地基行星大气掩星观测资料流程和数据整理

在中国火星探测萤火一号(YH-1)计划中, 包括了地基掩星观测反演火星大气的科研任务. 观测资料整理是反演流程的第一步. 本文描述了地基火星大气掩星观测处理软件系统的观测数据流程和观测资料整理模块,并详细介绍了观测资料整理模块的结构和功能, 其中包括时间系统转换、历表插值、坐标系变换、信号时延改正以及掩星平面建立. 利用行星数据系统公布的火星快车无线电科学数据和由SPICE得到的地球、火星历表以及火星快车的轨道数据, 结合本文的算法, 得到了一些实验结果.      

Orbit perturbation: Evolution of a keplerian disk

The dynamical evolution of a keplerian disk of matter around a star or around a planet is a crucial point for the understanding of the formation of solar systems or for the formation of planetary rings. In this paper we will adopt a general point of view : we describe the disk by a distribution function in phase space and follow its evolution in presence of a stochastic force or binary collisions.     

Comets in other planetary systems?

Comets in our solar system appear to have provided a bridge between the cold, volatile-rich outer solar system, and the warm, but volatile-poor inner solar system. Excluding tidal and possible extinct radionuclide heating sources, only in the inner solar system are temperatures high enough for liquid water, and therefore life as we know it, to exist for times comparable to the age of the solar system. Comets may have been crucial for providing biogenic volatiles and perhaps organic molecules to this warm environment. It is therefore interesting from an exobiological point of view to ask if comets exist in other planetary systems. Most attempts to detect comets around other stars or in interstellar space have failed. However, there is growing spectroscopic evidence for comet-like bodies orbiting the star Beta Pictoris.     

Synergies of Earth science and space exploration

A more flexible policy basis from which to manage our planet in the 21st century is desirable. As one contribution, we note that synergies between space exploration and the preservation of our habitat exist, and that protecting life on Earth requires similar concepts and information as investigations of life beyond the Earth, including the expansion of human presence in space. Instrumentation and data handling to observe both planetary objects and planet Earth are based on similar techniques. Moreover, while planetary surface operations are conducted under different conditions, the technology to probe the surface and subsurface of both the Earth and other planets requires similar tools, such as radar, seismometers, and drilling devices. The Earth observation community has developed some exemplary tools and has featured successful international cooperation in data handling and sharing that could be equally well applied to robotic planetary exploration. Here we propose a network involving both communities that will enable the interchange of scientific insights and the development of new policies and management strategies. Those tools can provide a vital forum through which the management of this planet can be assisted, and in which a new bridge between the Earth-centric and space-centric communities can be built.     

一种基于三正交主轴平面的快速星图模拟方法

为在地面上进行星图模拟以测试星图识别算法和星敏感器性能, 提出了一种新的快速星图模拟方法. 该方法利用截面圆和固定区域的性质, 将单位天球按照三个正交主轴平面进行等分, 根据赤纬值求解恒星矢量与主轴平面夹角来确定选星条件, 进而根据坐标变换原理进行星图模拟. 为验证所提方法与现有方法相比的优势, 进行了大量的仿真实验. 实验结果表明, 与现有星图模拟方法相比, 本文方法在保证星图模拟精度的条件下, 选星速度提高了3倍以上.      

Europa, tidally heated oceans, and habitable zones around giant planets.

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. In our own solar system, Europa, one of the Galilean satellites of Jupiter, could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such case, sufficient solar energy could reach liquid water that organisms similar to those found under Antarctic ice could grow. In other solar systems, larger satellites with more significant heat flow could represent environments that are stable over an order of Aeons and in which life could perhaps evolve. We define a zone around a giant planet in which such satellites could exist as a tidally-heated habitable zone. This zone can be compared to the habitable zone which results from heating due to the radiation of a central star. In our solar system, this radiatively-heated habitable zone contains the Earth.     

Interstellar planetary protection

In the coming decades the detection of Earth-like extrasolar planets, either apparently lifeless or exhibiting spectral signatures of life, will encourage design studies for craft to visit them. These missions will require the elaboration of an interstellar planetary protection protocol. Given a specific dose required to sterilize microorganisms on a spacecraft, a critical mean velocity can be determined below which a craft becomes self-sterilizing. This velocity is calculated to be below velocities previously projected for interstellar missions, suggesting that an active sterilization protocol prior to launch might be required. Given uncertainties in the surface conditions of a destination extrasolar planet, particularly at microscopic scales, the potential for unknown biochemistries and biologies elsewhere, or the possible inoculation of a lifeless planet that is habitable, then both lander and orbiter interstellar missions should be completely free of all viable organisms, necessitating a planetary protection approach applied to orbiters and landers bound for star systems with unknown local conditions for habitability. I discuss the case of existing craft on interstellar trajectories – Pioneer 10, 11 and Voyager 1 and 2.     

FY-3D卫星的北斗掩星分布特征与误差特性

利用2018年1－3月FY-3D卫星的掩星折射率数据,研究了北斗导航卫星系统的掩星分布特点、数据精度以及误差统计特征。北斗导航卫星系统同步静止轨道掩星沿卫星轨道呈弧状分布在南北两极地区,倾斜轨道掩星在东西半球低纬度地区分别形成一小一大两个空洞,中地球轨道掩星则全球均匀分布。北斗掩星折射率数据精度在探测核心区域,即12~32 km范围内,与ERA5再分析资料计算的折射率相比,平均偏差的标准差约为1.5%,在核心区外,标准差从1.5%逐渐增大到6%。静止轨道掩星的平均偏差在高层略大于倾斜轨道和中地球轨道掩星。下降掩星在20 km以上区域的标准差大于上升掩星,20 km以下区域小于上升掩星。高纬地区北斗掩星标准差最小,低纬地区最大,对流层中下层尤其明显。分析结果表明,北斗掩星的数据精度和误差特征与GPS掩星数据相似。      

Distinguishing features of CCD astrometry of faint GEO objects

The problem of initial reduction of CCD observations of faint GEO objects is studied. Several difficulties and questions arising in connection with image processing for this kind of observations are mentioned. A natural way to apply the PSF fitting technique to line-shaped trails of stars or GEO objects is described and illustrated by its implementation in the Apex II astronomical image processing package. Logical filtering technique to improve the automatic object detection is proposed. These rather convenient and versatile methods can increase astrometric and, to a lesser degree, photometric accuracy of faint GEO object observations.     

Limits of photosynthesis in extrasolar planetary systems for earth-like planets.

We present a general modeling scheme for investigating the possibility of photosynthesis-based life on extrasolar planets. The scheme focuses on the identification of the habitable zone in main-sequence-star planetary systems with planets of Earth mass and size. Our definition of habitability is based on the long-term possibility of photosynthetic biomass production as a function of mean planetary surface temperature and atmospheric CO2-content. All the astrophysical, climatological, biogeochemical, and geodynamic key processes involved in the generation of photosynthesis-driven life conditions are taken into account. Implicitly, a co-genetic origin of the central star and the orbiting planet is assumed. The numerical solution of an advanced geodynamic model yields realistic look-up diagrams for determining the limits of photosynthesis in extrasolar planetary systems, assuming minimum CO2 levels set by the demand of C4 photosynthesis.     

星敏感器中快速星跟踪技术

跟踪模式是星敏感器的主要工作模式之一,跟踪过程的快速性直接影响星敏感器的整体性能.提出了一种快速星跟踪算法.在跟踪算法的3个耗时环节,分别采用了分区星表、阈值映射、先排序后匹配识别这3种方法,以提高跟踪过程的快速性.其中分区星表法将整个天区分成了若干个子天区,使得在星体映射时,只是搜索星敏感器视轴指向附近的部分子天区,而不是搜索整个天区,减少了搜索星体的数量;阈值映射法,在满足精确度的情况下,设置被跟踪星体的数量阈值,只有被跟踪的星体数目少于此阈值时,才进行星体映射,减少了映射次数;先排序后匹配识别法,先根据星体在星图中的坐标值进行排序,然后再进行匹配识别,减少了那些距离较大的无谓的星体间的匹配识别.仿真测试结果表明,这3种方法的采用,提高了跟踪算法的快速性,提高了星敏感器的整体效能.      

Revised planetary protection policy for solar system exploration

In order to control contamination of planets by terrestrial microorganisms and organic constituents, U.S. planetary missions have been governed by a planetary protection (or planetary quarantine) policy which has changed little since 1972. This policy has recently been reviewed in light of new information obtained from planetary exploration during the past decade and because of changes to, or uncertainties in, some parameters used in the existing quantitative approach. On the basis of this analysis, a revised planetary protection policy with the following key features is proposed: deemphasizing the use of mathematical models and quantitative analyses; establishing requirements for target planet/mission type (i.e., orbiter, lander, etc.) combinations; considering sample return missions a separate category; simplifying documentation; and imposing implementing procedures (i.e., trajectory biasing, cleanroom assembly, spacecraft sterilization, etc.) by exception, i.e., only if the planet/mission combination warrants such controls.     

适用于弹道导弹的导航星选取方法

针对当前适用于弹道导弹的导航星选取方法较少,研究了一种根据标准弹道均匀化选取导航星的方法.通过分析导弹飞行弹道及飞行参数的影响,在构建"伪天球"坐标系的基础上,确定了导航星的选取区域,简化了因射向变化、坐标越界等造成计算和搜索比较复杂的问题;基于天球上的基准点及提出的"距离-星等"加权法,均匀化地筛选导航星,并按照k-vector索引方式构建适用性较好的弹载导航星表,可以有效地提高星图识别算法的效率.仿真结果表明,该方法简单易行,选取的导航星数量少,分布均匀性好,在不降低星图识别成功率的基础上,能够有效地减少识别时间.     

高精度高动态星模拟器研究

给出了高精度高动态星模拟器系统组成和总体结构, 提出一种基于亚像素显示技术提高动态星模拟器星点位置显示精度从而减小星点几何中心与能量中心位置偏差的新方法. 针对动态星模拟器高动态性要求, 提出一种快速精确检索全天球导航星的新方法, 将已知SAO (Smithsonian Astrophysical Observatory) 星表进行分区, 计算当前姿态四元数并分析航天器视轴所在区域, 对于距离视轴较远区域停止搜索和匹配, 很大程度上缩小了导航星搜索范围, 进而缩短了全天球恒星遍历所需时间. 软件测试结果表明, 此快速检索法可使全天球导航星的选取速度提高近10倍.      

A proposed new policy for planetary protection

In order to control contamination of planets by terrestrial microorganisms and organic constituents, U.S. planetary missions have been governed by a planetary protection (or planetary quarantine) policy which has changed little since 1972. This policy has recently been reviewed in light of new information obtained by planetary exploration during the past decade and because of changes to, or uncertainties in, some parameters used in the existing quantitative approach. On the basis of this analysis, a new planetary protection policy, with the following key features, is proposed: deemphasizing the use of mathematical models and quantitative analyses; establishing requirements for target planet/mission type (i.e., orbiter, lander, etc.) combinations; considering sample return missions a separate category; simplifying documentation; and imposing implementing procedures (i.e., trajectory biasing, cleanroom assembly, spacecraft sterilization, etc.) by exception, i.e., only if the planet/mission combination warrants such controls. Interpretation of the new policy for missions like Galileo, Mars Surface Sample Return, Saturn Orbiter with Twin Probes, and missions to comets are considered. In general, the new policy proposes elimination of all but documentation requirements for most planetary missions and simplification of the remaining compliance procedures.     

Analytic and numerical treatment of motion of dust grain particle around triangular equilibrium points with post-AGB binary star and disc

This paper investigates the motion around the triangular equilibrium points, of a passively gravitating dust particle in the gravitational field of a low-mass post-AGB binary system, surrounded by circumbinary disc. The two bodies of the binary are modeled as a triaxial star and a radiating-oblate star. Due to small deviation of disc stars on circular orbits, we have assumed that the Coriolis and centrifugal forces of the stars are slightly perturbed. The triangular equilibrium points of the particle are found. These points are defined by, triaxiality of the primary star, oblateness and radiation of the secondary one and the gravitational potential from the disc mass. Further, when the disc mass increases, the particle moves nearer to the stars and farther away from the disc. In general, these equilibrium points are linearly stable when μ < μ_C; where μ is the mass ratio and μ_C is the critical mass function, defined by the parameters of the system. The effects of each of these parameters on the size of the stability region are stated, and the periodic motion around the stable points is examined. It is seen that the orbits are ellipses, and the orientation, eccentricities, lengths of the semi-major and semi-minor axes are influenced by the parameters of the problem. In particular, for our numerical linear stability analysis, we have taken an extremely depleted pulsating star, IRAS 11472-0800 as the post-AGB triaxial star, with a weakly-radiating young white dwarf star; G29-38 as the secondary. For this system, the stability result of the triangular points comes out different. Here, μ_C < μ throughout the entire range of the mass ratio and the critical mass function. Hence, the triangular equilibrium points are unstable. The stability of the orbits is tested using the Poincaré surfaces of section (Pss). The region of stability is controlled by the introduced parameters and the Jacobi constant.     

一种基于恒星分布的星敏感器导航星库制作方法

为了进一步提升星敏感器的定姿精度,分析了恒星的空间几何分布对姿态精度的影响,提出了一种基于恒星分布的星敏感器导航星库制作方法。在分析对比了现有星库的基础上,选择了精度较高的依巴谷星库作为基础星库,并剔除了暗星和双星以精简星库。遍历全天球对每个视场的星库中的空间几何分布进行分析对比,删除定姿较差的3颗星。对星库进行遍历补偿,保证了星库中恒星分布的均匀性。实验结果表明,全天球轨道测试中,本文方法制作星库的全天识别率为100%,且星颗分布数10颗以上天区占到97.64%,具有良好的覆盖性,同时有效提升了姿态精度。     

Star angle/ double-differenced pulse time of arrival integrated navigation method for Jupiter exploration

Pulsar navigation is a promising autonomous navigation system for spacecraft, which is applicable to the entire solar system. However, the pulsar’s directional error and the onboard clock error are two types of systematic errors that seriously reduce navigation accuracy. To solve this problem, a star angle/double-differenced pulse time of arrival(SA/DDTOA) integrated navigation method is proposed. Since measurements obtained by observing different pulsars contain the same clock errors, the measurements can be differed to eliminate the common clock error. Then, the pulsar-differenced measurements at neighbor filtering time can be differed to suppress the effect of the pulsar’s directional error on navigation precision. Star angle is used to obtain absolute navigation information, which denotes the angles between the light of sight of Jupiter and that of its background stars. Simulation results demonstrate that the proposed method can eliminate the influence of the onboard clock error and greatly weaken the effects of the pulsar’s directional error. The navigation accuracy is better than the traditional star angle/pulse time of arrival integrated navigation method and star angle/pulse time difference of arrival integrated navigation method. In addition, the navigation accuracy of the SA/DDTOA integrated navigation method is less affected by Jupiter’s ephemeris error. This work greatly reduces the influence of common systematic errors in pulsar navigation on navigation accuracy.     

Autonomous satellite navigation using starlight refraction angle measurements

An on-board autonomous navigation capability is required to reduce the operation costs and enhance the navigation performance of future satellites. Autonomous navigation by stellar refraction is a type of autonomous celestial navigation method that uses high-accuracy star sensors instead of Earth sensors to provide information regarding Earth’s horizon. In previous studies, the refraction apparent height has typically been used for such navigation. However, the apparent height cannot be measured directly by a star sensor and can only be calculated by the refraction angle and an atmospheric refraction model. Therefore, additional errors are introduced by the uncertainty and nonlinearity of atmospheric refraction models, which result in reduced navigation accuracy and reliability. A new navigation method based on the direct measurement of the refraction angle is proposed to solve this problem. Techniques for the determination of the refraction angle are introduced, and a measurement model for the refraction angle is established. The method is tested and validated by simulations. When the starlight refraction height ranges from 20 to 50 km, a positioning accuracy of better than 100 m can be achieved for a low-Earth-orbit (LEO) satellite using the refraction angle, while the positioning accuracy of the traditional method using the apparent height is worse than 500 m under the same conditions. Furthermore, an analysis of the factors that affect navigation accuracy, including the measurement accuracy of the refraction angle, the number of visible refracted stars per orbit and the installation azimuth of star sensor, is presented. This method is highly recommended for small satellites in particular, as no additional hardware besides two star sensors is required.