Application of GPS to space vehicles: analysis of space environmentand errors

GPS based systems become extremely competitive for space applications because of their all-weather capabilities and continual information on position, velocity, precise time and even attitude to increase mission effectiveness, reduce mission cost, minimize requirements of on-board devices. In this paper, space application environment and error sources have been systematically analyzed, including geometric location of user with GPS satellites, dynamic state, physical environment and the effects on positioning accuracy. Several special differential GPS technologies to space use are proposed     

AE9, AP9 and SPM: New Models for Specifying the Trapped Energetic Particle and Space Plasma Environment

The radiation belts and plasma in the Earth’s magnetosphere pose hazards to satellite systems which restrict design and orbit options with a resultant impact on mission performance and cost. For decades the standard space environment specification used for spacecraft design has been provided by the NASA AE8 and AP8 trapped radiation belt models. There are well-known limitations on their performance, however, and the need for a new trapped radiation and plasma model has been recognized by the engineering community for some time. To address this challenge a new set of models, denoted AE9/AP9/SPM, for energetic electrons, energetic protons and space plasma has been developed. The new models offer significant improvements including more detailed spatial resolution and the quantification of uncertainty due to both space weather and instrument errors. Fundamental to the model design, construction and operation are a number of new data sets and a novel statistical approach which captures first order temporal and spatial correlations allowing for the Monte-Carlo estimation of flux thresholds for user-specified percentile levels (e.g., 50th and 95th) over the course of the mission. An overview of the model architecture, data reduction methods, statistics algorithms, user application and initial validation is presented in this paper.     

An Overview of Ionosphere—Thermosphere Models Available for Space Weather Purposes

Our objective is to review recent advances in ionospheric and thermospheric modeling that aim at supporting space weather services. The emphasis is placed on achievements of European research groups involved in the COST Action 724. Ionospheric and thermospheric modeling on time scales ranging from a few minutes to several days is fundamental for predicting space weather effects on the Earth’s ionosphere and thermosphere. Space weather affects telecommunications, navigation and positioning systems, radars, and technology in space. We start with an overview of the physical effects of space weather on the upper atmosphere and on systems operating at this regime. Recent research on drivers and development of proxies applied to support space weather modeling efforts are presented, with emphasis on solar radiation indices, solar wind drivers and ionospheric indices. The models are discussed in groups corresponding to the physical effects they are dealing with, i.e. bottomside ionospheric effects, trans-ionospheric effects, neutral density and scale height variations, and spectacular space weather effects such as auroral emissions. Another group of models dealing with global circulation are presented here to demonstrate 3D modeling of the space environment. Where possible we present results concerning comparison of the models’ performance belonging to the same group. Finally we give an overview of European systems providing products for the specification and forecasting of space weather effects on the upper atmosphere, which have implemented operational versions of several ionospheric and thermospheric models.     

Intrinsic evolution of safe control strategies for autonomous spacecraft

Autonomous space vehicles need adaptive control strategies that can accommodate unanticipated environmental conditions. Although it is not difficult to construct alternative control strategies, a proper evaluation frequently can only be done by actually trying them out in the real physical environment. It therefore becomes imperative that any candidate control strategy be deemed safe - i.e., it does not damage any systems - prior to being tested online. How to do this has been a challenging problem. We propose a solution to this problem. Our approach uses an evolutionary algorithm (EA) to intrinsically evolve new control strategies. All candidate strategies are checked for safety using formal methods. More specifically, an EA evolves a series of finite state machines (FSMs), each of which encodes a unique control strategy. Model checking guarantees whether all safety properties are satisfied in the strategy. A numerical example is included to illustrate our approach.     

The Radar Support System (RSS): a tool for siting radars andpredicting their performance

The Radar Support System (RSS) is a software tool for assisting the radar siting engineer in determining the best location for a sensor, evaluating performance in a site-specific environment and analyzing operational problems. The RSS uses detailed sensor models, along with digital topographic, land cover and cultural databases, to provide accurate performance predictions. The RSS is currently being used by the FAA to assist in their sensor deployments and analyses     

我国中继卫星系统在交会对接任务中的应用

我国中继卫星系统在交会对接任务中得到了成功应用,显著提升了测控通信覆盖率,充分体现了中继卫星系统在交会对接任务测控通信中发挥的重要作用.结合中继卫星系统特点,分析交会对接任务对中继卫星系统的任务需求;探讨了中继卫星捕获跟踪用户目标、对用户目标测定轨、着陆场直升机缝隙通信等关键技术;最后总结了中继卫星系统抗雨衰影响、多目标支持能力、应急支持能力等方面的经验做法,并提出了后续改进和完善的建议.     

可视化航空发动机性能仿真模型

介绍了一种图形化的发动机总体性能仿真模型.该模型可以通过模块化的图形方式,由用户选择发动机部件,自由灵活地组装任意合理结构形式的航空发动机,方便地进行航空发动机稳态与过渡态性能的计算.对该仿真模型的编程思想和编程方法做了具体介绍,并通过对具体发动机进行试计算证明通用模型计算逻辑的正确性以及可视化图形界面的实用性.      

可扩充DBMS的用户扩充存取方法的研究

介绍可扩充 DBMS- NHEDB的用户扩充新索引方法的原理、方法及实现策略。提出一种以数据字典为联接媒介 ,借助于存取数据库的函数库的实现方案 ,并详细讨论了对 DBMS的各种主要影响及处理方法     

Remote Sensing: The Next 50 Years

Remote sensing is viewed as a complete system for observing and managing man's environment on Earth. The sensing portion of the system consists of space, air, and ground observing platforms, sensors, communications, computational capabilities and interpretation, and recognition algorithms. The data are put to use using environmental models, geographic information systems, and management models. Progress and potentials in each of these system elements are reviewed and projected. Existing and potential uses are sketched and certain institutional, diplomatic, and national security issues are identified. Some underlying reasons for belief in continued progress are mentioned and the need for certain crucial choices in the future stated which will determine which of a number of possible futures will be realized.     

Reducing lag in virtual displays using multiple model adaptiveestimation

A multiple model adaptive predictor is applied to a virtual environment flight simulator to remove the effect of computational and scene-rendering delay time. Angular orientation of the user's head is predicted a period of time into the future, so that the scene can be rendered appropriately by the time the user actually looks in that direction. Single nonadaptive predictors cannot adequately cover the dynamic range of head motion. By using three dissimilar models of head motion upon which to base the individual elemental filters within the multiple model adaptive estimator (MMAE) algorithm, an MMAE is designed which outperforms the nonadaptive Kalman predictor proposed by Liang (1991)     

空间摩擦学及其材料的研究进展

综述了空间苛刻服役环境及其对空间摩擦学材料性能影响的研究,深入分析了空间环境对空间摩擦材料、空间耐磨材料和空间减摩材料摩擦磨损机理的影响。空间摩擦材料主要应用于空间对接机构及空间机械臂中,应具有稳定的摩擦力矩与优良的抗黏着磨损性能。空间耐磨材料主要应用于空间轴承、齿轮和密封件等部件中,如FeAl金属间化合物在高温下抗蠕变性急剧下降,常通过添加金属元素(Ce,Cr,Mn,Mo,Nb,W等)及固体润滑剂提高材料抗蠕变性能;Ti及其合金常通过表面改性改善黏着性;与基体结合性良好的耐磨涂层可以较大程度的改善材料的耐磨性。空间减摩材料主要指润滑剂与自润滑材料,如软金属Pb、高分子材料PI和PTFE等,以及某些金属的氧化物,氟化物和硫化物等,能较好地降低材料表面的摩擦因数。随着航天科技的发展,亟须开发新型高性能空间摩擦学材料,建立摩擦学材料数据库,以应对国际航天技术发展的挑战。     

Airborne/spacebased radar STAP using a structured covariance matrix

It is shown that partial information about the airborne/spacebased (A/S) clutter covariance matrix (CCM) can be used effectively to significantly enhance the convergence performance of a block-processed space/time adaptive processor (STAP) in a clutter and jamming environment. The partial knowledge of the CCM is based upon the simplified general clutter model (GCM) which has been developed by the airborne radar community. A priori knowledge of parameters which should be readily measurable (but not necessarily accurate) by the radar platform associated with this model is assumed. The GCM generates an assumed CCM. The assumed CCM along with exact knowledge of the thermal noise covariance matrix is used to form a maximum likelihood estimate (MLE) of the unknown interference covariance matrix which is used by the STAP. The new algorithm that employs the a priori clutter and thermal noise covariance information is evaluated using two clutter models: 1) a mismatched GCM, and 2) the high-fidelity Research Laboratory STAP clutter model. For both clutter models, the new algorithm performed significantly better (i.e., converged faster) than the sample matrix inversion (SMI) and fast maximum likelihood (FML) STAP algorithms, the latter of which uses only information about the thermal noise covariance matrix.     

Space weather: its effects and predictability

Terrestrial technology is now, and increasingly, sensitive to space weather. Most space weather is caused by solar storms and the resulting changes to the Earth's radiation environment and the magnetosphere. The Sun as the driver of space weather is under intense observation but remains to be adequately modelled. Recent spacecraft measurements are greatly improving models of solar activity, the interaction of the solar wind with the magnetosphere, and models of the radiation belts. In-situ data updates the basic magnetospheric model to provide specific details of high-energy electron flux at satellite orbits. Shock wave effects at the magnetopause can also be coarsely predicted. However, the specific geomagnetic effects at ground level depend on the calculation of magnetic and electric fields and further improvements are needed. New work on physical models is showing promise of raising geomagnetic and ionospheric predictability above the synoptic climatological level. This revised version was published online in August 2006 with corrections to the Cover Date.     

燃气轮机分布式仿真的现状与展望

燃气轮机仿真正在从单一的、定制的仿真程序,向多学科综合的、适应各种需要的仿真环境发展。由于燃气轮机各个部件模型通常使用不同的编程模型和编程语言,需要在系统层次采用统一的编程模型来进行集成,同时屏蔽编程语言的差异。本文从编程模型的角度出发,对各种分布式仿真技术进行比较分析,并介绍了国内外燃气轮机仿真的发展现状。最后对国内燃气轮机分布式仿真的发展方向进行了探讨。     

Towards a Global Unified Model of Europa’s Tenuous Atmosphere

Despite the numerous modeling efforts of the past, our knowledge on the radiation-induced physical and chemical processes in Europa’s tenuous atmosphere and on the exchange of material between the moon’s surface and Jupiter’s magnetosphere remains limited. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios and considerations has resulted in a fragmentary understanding of the interactions of the magnetospheric ion population with both the moon’s icy surface and neutral gas envelope. Models show large discrepancy in the source and loss rates of the different constituents as well as in the determination of the spatial distribution of the atmosphere and its variation with time. The existence of several models based on very different approaches highlights the need of a detailed comparison among them with the final goal of developing a unified model of Europa’s tenuous atmosphere. The availability to the science community of such a model could be of particular interest in view of the planning of the future mission observations (e.g., ESA’s JUpiter ICy moons Explorer (JUICE) mission, and NASA’s Europa Clipper mission). We review the existing models of Europa’s tenuous atmosphere and discuss each of their derived characteristics of the neutral environment. We also discuss discrepancies among different models and the assumptions of the plasma environment in the vicinity of Europa. A summary of the existing observations of both the neutral and the plasma environments at Europa is also presented. The characteristics of a global unified model of the tenuous atmosphere are, then, discussed. Finally, we identify needed future experimental work in laboratories and propose some suitable observation strategies for upcoming missions.     

Designing security systems: changing the way we think

Today's world demands new ways of thinking about security solutions. The problem space is complex and ambiguous. Solutions must be multi-dimensional, incorporating not only technology, but the social, economic, political, and religious dynamics of a security intervention. A facilitator-led experiential training program was designed for our technical staff that leads them out of the box. The course design is based upon the theories of cognitive flexibility and situated cognition, and uses a socio-constructivist approach. Participants are led by a senior systems engineer/facilitator through a series of exercises in which they observe contextually relevant right way/wrong way videos, engage in critical thinking assessments about what they observed, and solve logic puzzles. Group interaction and problem-solving is emphasized. As in the real world, there is no one "right" solution. Outcomes can include a broader understanding of the threat space, creative solutions that enable survival in spite of an evolving enemy, and a deeper sense of the complex dynamics involved in any security decision. Training impact is being evaluated using a mixed qualitative/quantitative approach. Survey data combined with ethnographic interviewing techniques will determine whether participants have transferred their new understandings to the work environment.     

小空间碎片环境数据库的生成

首先对空间碎片的产生和演化过程进行了详细分析，在此基础上提出了一种小空间碎片环境数据库的生成方法。最后选择合适的碎裂模型和参数对1961年至1999年之间发生的154次历史碎裂事件进行了仿真计算，生成了当前的小空间碎片环境数据库。     

In search of other planetary systems

Numerous recent developments have led to an increasing awareness of and interest in the detection of other planetary systems. A brief review of the modern history of this subject is presented with emphasis on the status of data concerning Barnard's star. A discussion is given of plausible observable effects of other planetary systems with numerical examples to indicate the nature of the detection problem. Possible types of information (in addition to discovery) that observations of these effects might yield (e.g., planetary mass and temperature) are outlined. Also discussed are various candidate detection techniques (e.g., astrometric observations) which might be employed to conduct a search, the current state-of-the-art of these techniques in terms of measurement accuracy, and the capability of existing or planned facilities (e.g., space telescope) to perform a search. Finally, consideration is given to possible search strategies and the scope of a comprehensive search program.