Integrating BDS and GPS for precise relative orbit determination of LEO formation flying

Low-Earth-Orbit(LEO) formation-flying satellites have been widely applied in many kinds of space geodesy. Precise Relative Orbit Determination(PROD) is an essential prerequisite for the LEO formation-flying satellites to complete their mission in space. The contribution of the BeiDou Navigation Satellite System(BDS) to the accuracy and reliability of PROD of LEO formation-flying satellites based on a Global Positioning System(GPS) is studied using a simulation method. Firstly, when BDS is added to GPS, the mean number of visible satellites increases from9.71 to 21.58. Secondly, the results show that the 3-Dimensional(3 D) accuracy of PROD, based on BDS-only, GPS-only and BDS + GPS, is 0.74 mm, 0.66 mm and 0.52 mm, respectively. When BDS co-works with GPS, the accuracy increases by 29.73%. Geostationary-Earth-Orbit(GEO) satellites and Inclined Geosynchronous-Orbit(IGSO) satellites are only distributed over the Asia-Pacific region; however, they could provide a global improvement to PROD. The difference in PROD results between the Asia-Pacific region and the non-Asia-Pacific region is not apparent. Furthermore, the value of the Ambiguity Dilution Of Precision(ADOP), based on BDS + GPS, decreases by 7.50% and 8.26%, respectively, compared with BDS-only and GPS-only. Finally, if the relative position between satellites is only a few kilometres, the effect of ephemeris errors on PROD could be ignored. However, for a several-hundred-kilometre separation of the LEO satellites, the SingleDifference(SD) ephemeris errors of GEO satellites would be on the order of centimetres. The experimental results show that when IGSO satellites and Medium-Earth-Orbit(MEO) satellites co-work with GEO satellites, the accuracy decreases by 17.02%.     

Presolar Grains in Meteorites and Their Compositions

Small amounts of pre-solar “stardust” grains have survived in the matrices of primitive meteorites and interplanetary dust particles. These grains—formed directly in the outflows of or from the ejecta of stars—include thermally and chemically refractory carbon materials such as diamond, graphite and silicon carbide; as well as refractory oxides and nitrides. Pre-solar silicates, which have only recently been identified, are the most abundant type except for possibly diamond. The detailed study with modern analytical tools, of isotopic signatures in particular, provides highly accurate and detailed information with regard to stellar nucleosynthesis and grain formation in stellar atmospheres. Important stellar sources are Red Giant (RG) and Asymptotic Giant Branch (AGB) stars, with supernova contributions apparently small. The survival of those grains puts constraints on conditions they were exposed to in the interstellar medium and in the early solar system.     

正电子素的络合作用

本文介绍了正电子素及其络合作用。讨论了正电子素络合反应速度常数的测量方法。比较了几种线性自由能的表达式。     

工艺参数对2A12铝合金微弧氧化陶瓷层生长的影响

研究了正／负向电流密度、频率和正／负向占空比对铝合金微弧氧化陶瓷层生长的影响,采用扫描电子显微镜（SEM）和X射线衍射仪（XRD）,分析了不同氧化时间陶瓷层表面和截面形貌、成分和相组成,讨论了陶瓷层的生长过程。研究表明,正／负向电流密度相同时,随电流密度的增加膜层厚度增大;而当正向电流密度相同时,负向电流密度增加有利于膜层的生长;成膜速率随脉冲频率和负向占空比增加,均呈现先增大后减小的趋势;陶瓷层总厚度随氧化时间接近于线性增长,致密层占总膜层的比例先快速增加,其后略微下降。SEM结果显示,随氧化时间延长,样品表面膜厚度趋于均匀,界面处氧化膜变得比较平坦。陶瓷层主要由α—Al2O3和γ-Al2O3相组成,随氧化时间的延长,γ-Al2O3相在陶瓷层中的相对含量逐渐减少。而α-Al2O3相的含量逐渐提高。     

航天器编队飞行轨道构型研究

通过研究航天器编队飞行轨道动力学和推力控制,提出编队飞行轨道构型有自然式、强迫式和混合式三类,并对每类都举例说明若干种典型构型,以及构型如何建造与所需燃料,文章总共提出14种典型轨道构型,这些构型可供今后编队飞行应用参考。     

Extragalactic observations and future missions

We briefly review some questions of extragalactic astrophysics and cosmology that would most benefit from future missions outside the Earth's atmosphere in the IR and submillimeter. These include the formation and early evolution phases in galaxies and the probably related question of quasar formation; the observation of Active Galactic Nuclei embedded in thick dusty structures (torii) and its impact on the still debated unified model of AGN activity; the observability of radiation processes occurring at very highz through background measurements; the investigation of the large scale structure and velocity field in the distant universe; and studies of the interstellar medium in galaxies. Some more emphasis is given on the galaxy formation problem, because we believe that IR-mm sensitive observations will be crucial to its final solution.     

Robust image-based coordinated control for spacecraft formation flying

This paper addresses a coordinated control problem for Spacecraft Formation Flying (SFF). The distributed followers are required to track and synchronize with the leader spacecraft. By using the feature points in the two-dimensional image space, an integrated 6-degree-of-freedom dynamic model is formulated for spacecraft relative motion. Without sophisticated three-dimensional reconstruction, image features are directly utilized for the controller design. The proposed image-based controller can drive the follower spacecraft in the desired configuration with respect to the leader when the real-time captured images match their reference counterparts. To improve the precision of the formation configuration, the proposed controller employs a coordinated term to reduce the relative distance errors between followers. The uncertainties in the system dynamics are handled by integrating the adaptive technique into the controller, which increases the robustness of the SFF system. The closed-loop system stability is analyzed using the Lyapunov method and algebraic graph theory. A numerical simulation for a given SFF scenario is performed to evaluate the performance of the controller.     

卫星编队飞行相对轨道的摄动研究综述

卫星在编队飞行过程中总受到各种摄动力的作用，由于轨道摄动直接影响卫星的寿命，研究卫星编队飞行相对轨道的摄动问题具有重要的意义。本文综述了国内外有关编队飞行卫星相对轨道摄动问题的研究方法以及进展，最后针对目前相对轨道摄动的研究现状，指出了进一步的研究方向。     

用最小二乘法确定卫星间相对姿态

为确定多卫星编队飞行时卫星的相对姿态，采用坐标系的空间变换，用最小二乘法计算两卫星本体坐标系间的旋转参数。给出了方法的基本原理、步骤、方法和适用条件。     

分布式航天器应用系统编队构形优化设计一般方法

从系统性能优化角度出发，提出分布式航天器应用系统编队优化设计一般方法，将其概括为求解一个优化问题。根据系统任务要求，建立通用目标函数，用性能指标的加权和来表示；选取优化问题的决策变量，用编队设计变量的矢量组合表示；以基线矢量过渡，建立性能指标与编队卫星设计变量的关系，进而建立优化问题的目标函数与决策变量的关系，得到完整的优化问题描述；采用遗传算法求解。针对主星带辅星群体制分布式干涉合成孔径雷达（interferometric synthetic aperture radar，InSAR）系统进行仿真编队优化设计，并与干涉车轮和钟摆进行仿真分析比较，仿真结果验证了优化设计方法的有效性和正确性。