利用参照轨道要素设计卫星编队轨道

利用球面几何方法,引入参照轨道要素概念,给出其定义,并推导了用其描述的从星相对运动方程的精确形式。在主星偏心率为零时,根据编队飞行中相对轨道倾角很小的特点,对相对运动方程进行简化并分析相对运动的特征。设计了由一颗主星和六颗从星构成合成孔径静止侦察卫星的编队应用,给出编队队形特征和主星的轨道参数,从相对运动方程计算出编队中的六颗从星的轨道要素。数值仿真结果表明:简化的方程具有很高的精度,对编队队形设计问题完全适用。     

利用参照轨道要素研究卫星星座/编队的几何特征

星座/编队的对地遥感应用要求对卫星群的空间几何特征进行量度和评价。定义了用于刻画卫星星下点球面几何特征的两个指标:平均星间角距和星下点分布面积。作为星下点分布的一维量度,星间角距可由参照轨道要素解析地表达。继而给出星下点最小凸球面多边形的判定算法;与传统方法不同,该算法无需求解星下点坐标即可判定星下点几何特征,进而计算卫星分布面积,作为星下点几何特征的二维表征。最后以四星编队任务为例,通过计算判定星下点几何,分析了编队轨道设计要素与编队几何特征的半定量关系。     

太阳帆悬浮轨道附近的编队

研究了利用太阳帆实现悬浮轨道附近编队飞行。考虑到相对运动距离与帆到太阳距离相比非常小,一阶近似得出了在主星坐标系中线性定常的相对运动方程。通过分析线性化的运动方程得出了相对运动的定性特征,给出了不同轨道附近的相对运动的稳定特性。讨论了原非线性相对运动方程的周期解的计算算法,给出了编队的初值条件。     

大偏心率远距离航天器编队飞行设计

为寻求适用于大偏心率远距离航天器编队设计的方法,摒弃了简化航天器相对运动非线性微分方程的传统思路,致力于简化该微分方程的解析解。首先,在同周期假定前提下,由运动学得到了椭圆轨道航天器相对于圆轨道航天器运动的封闭解析解,然后将其展开成傅立叶级数,证明了在特定条件下,单倍频项是最主要的,从而导出了编队飞行设计的新公式。最后以空间圆形编队设计为例,阐明了利用新公式进行编队设计的步骤,并用精确的数值计算验证了设计结果的正确性。与C-W方法和一般轨道参数设计方法相比,推导过程中并未采用小偏心率近距离假定,因此导出的新公式可适用于大偏心率远距离航天器的编队设计。     

三体绳系卫星面内编队飞行的回收控制

绳系卫星编队飞行根据构形的不同可分为三种类型:轴-辐型(Hub-and-Spoke),闭轴-辐型(Closed-Hub-and-Spoke)和串型(Series Configuration).研究了面内轴-辐型三体绳系卫星编队飞行时的最优回收控制问题.通过伪线性化和Legendre伪谱算法,将绳系卫星编队系统的非线性最优控制转化为一组线性方程的迭代求解问题.针对不同回收初值及回收初值受扰情况,计算了绳系卫星编队飞行的最优控制张力和飞行轨迹.     

椭圆参考轨道的卫星编队队形保持控制设计

以Lawden方程作为动力学模型,建立状态方程,利用线性二次型性能指标最优控制(LQR)对卫星编队队形进行保持控制。首先对近地轨道的主要摄动J2项摄动和大气阻力摄动进行描述,得到了影响编队队形的相对摄动的表达式。然后通过合理的选择控制加权矩阵R,用无约束的线性二次型优化方法解决了电推力火箭有控制约束的优化问题。仿真结果表明,这种控制方法是有效的。     

复合材料推动织女星卫星发射器

欧洲航天局(ESA)最近完成了新的织女星卫星发射器的首次飞行,这是第一个采用碳素纤维复合外壳的新型运载火箭。织女星被设计成一个小型的运载工具,对于多个发射到极地和低地轨道中用于科学和地球观测任务的欧洲卫星而言是非常经济的投入。设计工程师正在把碳素纤维复合材料(CFR)应用于卫星发射器也     

在轨卫星红外辐射周期性特征建模与计算

卫星红外特征的研究对于卫星红外探测与识别以及卫星红外隐身设计都具有重要意义。论文首先计算了卫星的轨道辐射热流,接着计算了卫星各个表面的温度随轨道运行周期的变化,在此基础上计算了卫星各表面的自身辐射的红外辐射以及反射环境的红外辐射,由此建立了计算卫星红外周期性特征的数理模型,并以某轨道上六面体卫星为例,数值计算了卫星六个不同表面在中红外和远红外两个大气窗口的周期性红外辐射特征,相关结果可为卫星红外探测识别以及红外隐身设计提供参考数据。     

小卫星领域应用电推进技术的评述

小功率电推进已成为最有技术竞争力的小卫星应用的推进系统选择。在总结美国、俄罗斯、欧洲、日本等国家离子、霍尔、电热、PPT、FEEP等小功率电推进技术发展和小卫星应用情况的基础上,从轨道转移、大气阻尼补偿、位置保持、姿态控制、编队飞行等任务方面分析了小卫星对电推进系统的需求和应用电推进的必要性及可行性,针对小卫星主要任务需求分别提出了10 W、100 W和500 W级小功率电推进技术发展和应用方面的具体建议。     

高速铁路无砟轨道不平顺分形特征分析EI北大核心CSCD

无砟轨道不平顺作为线路服役状态的直接表征,一直是高速铁路检查与维修作业的核心。为了深刻而有效地掌握无砟轨道不平顺的时空分布特征,从分形几何的基础物理含义出发,确定轨道不平顺的分形特征,比选轨道不平顺分形维数的计算方法,通过分析典型高速铁路无砟轨道不平顺的累计检测数据,讨论分形维数进行轨道区段质量管理的可行性与合理性。结果表明:轨道不平顺具有典型自相似性和标度不变性的分形特征,各种分形维数方法进行轨道不平顺计算的结果具有显著差异性,变差法计算的精度、鲁棒性好,适合进行轨道不平顺分形维数计算;分形维数可以有效地表征线路轨道服役状态逐渐恶化的趋势,受线路作业维修干扰较少。建议结合线路具体养修条件,进一步对不同线路条件下轨道不平顺分形维数特征进行深入研究。     

Attitude and orbit control for satellite broadcasting missions

This paper gives a broad introduction to the problems of attitude and orbit control of geostationary communications satellites. It specifically discusses the relationships between the satellite user’s requirements for a broadcasting mission and the design of the attitude and orbit control system. To put the subject in perspective, a brief review of past and present satellites is presented first. Then orbit control is described in terms of the forces that act on a satellite in geostationary orbit and the necessary station-keeping strategies. The design of attitude control systems for three-axis stabilised satellites is presented by considering the disturbance torques, attitude sensors and actuators and by identifying the various system problems and their solutions. Sources of error in pointing the satellite towards the earth are discussed together with the formulation of error budgets. Finally, the design approach for missions that require extremely accurate pointing is considered, and some remarks are given regarding the achievable accuracy for this class of satellite missions.     

Modelling radiation loads to detectors in a SNAP mission

In order to investigate the degradation of optical detectors of the Supernova Acceleration Project (SNAP) space mission because of irradiation, a three-dimensional model of the satellite has been developed. A realistic radiation environment at the satellite orbit, including both galactic cosmic rays and cosmic ray trapped in radiation belts, has been taken into account. The modelling has been performed with the MARS14 Monte Carlo code. In a current design, the main contribution to dose accumulated in the photo-detectors is shown to be due to trapped protons. The contribution of primary alpha particles is estimated. Predicted performance degradation for the photodetector for a four-year space mission is 40% and this can be reduced further by means of shielding optimisation.     

Rapid tooling manufacturability evaluation using fuzzy-AHP methodology

Rapid tooling (RT) processes driven by rapid prototyping (such as stereolithography and selective laser sintering) can reduce mold development lead-time by 50% or more, though there are certain limitations in terms of mold materials, accuracy, and surface finish. This paper presents a systematic approach for manufacturability analysis of molds produced by rapid tooling methods, based on three aspects: mold feature manufacturability, secondary elements compatibility, and cost effectiveness. The geometric features of functional elements of the mold (core, cavity, side core, etc.) are evaluated for manufacturability using fuzzy-analytic hierarchy process (Fuzzy-AHP) methodology. The secondary elements of mold (parting surface, ejectors, cooling lines, etc.) are checked for compatibility with RT mold properties (machinability, wear resistance, and surface evenness). Finally, the cost of RT mold is estimated using a semi-empirical model based on cost drivers and cost modifiers, and compared with that of a conventional mold. The methodology has been demonstrated with an experimental mold. It is useful not only for RT mold process selection, but also for identifying minor modifications to a mold design to improve its manufacturability and economy.     

Performance evaluation of multi-sensor data-fusion systems in launch vehicles

In this paper, the utilization of multi-sensors of different types, their characteristics, and their data-fusion in launch vehicles to achieve the goal of injecting the satellite into a precise orbit is explained. Performance requirements of sensors and their redundancy management in a typical launch vehicle are also included. The role of an integrated system level-test bed for evaluating multi-sensors and mission performance in a typical launch vehicle mission is described. Some of the typical simulation results to evaluate the effect of the sensors on the overall system are highlighted     

Tropospheric emission spectrometer for the Earth Observing System's Aura satellite

The Tropospheric Emission Spectrometer (TES) is an imaging infrared Fourier-transform spectrometer scheduled to be launched into polar Sun-synchronous orbit aboard the Earth Observing System's Aura satellite in June 2003. The primary objective of the TES is to make global three-dimensional measurements of tropospheric ozone and of the physical-chemical factors that control its formation, destruction, and distribution. Such an ambitious goal requires a highly sophisticated cryogenic instrument operating over a wide frequency range, which, in turn, demands state-of-the-art infrared detector arrays. In addition, the measurements require an instrument that can operate in both nadir and limb-sounding modes with a precision pointing system. The way in which these mission objectives flow down to the specific science and measurement requirements and in turn are implemented in the flight hardware are described. A brief overview of the data analysis approach is provided.     

PARASOL in-flight calibration and performance

Since 18 December 2004, the PARASOL satellite is a member of the so-called A-train atmospheric orbital observatory, flying together with Aqua, Aura, CALIPSO, CLOUDSAT, and OCO satellites. These satellites combine for the first time a full suite of instruments for observing aerosols and clouds, using passive radiometer complementarily with active lidar and radar sounders. The PARASOL payload is extensively derived from the instrument developed for the POLDER programs that performs measurements of bidirectionality and polarization for a very wide field-of-view and for a visible/near-infrared spectral range. An overview of the results obtained during the commissioning phase and the reevaluation after one year in orbit is presented. In-flight calibration methods are briefly described, and radiometric and geometric performances are both evaluated. All algorithms are based on a panel of methods using mainly natural targets previously developed for POLDER missions and adapted or redeveloped in the PARASOL context. Regarding performances, all mission requirements are met except for band 443 (not recommended for use). After one year in orbit, a perfect geometrical stability was found while a slight decrease of the radiometric sensitivity was observed and corrected through an innovative multitemporal algorithm based on observations of bright and scattered convective clouds. The scientific exploitation of PARASOL has now begun, particularly by coupling these specific observations with other A-train sensor measurements.