多用途卫星等。与导航卫星相比,由合成孔径雷达卫星组成的SA-BiSAR系统具有以下的优点:信号处理过程相对简单、易于实现、可获得高分辨率图像。虽然,其应用领域和使用范围受到卫星过境时间和照射区域的限制。但是,SA-BiSAR利用现有的卫星资源(本国的或者国外的);可实现低成本、高可靠性、高稳定性的高分辨率成像。由于卫星和飞机在速度上存在着巨大的差异;与机载/星载BiSAR相比,SA-BiSAR的距离历程呈现出不同的特点,导致了SA-BiSAR具有明显的二维移变特性(距离向和方位向)。因此,对于SA-BiSAR系统而言,在成像机理、运动误差分析、成像算法以及运动补偿等方面还需要进一步的研究。本文的主要研究内容和创新点为:1.在成像机理与回波信号模拟方面,提出了用收发平台运动轨迹在地面投影的方法来表征双基地几何的差异程度;在此基础上,推导出了BiSAR技术指标(基线距离、双基地角和距离/方位分辨率)与双基地几何之间的解析表达式。在考虑地球自转的情况下,提出了用坐标变换的方法,解决了近地空间与外太空之间距离的计算问题;实现了任意几何模型下更为精确的SA-BiSAR回波模拟。同时,利用此方法,也可获得直达波信号。2.在方位向移变特性与多普勒特性方面,研究了平飞非等速BiSAR方位向移变的问题,运用Taylor多项式展开的分析方法,推导出了方位向移变量的解析表达式,仿真结果与理论分析相吻合。同时,研究了SA-BiSAR多普勒特性。提出了用矢量法来计算多普勒中心频率和多普勒调频率的方法;通过分析收发平台波束的运动轨迹,推导出了合成孔径时间和方位向分辨率的计算公式。文中给出相应的仿真并对结果进行了分析。3.在BiSAR成像算法方面,当双基地几何为非平飞模式情况下,研究了BP成像算法的聚焦效果;当双基地几何为平飞模式,提出了一种适合于SA-BiSAR改进的RD成像算法;在PPS参数估计方面,本文提出了用HAF方法来确定初始值,并且用NM单纯形算法作为搜索方法的极大似然参数估计方法。在较低SNR的情况下,改进了算法的估计性能。4.在运动误差分析与补偿方面,提出了可以同时描述速度误差和姿态误差的几何模型。在此基础上,推导出了距离误差方程的表达式;归纳出了三种误差类型并且分析了各类误差的来源及其特点。同时,利用仿真的回波数据,进行成像处理后,表明了误差分析结果的正确性。5.在相位噪声对SA-BiSAR成像的影响方面,研究了机载SAR在静态条件和振动条件下相位噪声的变化情况。基于两个独立非同分布的振荡器模型,推导了振荡器的单边带谱密度和SA-BiSAR积分旁瓣比的计算公式。文中给出相应的仿真并对结果进行了分析。更多还原

【Abstract】 Compared to mono-static SAR, bistatic SAR propose many advantages, for example, improving object scattering factor, increasing system survival, and enhancing anti-stealth etc. It can be widely applied to terrain mapping, battlefield surveillance, and detecting moving targets. Transmitter/receiver of BiSAR can be carried by different platforms such as satellite, aircraft and even the stationary on the high tower. SA-BiSAR is a subclass of bistatic SAR, which consists of a spaceborne transmitter and an airborne receiver.According to its application fields, satellite can be divided into communicate, weather, scout, navigation mapping, earth resource and multi-useful satellites etc. Compared to navigation satellite, spaceborne SAR possesses the potential ability of forming high-resolution images, and its signal processing is relatively easy. Although its application is limited by the satellite illuminating time and region, high resolution imaging can be implemented with low cost, high reliability and stability using native or foreign satellite resources at present.Due to the great discrepancy of satellite and aircraft speed, the configuration SA-BiSAR implies an essential asymmetry, which has different range history and obvious two-dimension spatial variant properties. Hence, some aspects of SA-BiSAR are deeply studied such as imaging mechanism, image formation algorithm and motion error and compensation. The contents in this paper are as follows:1. Imaging mechanism and echo simulation are investigated. General bistatic geometry model are presented by the projection of transmitter/receiver trajectory. The variation of technology parameters with bistatic geometry is exhibited by formulae and simulations including baseline range, bistatic angle and range/ azimuth resolution. Considering with earth rotation, we solved the range problem between near earth and out space by coordinate transform method, implemented the more accurate echo simulation at SA-BiSAR arbitrary geometry model. The direct path signal can also be simulated.2. Azimuth space-variant and Doppler properties are studied. Aiming at the parallel mode, the formulae of azimuth space-variant are derived with Taylor second order expansion. Theory analysis is coincided with simulations. At the same time, we put forward to compute Doppler centric frequency and Doppler frequency rate by vector method. From footprint trajectory of transmitter and receiver, we derived the formulae of integrated time and azimuth resolution. Simulations are given and results are analyzed in this chapter.3. Imaging formation algorithms of BiSAR are investigated. Under the non-parallel mode, we evaluated the focus performance of BP algorithm. Under the parallel mode, we presented the improved RD algorithm which is adapted to the SA-BiSAR system; on the estimation of polynomial phase signal, we formulate the maximum likelihood method which is to choose the initial value by HAF method and search the loacal minimum by NM simplex method. Under the lower SNR, it improved the estimated properties.4. Motion error and compensation of SA-BiSAR are demonstrated. We formulated the geometry model including velocity and attitude errors, derived range error equation, induced the three kind errors, and analyzed these properties. With simulating echo data, the results are coincided with theoretical analysis after imaging formation algorithms.5. Oscillator phase noise impacts on SA-BiSAR image quality. We studied phase noise change when airborne SAR is at static and vibration conditions. Based on independent and non-identical distribution oscillators, the formula of integration side lobe ratio of SA-BiSAR is derived when airborne SAR is at static conditions or perturbing by random vibration. Simulations are given and results are analyzed in this chapter.更多还原