CEMP星HE 1005-1439中子俘获元素天体物理来源的研究

HE1005-1439是一颗金属丰度极低([Fe/H]～-3.0)的碳增丰贫金属星(Carbon Enhanced Metal-Poor,CEMP),该星的s-过程元素显著超丰([Ba/Fe]=1.16±0.31,[Pb/Fe]=1.98±0.19),而r-过程元素温和超丰([Eu/Fe]=0.46±0.22),使用单一的s-过程模型和i-过程模型均不能拟合该星中子俘获丰度分布.采用丰度分解的方法探究该星化学元素的天体物理来源可有助于理解CEMP星的形成和化学演化.利用s-过程和r-过程的混合模型对其中子俘获元素的丰度分布进行拟合,发现该星的中子俘获元素主要来源于低质量低金属丰度AGB伴星的s-过程核合成,而r-过程核合成也有贡献.     

The 3-D lattice theory of Flower Constellations

Flower Constellations (FCs) have been extensively studied for use in optimal constellation design. The Harmonic FCs (HFCs) subset, representing the symmetric configurations, have recently been reformulated into 2-D Lattice Flower Constellations (2D-LFCs), encompassing the complete set of HFCs. Elliptic orbits are generally avoided due to the deleterious effects of Earth’s oblateness on the constellation, but here we present a novel concept for avoiding this problem and enabling more effective global coverage utilizing elliptic orbits. This new 3D Lattice Flower Constellations (3D-LFCs) framework generalizes the 2D-LFCs, Walker constellations, elliptical Walker constellations, and many of Draim’s global coverage constellations. Previous studies have shown FCs can provide improved performance in global navigation over existing Global Navigation Satellite Systems (GNSS). We found a 3D-LFC design that improved the average positioning accuracy by 3.5 % while reducing launch $\varDelta v$ Δ v requirements when compared to the existing Galileo GNSS constellation.     

A new proposition for redating the Mithraic tauroctony scene

Assuming that the figures of the central icon of the Mithraic cult – the scene of tauroctony (bull slaying) – represent equatorial constellations at the time when the spring equinox was placed somewhere between Taurus and Aries, it is difficult to explain why some equatorial constellations (Orion and Libra) were not included in the Mithraic icons A simulation of the sky at the times in which the spring equinox was in the constellation of Taurus, only a small area of spring equinox positions permits to exclude these two constellations, with all other representations of equatorial constellations (Taurus, Canis Minor, Hydra, Crater, Corvus, Scorpio) included. These positions of the spring equinox occurred at the beginning of the age of Taurus, and included Gemini as an equatorial constellation. Two of the main figures in the Mithraic icons are two identical figures, usually represented on the each side of the bull, wearing phrygian caps and holding torches. Their names, Cautes and Cautopates, and their looks may indicate that they represent the constellation of Gemini. In that case the main icon of Mithraic religion could represent an event that happened around 4000 BC, when the spring equinox entered the constellation of Taurus. Also, this position of equator contains Perseus as an equatorial constellation. Ulansey suggested that the god Mithras is identified with the constellation Perseus. In that case, all figures in the main scene would be equatorial constellations.     

二十八宿分野暨轸宿星名含义考证

二十八宿对应的天文地理分野,是以族群所崇拜的图腾为重要依据.分布在四方的二十八宿与地上四方的方国地域相对应,其中有部分不相符,是因族人迁移或异地建国,仍要依据其所崇祀的图腾.另外,作为二十八宿最后一宿的轸,其含义过去一直被释为车,仅是其一.轸宿星名还有更深的另一层含义,是象征南方楚地的轸国,轸国也即曾国.     

Regional positioning using a low Earth orbit satellite constellation

Global and regional satellite navigation systems are constellations orbiting the Earth and transmitting radio signals for determining position and velocity of users around the globe. The state-of-the-art navigation satellite systems are located in medium Earth orbits and geosynchronous Earth orbits and are characterized by high launching, building and maintenance costs. For applications that require only regional coverage, the continuous and global coverage that existing systems provide may be unnecessary. Thus, a nano-satellites-based regional navigation satellite system in Low Earth Orbit (LEO), with significantly reduced launching, building and maintenance costs, can be considered. Thus, this paper is aimed at developing a LEO constellation optimization and design method, using genetic algorithms and gradient-based optimization. The preliminary results of this study include 268 LEO constellations, aimed at regional navigation in an approximately 1000 km \(\times \) 1000 km area centered at the geographic coordinates [30, 30] degrees. The constellations performance is examined using simulations, and the figures of merit include total coverage time, revisit time, and geometric dilution of precision (GDOP) percentiles. The GDOP is a quantity that determines the positioning solution accuracy and solely depends on the spatial geometry of the satellites. Whereas the optimization method takes into account only the Earth’s second zonal harmonic coefficient, the simulations include the Earth’s gravitational field with zonal and tesseral harmonics up to degree 10 and order 10, Solar radiation pressure, drag, and the lunisolar gravitational perturbation.     

Time distributions in satellite constellation design

The aim of the time distribution methodology presented in this paper is to generate constellations whose satellites share a set of relative trajectories in a given time, and maintain that property over time without orbit corrections. The model takes into account a series of orbital perturbations such as the gravitational potential of the Earth, the atmospheric drag, the Sun and the Moon as disturbing third bodies and the solar radiation pressure. These perturbations are included in the design process of the constellation. Moreover, the whole methodology allows to design constellations with multiple relative trajectories that can be distributed in a minimum number of inertial orbits.     

Formation Flying Satellites: Control By an Astrodynamicist

Satellites flying in formation is a concept being pursued by the Air Force and NASA. Potential periodic formation orbits have been identified using Hill's (or Clohessy Wiltshire) equations. Unfortunately the gravitational perturbations destroy the periodicity of the orbits and control will be required to maintain the desired orbits. Since fuel will be one of the major factors limiting the system lifetime it is imperative that fuel consumption be minimized. To maximize lifetime we not only need to find those orbits which require minimum fuel we also need for each satellite to have equal fuel consumption and this average amount needs to be minimized. Thus, control of the system has to be addressed, not just control of each satellite. In this paper control of the individual satellites as well as the constellation is addressed from an astrodynamics perspective.     

卫星星座的结构演化

主要研究星座几何结构的演化问题，分析了在地球扁率摄动和卫星的入轨偏差影响下轨道的演化过程，以卫星的相位和升交点赤经为参数描述了星座结构演化的一般规律，又以星下点轨迹变化和星下点的相互位置关系的变化为参数描述了区域星座结构演化的地域特性．分析表明，地球扁率摄动将导致星座结构的整体漂移，而卫星的入轨偏差则会导致星座几何结构的紊乱，特别是轨道半长轴的偏差，将是影响星座几何结构稳定性的决定因素．     

Resonant dynamics of Medium Earth Orbits: space debris issues

The Medium Earth Orbit region is the home of the navigation constellations. It is shown how the orbits of these constellations of satellites are strongly affected by the so-called inclination dependent luni-solar resonances. The analytical theory of these resonances is recalled and a large set of numerical integrations is used to investigate the stability of the orbits of the constellations over very long time spans. The stability issue is important in the definition of possible disposal strategies for the constellation spacecraft, after their end-of-life. Two possible disposal strategies are envisaged involving either stable or unstable orbits (from the eccentricity growth point of view). In particular it is shown how, to have disposal orbits with moderately short lifetime (of the orders of 40–50 years), a very large disposal maneuver would be required to rise the inital eccentricity to values above ~ 0.3.     

Lissajous trajectories for lunar global positioning and communication systems

This work proposes a Lunar Global Positioning System (LGPS) and a Lunar Global Communication System (LGCS) using two constellations of satellites on Lissajous trajectories around the collinear L ₁ and L ₂ libration points in the Earth–Moon system. This solution is compared against a Walker constellation around the Moon similar to the one used for the Global Positioning System (GPS) on the Earth to evaluate the main differences between the two cases and the advantages of adopting the Lissajous constellations. The problem is first studied using the Circular Restricted Three Body Problem to find out its main features. The study is then repeated with higher fidelity using a four-body model and higher-order reference trajectories to simulate the Earth-Moon-spacecraft dynamics more accurately. The LGPS performance is evaluated for both on-ground and in-flight users, and a visibility study for the LGCS is used to check that communication between opposite sides of the Moon is possible. The total ΔV required for the transfer trajectories from the Earth to the constellations and the trajectory control is calculated. Finally, the estimated propellant consumption and the total number of satellites for the Walker constellation and the Lissajous constellations is used as a performance index to compare the two proposed solutions.     

基于不同轨道高度iHCO通信卫星的CAPS星座优化研究

基于通信卫星的导航系统可以利用比地球静止轨道（Geostationary Earth Orbit,GEO）高约200 km的倾斜高圆轨道（inclined Highly Circular Orbit,iHCO）通信卫星组成导航星座.结合两种轨道高度的倾斜高圆轨道通信卫星,仿真分析了利用倾斜高圆轨道卫星组成的中国区域定位系统（Chinese Area Positioning System,CAPS）的导航性能,并讨论了利用倾斜高圆轨道卫星组成的中国区域定位系统实现中国区域覆盖的最佳星座布局.     

Exploiting Unstable Periodic Orbits of a Chaotic Invariant Set for Spacecraft Control

The purpose of this work is to show that chaos control techniques (OGY, in special) can be used to efficiently keep a spacecraft around another body performing elaborate orbits. We consider a satellite and a spacecraft moving initially in coplanar and circular orbits, with slightly different radii, around a heavy central planet. The spacecraft, which is the inner body, has a slightly larger angular velocity than the satellite so that, after some time, they eventually go to a situation in which the distance between them becomes sufficiently small, so that they start to interact with one another. This situation is called as an encounter. In previous work we have shown that this scenario is a typical situation of a chaotic scattering for some well-defined range of parameters. Considering this scenario, we first show how it is possible to find the unstable periodic orbits that are located in the chaotic invariant set. From the set of unstable periodic orbits, we select the ones that can be combined to provide the desired elaborate orbit. Then, chaos control technique based on the OGY method is used to keep the spacecraft in the desired orbit. Finally, we analyze the results and make considerations regarding a realistic scenario of space exploration.     

Errors in orbit determination

The gaussian noise which affects tracking measurements causes an error in the computed value of the orbit parameters. This study provides a method for evaluating: (a) the length of the arc over which the satellite must be tracked; (b) the number of measurements to be made along this arc; (c) the position of the arc with respect to the orbit, necessary to reach the desired accurary of the calculated orbit parameters for a given pointing error of the tracking antenna. It has been assumed that the errors of the tracking measurements have a known gaussian probability distribution, which may differ for each measurement. The equations relating the orbit parameters to the measurements performed have been linearized. It has been shown that the orbit parameters are gaussian random variables and their variance has been calculated as a function of (a), (b) and (c).This paper was presented at the AIAA/AAS meeting, Princeton University, August 1969.     

卫星星座与编队飞行问题综述

卫星应用的日益发展需要由多颗卫星组成的卫星系统(形成各种类型的星座)来替代单颗卫星，这就给卫星轨道力学的应用增加了一些新内容。重点阐述了卫星星座整体结构的演化、卫星星座编队飞行中的星-星相对几何构形及其保持问题，以及星座卫星定轨所涉及的测控方式和天地基网联合定轨问题。这些内容都是卫星星座轨道力学中的重要组成部分。     

Secular evolution of the orbits of hypothetical satellites of Uranus

The problem of the secular perturbations of the orbit of a test satellite with a negligible mass caused by the joint influence of the oblateness of the central planet and the attraction by its most massive (or main) satellites and the Sun is considered. In contrast to the previous studies of this problem, an analytical expression for the full averaged perturbing function has been derived for an arbitrary orbital inclination of the test satellite. A numerical method has been used to solve the evolution system at arbitrary values of the constant parameters and initial elements. The behavior of some set of orbits in the region of an approximately equal influence of the perturbing factors under consideration has been studied for the satellite system of Uranus on time scales of the order of tens of thousands of years. The key role of the Lidov–Kozai effect for a qualitative explanation of the absence of small bodies in nearly circular equatorial orbits with semimajor axes exceeding ~1.8 million km has been revealed.     

卫星星座时间同步中星间链路的设计和性能分析

在卫星星座内的时间同步中,为满足时间同步信号的传输要求,需要对星间链路进行分析和设计。对星间链路各参数之间的关系进行了分析,并对低地球轨道(LEO)卫星和中地球轨道(MEO)卫星的星间链路参数和性能分别进行了计算和分析。另外,还对受天线指向误差影响的LEO卫星和MEO卫星的星间链路性能进行了分析。在分析计算的基础上完成了符合要求的星间链路的参数设计。     

一种转发式卫星授时新方法

为了减少对GPS提供的高精度授时服务的依赖,建设拥有自主知识产权的且具有投入成本低、精度高的授时系统。提出了一种利用地面高稳定度原子钟作为时频基准,通过通信卫星转发实现卫星广播授时的新方法。详细研究了利用地面高稳定度原子钟和通信卫星组成的导航星座进行多星授时、单星授时的原理和测量方法,分析研究了影响授时精度的原因。由于转发式卫星授时系统的时频基准源稳定度高,其测距精度也会相应的提高。只要精确扣除时延误差,同样可以实现高精度授时。粗码的授时精度可达20ns以内,短精码的授时精度可达10ns左右。总之该系统具有组建灵活简便、应用面广等优势和特色。     

基于iHCO通信卫星的CAPS星座优化研究

中国区域定位系统(Chinese Area Positioning System,CAPS)把寿命末期的地球静止轨道(Geostationary Earth Orbit,GEO)通信卫星推到比GEO轨道高约200 km的倾斜高圆轨道(inclined Highly Circular Orbit,iHCO),卫星相对地球向西漂移。利用该类卫星组建CAPS导航星座,可以实现全球范围内的导航通信覆盖。重点开展基于iHCO通信卫星的CAPS星座优化研究,结果表明:利用GEO通信卫星和iHCO通信卫星组成的星座可以实现较好的空间星座布局,可以满足一般导航用户的需要。     

3-Dimensional Necklace Flower Constellations

A new approach in satellite constellation design is presented in this paper, taking as a base the 3D Lattice Flower Constellation Theory and introducing the necklace problem in its formulation. This creates a further generalization of the Flower Constellation Theory, increasing the possibilities of constellation distribution while maintaining the characteristic symmetries of the original theory in the design.     

Relativistic positioning: errors due to uncertainties in the satellite world lines

Global navigation satellite systems use appropriate satellite constellations to get the coordinates of an user—close to Earth—in an almost inertial reference system. We have simulated both GPS and GALILEO constellations. Uncertainties in the satellite world lines lead to dominant positioning errors. In this paper, a detailed analysis of these errors is developed inside a great region surrounding Earth. This analysis is performed in the framework of the so-called relativistic positioning systems. Our study is based on the Jacobian (J) of the transformation giving the emission coordinates in terms of the inertial ones. Around points of vanishing J, positioning errors are too large. We show that, for any 4-tuple of satellites, the points with J=0 are located at distances, D, from the Earth centre greater than about 2R/3, where R is the radius of the satellite orbits which are assumed to be circumferences. Our results strongly suggest that, for D-distances greater than 2R/3 and smaller than 10⁵ km, a rather good positioning may be achieved by using appropriate satellite 4-tuples without J=0 points located in the user vicinity. The way to find these 4-tuples is discussed for arbitrary users with D<10⁵ km and, then, preliminary considerations about satellite navigation at D<10⁵ km are presented. Future work on the subject of space navigation—based on appropriate simulations—is in progress.