【作者】 李建勋；

【导师】 柯熙政；

【作者基本信息】 西安理工大学 ， 微电子学与固体电子学， 2009， 博士

【摘要】 脉冲星是一类具有超核密度、强引力场、强电磁场的快速自转的中子星,被广泛用于天体演化、引力波探测等前沿课题的研究。其最重要的观测特征是在射电、红外、可见光、紫外、χ射线和γ射线频段具有极其稳定的周期性脉冲辐射,辐射信号的长期稳定度可与原子钟相当,用于精确定时和导航的潜力巨大。相比于卫星导航系统和地基探测网的昂贵维护成本,有限作用区域,脆弱抗击毁能力,脉冲星可为近地空间、深空领域的人造卫星、飞船、探测器等提供时间、姿态、位置和速度信息,有望实现自主、安全、高精度、高性价比的三维导航服务,这不仅具有重大军事意义,也适合人类深空探测的导航需求。相对于射电脉冲星,以X射线脉冲星作为导航源有助于探测器的小型化和信号检测。因此,近几年来,基于X射线脉冲星的导航技术引起了各国的极大关注。然而,该新型技术目前仍处于理论探索和可行性验证阶段,系统的真正实现尚面临众多挑战,尤其在国内,实质性的研究工作才刚刚起步。基于此,本论文从利用X射线脉冲星信号实现航天器定位功能的过程和系统框架着眼,涉及以下紧密联系、不可分割的几个方面：X射线探测器选择以及X射线光子探测、到达时间(TOA)测量、星源信息库建立、空间光行时方程求解、导航算法实现等,对其中的一些关键技术进行了理论分析和仿真,对所需的数学原理进行了必要的探索和推导。由于高精度的脉冲到达时刻信息是脉冲星导航的基础,因此,本文重点研究信号的TOA估计与其定时性能分析,以及可行的定位算法。论文的研究工作主要包含以下几方面：1、从深空探测需求、卫星导航的局限性、天文导航的优势出发,讨论了研究脉冲导航技术的必要性,分析了脉冲星导航的特点和优势,特别展望了脉冲星导航在行星际自主编队航天器设计中的应用前景,总结了国内外研究现状,简述了该技术目前面临的若干难题。2、比较了多种x射线探测器的性能,并参考了各国X射线观测卫星上使用的探测设备,提出以硅微条探测器和量热器作为探测器原型用于X射线脉冲星导航的观点,并据此设想了一个包含有编码盘、准直器等辅助设备的综合探测器系统,探讨了在空间核辐射环境下,探测器微电子器件的防护问题。该部分工作对探测器的选择和复合探测器的设计具有一定的参考价值。3、从X射线脉冲星导航的需求和特殊性出发,提出了应纳入星载数据库框架的脉冲星特征参数,包括位置、距离、流量、周期、有效脉冲比、脉冲轮廓等,并对上百颗X射线脉冲星的流量、周期变化率等特征进行统计分析,探讨了毫秒级X射线脉冲星在导航中的优势,依据周期变化率参数推荐了性能较好的15颗X射线脉冲星,为数据库的构建打下了一定基础。4、提出了基于非齐次泊松模型的X射线脉冲TOA的最大似然估计,给出了其低信噪比下的近似表达,并利用B1921-24和Crab两颗脉冲星,分别在不同观测时间和不同信噪比的条件下进行了仿真,验证了该方法的可行性和有效性。另外,系统探讨了基于脉冲轮廓特征的TOA精度分析,并利用拟合的具有解析形式的高斯脉冲、双高斯脉冲、指数型脉冲等进行了对比研究。理论分析和仿真结果表明,在相同的流量和占空比假设下,不同的形状特征得到的TOA估计性能不同。为了精确化距离估计误差,作者认为为每颗脉冲星建立解析的轮廓模板有一定意义。5、基于二阶循环平稳过程,提出了一种称为最大相关方差搜索法的射电脉冲星周期估计方法,若不考虑探测器的光子接收过程,该方法完全可推广到X射线脉冲星的周期估计。分析和仿真表明,新方法运算速度快,对数据量要求不高,效果明显,对于微弱脉冲星信号的实时周期估计具有参考价值。同时,提出了一种标准轮廓生成的新方法,不需要事先设计模板脉冲,在低信噪比以及累积脉冲个数较少的情况下,可以得到高品质的标准脉冲轮廓。6、深入讨论了脉冲星信号的时间测量步骤,推导了观测时间向太阳系质心传递的高阶广义相对论修正模型,修正项包含Roemer延迟、Shapiro延迟效应等。在特定空间点,数值分析了简化模型中各项时延的贡献。7、发展了一种基于定时模型的位置误差修正迭代法,给出了线性化形式,并对误差源进行了讨论。另外,改进了绝对定位中的一种模糊度解析方法,假设已知航天器的粗略位置信息和足够稳定的星钟时间,通过4颗并逐步加入更多的大周期脉冲星求解相位观测方程,利用定时或定位精度作为阈值,可由最大似然法估计出一个最可能的整周数取值集合。定位仿真结果表明了算法的可行性和有效性。更多还原

【Abstract】 Pulsars are a unique class of neutron stars that spin rapidly and stably, being of super-nuclear density, strong electro-magnitic field and strong gravitational field. They have been broadly used in the study on celestial bodies evolution, gravitational wave detection and many other frontiers. The most attractive observation characteristic of pulsars is their pulsed emission with a highly stable repetition rate in the radio, infrared and higher energy regions of the electromagnetic spectrum. The period between pulse peaks is equal to the spin period of the pulsar, whose timing stability rivals that of conventional atomic clocks, thus having tremendous potential in timing and navigation. Compared to the satellite navigation system and the earth-based tracking network that have restrictions of huge maintenance cost, limited coverage area and vulnerable anti-destroy ability, pulsars can determine time, attitude, position and velocity information for satellites, spacecrafts and explorers flying in near-ground space and deep-space, and demonstrate the potentiality of providing 3D navigation service with autonomous, safe, precise and high price-performance ratio properties, whcih not only is of important value in military, but can suit the demand in deep space detection. For X-ray pulsars are advantageous when considering the detector size and signal detection compared to radio pulsars, in the last few years, the X-ray pulsar-based navigation (XNAV) technology is having been paid close attention by many countries. However, the new technology is still on the stage of theory exploration and feasibility validation with many challenges to be handled. Especially, the substantive research has just started in China. Based on the background, the thesis commences on the system framework and the process of using X-ray pulsars to implement vehicle positioning, involving the following aspects:selection of X-ray detectors and detection of X-ray photons, measurement of time of arrival (TOA), establishment of pulsar resources database, derivation of light-time equations, design of navigation algorithms and so on. Some of key technologies are analysed in theory and simulation, and several required mathematics principles are explored and deduced appropriately. Since the precise TOAs information of received pulses is the foundation of the pulsar navigation, the thesis places emphasis on the research about TOA estimation and its accuracy analysis, and feasible positioning algorithms.The main work presented in the dissertation can be summarized as follows:1. Beginning from the needs of deep space explore, the limits of satellite navigation system and the advantages of the celestial navigation, the necessities of the pulsar navigation are discussed. The specialties and merits of this technology are analysed and especially, the application prospect of the autonomous formation flyer orbiting Mars or other target bodies is expected. Meanwhile, the development of XNAV is reviewed and some of challenges existed are described.2. Through comparing the performances of several kinds of X-ray detector and refering to the style of instruments on X-ray observation satellites owned by different countries, the view that silicon-microstrip detector and calorimeter should be taken as detector prototype for XNAV is proposed. Based on this consideration, an integrated detector system with the coded-aperture mask and collimator is conceived and the protection technologies for microelectronic devices under nuclear radiation environment are discussed. All of these works are of reference value for detector selection and composite detector design.3. Starting from the demands and particularities of XNAV, Many characteristic parameters of pulsars including position, distance, flux, period, pulse profile and so on that should be bright into onboard database are proposed, among which, flux and 1st period derivative of more over 100 X-ray pulsars are analysed statistically. Based on the analysis,15 pulsars are recommended as navigation resources for their good performance when considering comprehensively both the period stability and the high flux. Additionally, the navigation advantages of X-ray pulsars with period on the order of several milliseconds are discussed for reason of their excellent timing ability. This work may help to establish the database in engineering effectively.4. The ML estimate for pulse TOA based on nonhomogeneous stationary poission process is proposed and its approximate expression under low SNR is presented. The feasibility and effectiveness are verified using pulsar B1921-24 and Crab respectively on two conditions of defferent observation time and different SNRs. Moreover, the estimation of TOA accuracy considering the specific shape of the pulse is proposed, and a comparative study among different types of pulse which is in analytical form got by fitting the observed pulses, including the gaussian pulse, the double-gaussian pulse and the exponential pulse, is performed. Theoretical research and simulation experiment results show that the error of TOA measurement depends tightly on the shape of pulses. So to establish an analytical profile template for one pulsar is necessary to ensure the accuracy estimation of TOA more exact, thus improving range estimate.5. A novel method called maximum correction variance search algorithm to estimate the period of radio pulsar signal is proposed based on the second-order cyclostationary process, which is totally applicable to X-ray pulsars if the receiving process for photons is ignored. Analysis and simulation results indicate that the method is of low computation complexity, loose demand for data quantity but obvious effect, which can be popularized in real-time period estimate for weak pulsar signal. Meanwhile, a new profile cumulation method using Wavelet-Modulus-Maxima correlation information is proposed, which don’t need to estimate one approximate pulse template and can get a standard profile with high quality even under the condition of low SNR and less pulses cumulated.6. The steps of TOA measurement for pulsars are discussed in a deepgoing way and the time transfer model to Solar System Barycentre is provided with high-order general relativistic corrections including Roemer time delay, Shapiro time delay and other delay effects, whose numerical contributions to the simplified light-time model are computed and evaluated for some space position.7. An iterative error correction method for spacecraft position is developed based on pulsar timing model and the linear form of the position offset equation is evolved. Modeling errors and other factors influencing the positioning error are also discussed. Additionally, An improved method for the ambiguity resolution in absolute positioning was put forward when the internal clock of the spacecraft can be considered stable enough to be a valid reference and a previous coarse knowledge of the spacecraft position is acquired. Taking four pulsars with larger period as the initial estimation set, additional pulsars are substituted in observation equations one by one. Using known variance of TOA or range estimate as the threshold, the technique can eliminate a great number of possible ambiguity points, and finally find the 3D location of the spacecraft by MLE. Simulation results of positioning for single point and satellite trajectory demonstrate its feasibility and effectiveness.更多还原