【作者】 刘震昆；

【导师】 黄顺吉；

【作者基本信息】 电子科技大学 ， 通信与信息系统， 2008， 博士

【摘要】 分布式SAR卫星是近年才发展起来的。与单基SAR相比，它有许多明显的优势：如生存能力大大提高，基线组合和工作模式灵活多样，对目标的分类和识别有显著改善等等。因此分布式SAR卫星已成为目前研究的热点。我们教研室也适时开展了“分布式卫星SAR系统总体技术研究”的863课题。为保证分布式SAR卫星的成像质量，系统对单星姿态和编队相对运动的测量提出了更高要求。SAR卫星处于高动态环境中，对测量的精度和实时性要求较高。本文围绕如何提高测量精度和实时性展开了研究。GPS载波相位技术高动态应用的难点主要在整周模糊度和周跳的快速解算，这是影响实时性的主要因素。为了提高测量精度，在CDMA体制的导航系统中采用超长码已成为趋势，故伪码的快速捕获也成为影响实时性的重要因素。就SAR卫星本身而言，空间摄动是影响姿态和编队相对运动的主要因素。本文主要研究如何利用GPS技术精密确定单星姿态和利用类GPS技术精密确定编队相对运动，以及整周模糊度和周跳的快速解算，GPS（类GPS）信号的快速捕获等关键技术问题。在前期研究的基础上，本文进一步探讨了主要摄动因素对卫星运动的影响，使研究更具实际应用价值。本文首先简要回顾了近年来GPS和类GPS测量技术的现状及发展。接着全面介绍了表达卫星姿态的几种方式和GPS载波相位技术测量卫星姿态的基本原理。然后介绍了GPS观测量及误差源。第三至七章是本文的重点，主要内容如下：1．第三章在考虑SAR卫星典型姿态控制方式和重力梯度力矩的影响下，构建了较准确的卫星受摄姿态动力学模型。然后采用极大似然估计卡尔曼滤波算法解算卫星姿态，最后进行了仿真，验证了算法的正确性。2．第四章深入分析了地球扁率J₂摄动项对卫星编队的影响，总结出了J₂项对编队卫星相对距离的影响规律。并指出，对于分布式SAR卫星这样对相对定位精度要求很高的应用而言，必须进行控制，使编队稳定地保持在要求的精度范围内。在类GPS技术确定卫星相对运动的研究中，采用了改进的采样卡尔曼（UKF）算法并结合平方根滤波解算其相对运动。最后进行了计算机仿真，与标准扩展卡尔曼（EKF）算法和基本UKF进行了比较，结果证明了该算法的优越性。3．快速解算整周模糊度是高动态下GPS测量的关键，第五章提出了三差法辅助快速求解整周模糊度的新算法。该算法利用三差辅助加快模糊度浮点解的解算，并提高了浮点解的精度，同时提出了全新的降维去相关算法，克服了整数高斯变换中可能的去相关失败，最后采用了高效的模糊度搜索策略。与著名的LAMBDA算法相比，结果表明该算法具有更高的可靠性和实时性。4．周跳的快速检测与修复是高动态下GPS测量的又一关键问题，到目前为止并没有一个普遍适合的算法。第六章提出了利用三差观测量和基线长度约束快速检测修复周跳的算法。理论分析和仿真结果表明，该算法能适用于高动态环境，可检测到1～2周的小周跳。5．GPS信号（或类GPS信号）的捕获是一切基于GPS（或类GPS）测量的前提。在测量中出现周跳又没有及时修复，则必须重新捕获信号进行新的观测。随着CDMA体制导航系统的发展，为提高精度而使用超长码成为一个趋势。在SAR卫星处于高动态环境下，因而伪码的快捕也成为影响实时性的重要因素。第七章主要研究了如何快速捕获伪码。本章提出了基于快速Walsh变换的相关新算法。该算法从伪随机编码理论出发，提出了复合码的子码同步移相算法，从而解决了复合码无法直接利用快速Walsh变换提高相关运算速度的难题。性能分析表明，该算法能显著缩短相关运算时间。本章还提出了改进自动控制多级门限的新算法。理论分析和仿真结果表明，该算法在判决捕获时，虚警概率很低而检测概率很大，这与用恒虚警算法相比是个显著的优势。该算法还允许预置较多门限，能精确调整门限，同时又不会明显增加捕获时间，因此也增强了伪码捕获的实时性。最后对全文进行了总结，并对下一步研究提出了建议。更多还原

【Abstract】 Recently, the distributed SAR satellites develop rapidly. Compared with the single SAR satellite, the distributed SAR satellites have many advantages: The viability has been enhanced obviously; The combination of baselines and models of observation are flexible; The target identification ability is improved. So the distributed SAR satellites have already become a research focus. Our research lab also starts the 863 project of the "Research on the Overall Distributed Satellites SAR System Technique ".In order to ensure the image quality of SAR, higher precision of the attitude and the formation movement of SAR satellites are required. All measurement must be performed in high dynamic environment, so the real-time of the measurement must be improved. Our research is developed around these difficulties. The emphases of GPS phase measurement include integer ambiguity resolution and cycle slip, which are the primary effect factors of precision and real-time. In order to improve precision, a long PN code has been uesd. So the capture of long PN code will effect the real-time. To SAR satellites themselves, the disturbing force is the primary factor which effects the attitude and the formation movement. Our research includes: how to precisely measure the attitude and the relative movement of SAR satellites. Integer ambiguity, cycle slip, capture of GPS signal （GPS-like signal） are also researched. On the basis of the prophase work, disturbing force is researched, so the paper has more engineering sense.The development of GPS （GPS-like） measurement technique in recent years is reviewed in this paper firstly. Then the measurement principle and the several describing manners of satellite attitude are discussed completely. The observables and the error sources in GPS measurement are introduced. The primary content is in the third chapter to the seventh chapter. The primary contents are as follows:1. In chapter 3, the typical attitude control mode and the gravity grads moment of SAR satellite are researched, and the disturbed attitude dynamics mode is established. The Maximum-likehood Kalman filtering is proposed to resolve the attitude. The simulation shows the advantage of the proposed algorithm.2. In chapter 4, J₂ disturbing force which effects the SAR satellites formation is researched. The relative range change rules are summed up, and the magnitudes of the change are worked out. The rules show that the appropriate correction must be adopted to ensure the SAR satellites formation to be in the need precision bound steadily. Then the improved UKF square root filtering is adopted to resolve the relative movement of SAR satellites formation. Finally, the simulation is made. The result shows the advantages of the proposed algorithm compared with EKF and UKF.3. Integer ambiguity resolution is very important to GPS measurement under the high dynamic condition. In chapter 5, a new ambiguity resolution algorithm aided with the triple difference observables is proposed. This algorithm can work out the ambiguity floating resolution faster, and improves the precision. A new decorrelation method is proposed to avoid possible sick factorization in Gauss transformation. Finally, an efficient search strategy is used. Compared with the famous LAMBDA （Least-Squares Ambiguity Decorrelation Adjustment） algorithm, the proposed algorithm has higher real-time and reliability.4. Cycle slip is another important problem to GPS dynamic measurement. There is not a common adaptive algorithm. In chapter 6, an algorithm is proposed which utilizes the triple difference observables to detect the cycle slip and corrects cycle slip by baseline length constraint. The simulation shows the effectiveness of this algorithm.5. The capture of GPS（GPS-like） signal is the precondition of GPS measurements. Along with the development of CDMA navigation system, the using of long PN code is a current. SAR satellites are under high dynamic, which the capture of long PN code will effect the real-time seriously. In chapter 7, a new fast correlation algorithm based FWT is proposed. The synchronization method of sub-code of multiple code is proposed to overcome the difficulty which blocks the use of FWT. Performance analysis shows that this algorithm can reduce correlation time greatly. In addition, a new improved algorithm, which is Auto Decision Threshold Level Control algorithm, is also proposed. Theoretics analysis and simulation show that this algorithm makes false alarm lower and the detection probability higher, and the acquisition time is reduced greatly.At last, the summarizer is made. The advice of the farther research is put forward.更多还原