首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   710篇
  免费   134篇
  国内免费   46篇
航空   479篇
航天技术   158篇
综合类   41篇
航天   212篇
  2024年   2篇
  2023年   13篇
  2022年   18篇
  2021年   19篇
  2020年   26篇
  2019年   21篇
  2018年   20篇
  2017年   15篇
  2016年   24篇
  2015年   15篇
  2014年   42篇
  2013年   34篇
  2012年   37篇
  2011年   54篇
  2010年   36篇
  2009年   24篇
  2008年   37篇
  2007年   48篇
  2006年   64篇
  2005年   55篇
  2004年   52篇
  2003年   36篇
  2002年   18篇
  2001年   22篇
  2000年   29篇
  1999年   19篇
  1998年   15篇
  1997年   13篇
  1996年   11篇
  1995年   10篇
  1994年   11篇
  1993年   9篇
  1992年   10篇
  1991年   7篇
  1990年   7篇
  1989年   9篇
  1988年   4篇
  1987年   4篇
排序方式: 共有890条查询结果,搜索用时 15 毫秒
671.
本文首先简述了MLS模拟器及其计量性能要求,然后对研制的MLS模拟器自动计量系统进行了详细介绍,包括硬件分系统组成及其关键技术,软件分系统架构及其核心流程;最后将自动计量系统应用于多台MLS模拟器的校准/检定。应用表明,MLS模拟器自动计量系统能够解决MLS模拟器计量保障中存在的问题,确保MLS机载设备各项参数准确一致,进而保证了飞行安全。  相似文献   
672.
This paper is one of the components of a larger framework of activities whose purpose is to improve the performance and productivity of space mission systems, i.e. to increase both what can be achieved and the cost effectiveness of this achievement. Some of these activities introduced the concept of Functional Architecture Module (FAM); FAMs are basic blocks used to build the functional architecture of Plan Management Systems (PMS). They also highlighted the need to involve Science Operations Planning Expertise (SOPE) during the Mission Design Phase (MDP) in order to design and implement efficiently operation planning systems. We define SOPE as the expertise held by people who have both theoretical and practical experience in operations planning, in general, and in space science operations planning in particular. Using ESA’s methodology for studying and selecting science missions we also define the MDP as the combination of the Mission Assessment and Mission Definition Phases. However, there is no generic procedure on how to use FAMs efficiently and systematically, for each new mission, in order to analyse the cost and feasibility of new missions as well as to optimise the functional design of new PMS; the purpose of such a procedure is to build more rapidly and cheaply such PMS as well as to make the latter more reliable and cheaper to run. This is why the purpose of this paper is to provide an embryo of such a generic procedure and to show that the latter needs to be applied by people with SOPE during the MDP. The procedure described here proposes some initial guidelines to identify both the various possible high level functional scenarii, for a given set of possible requirements, and the information that needs to be associated with each scenario. It also introduces the concept of catalogue of generic functional scenarii of PMS for space science missions. The information associated with each catalogued scenarii will have been identified by the above procedure and will be relevant only for some specific mission requirements. In other words, each mission that shares the same type of requirements that lead to a list of specific catalogued scenarii can use this latter list of scenarii (regardless of whether the mission is a plasma, planetary, astronomy, etc. mission). The main advantages of such a catalogue are that it speeds-up the execution of the procedure and makes the latter more reliable. Ultimately, the information associated to each relevant scenario (from the catalogue or freshly generated by the procedure) will then be used by mission designers to make informed decisions, including the modification of the mission requirements, for any missions. In addition, to illustrate the use of such a procedure, the latter is applied to a case study, i.e. the Cross-Scale mission. One of the outcomes of this study is an initial set of generic functional scenarii. Finally, although border line with the above purpose of this paper, we also discuss multi-spacecraft specific issues and issues related to the on-board execution of the plan update system (PUS). In particular, we show that the operation planning cost of N spacecraft is not equal to N times the cost of 1 spacecraft and that on-board non-synchronised operation will not require inter-spacecraft communication. We also believe that on-board PUS should be made possible for all missions as a standard.  相似文献   
673.
在SI国际单位制中,平面角是导出量,平面角的单位“弧度”是辅助单位。但是,平面角既不能由长度基本单位导出,又不能溯源至长度基本单位,因此不能确立导出关系。平面角应用广泛,作用重要,具有独特的性能,符合基本量的要求,因此提出:平面角,简称“角度”,是基本量;平面角的单位“度、(角)分、(角)秒”,是基本单位,以供探讨。  相似文献   
674.
针对近年来制造企业管理创新单点多、体系少,模仿多、自主创新少的结构失衡问题,借鉴世界先进企业管理提升的经验,提出管理创新要发挥后发优势并基于AOS运营体系构建来推进管理提升的思路,研究了基于AOS的运营体系顶层框架、展开层和体系层的构建方法,同时对基于AOS的体系构建策略与管理提升方式等相关问题进行了探讨.  相似文献   
675.
在介绍了弹头的突防技术的基础上,通过计算探讨了实现突防措施及破坏防御的目标拦截技术。提出了远程弹道导弹突防的策略,分析了现阶段可采用的技术手段。  相似文献   
676.
根据某空间机械臂的总线拓扑结构以及指令与数据特性,对空间数据系统咨询委员会(CCSDS)提出的指令与数据获取业务(CDAS)模型进行了研究,结合设备虚拟化业务,采用面向对象的方法进行设备抽象,阐述了设备数据池业务设计时的遥测数据更新、上层业务数据获取、数据有效性的判别等关键问题。提出了基于指令与数据获取业务模型的机械臂数据管理软件构架,并已在某机械臂中央控制器上成功应用。  相似文献   
677.
全球导航卫星系统(GlobalNavigationSatelliteSystem,GNSS)应用于高轨航天器时,因轨道高于导航卫星,可见星数量急剧减少,空间信号功率微弱,信号的快速捕获和跟踪十分困难。文章对高轨地球同步轨道(GeosynchronousEarthOrbit,GEO)接收技术进行了研究。以中国实践十七号卫星为研究对象,采用官方正式发布的发射天线方向图对GEO下GNSS信号特征及可用性开展研究分析,并针对高轨道航天器GNSS信号微弱的特点,采用长时间积分处理的梳状滤波方法、差分相干累加比特同步算法和基于动力学模型补偿的扩展卡尔曼滤波自主定轨算法设计GNSS接收机,并在半物理仿真平台进行了测试验证。试验结果表明:GNSS接收机捕获灵敏度优于-173dBW,跟踪灵敏度优于-175dBW,定轨位置精度优于50m,速度精度优于0.01m/s。  相似文献   
678.
Various studies have been performed to investigate the accuracy of troposphere zenith wet delays (ZWDs) determined from GPS. Most of these studies use dual-frequency GPS data of large-scale networks with long baselines to determine the absolute ZWDs. For small-scale networks the estimability of the absolute ZWDs deteriorates due to high correlation between the solutions of the ZWDs and satellite-specific parameters as satellite clocks. However, as relative ZWDs (rZWDs) can always be estimated, irrespective of the size of the network, it is of interest to understand how the large-scale network rZWD-performance of dual-frequency GPS using an ionosphere-float model compares to the small-scale network rZWD-performance of single-frequency GPS using an ionosphere-weighted model. In this contribution such an analysis is performed using undifferenced and uncombined network parametrization modelling. In this context we demonstrate the ionosphere weighted constraints, which allows the determination of the rZWDs independent from signals on the second frequency. Based on an analysis of both simulated and real data, it is found that under quiet ionosphere conditions, the accuracy of the single-frequency determined rZWDs in the ionosphere-weighted network is comparable to that of the large-scale dual-frequency network without ionospheric constraints. Making use of the real data from two baselines of 15?days, it was found that the absolute differences of the rZWDs applying the two strategies are within 1?cm in over 90% and 95% of the time for ambiguity-float and -fixed cases, respectively.  相似文献   
679.
This study characterizes total electron content (TEC) measured by Global Positioning System (GPS) over African equatorial ionization anomaly (EIA) region for 2009–2016 period during both quiet geomagnetic conditions (Kp?≤?1) and normal conditions (1?>?Kp?≤?4). GPS-TEC data from four equatorial/low-latitude stations, namely, Addis Ababa (ADIS: 9.04°N, 38.77°E, mag. lat: 0.2°N) [Ethiopia]; Yamoussoukro (YKRO: 6.87°N, 5.24°W, mag. lat: 2.6°S) [Ivory Coast]; Malindi (MAL2; 3.00°S, 40.19°E, mag. lat: 12.4°S) [Kenya] and Libreville (NKLG; 0.35°N, 9.67°W, mag. lat: 13.5°S) [Gabon] were used for this study. Interesting features like noontime TEC bite-out, winter anomaly during the ascending and maximum phases of solar cycle 24, diurnal and seasonal variations with solar activity have been observed and investigated in this study. The day-to-day variations exhibited ionospheric TEC asymmetry on an annual scale. TEC observed at equatorial stations (EIA-trough) and EIA-crest reach maximum values between ~1300–1600 LT and ~1300–1600 LT, respectively. About 76% of the high TEC values were recorded in equinoctial months while the June solstice predominantly exhibited low TEC values. Yearly, the estimated TEC values increases or decreases with solar activity, with 2014 having the highest TEC value. Solar activity dependence of TEC within the EIA zone reveals that both F10.7?cm index and EUV flux (24–36?nm) gives a stronger correlation with TEC than Sunspot Number (SSN). A slightly higher degree of dependence is on EUV flux with the mean highest correlation coefficient (R) value of 0.70, 0.83, 0.82 and 0.88 for quiet geomagnetic conditions (Kp?≤?1) at stations ADIS, MAL2, NKLG, and YKRO, respectively. The correlation results for the entire period consequently reveals that SSN and solar flux F10.7?cm index might not be an ideal index as a proxy for EUV flux as well as to measure the variability of TEC strength within the EIA zone. The estimated TEC along the EIA crest (MAL2 and NKLG) exhibited double-hump maximum, as well as post-sunset peaks (night time enhancement of TEC) between ~2100 and 2300 LT. EIA formation was prominent during evening/post-noon hours.  相似文献   
680.
《中国航空学报》2021,34(12):99-109
This paper presents a practical and efficient design method for aircraft Mission Success Space (MSS) based on the entropy measurement (EM). First, fundamentals regarding MSS, Inverse Design (ID) and entropy are discussed. Then, two EM schemes of entropy-based ID and the whole MSS ID procedure are given to demonstrate alternative ways of entropy quantification and MSS design. After that, Genetic Algorithm (GA) is utilized as a search algorithm to find the optimal MSS design with the minimum objective, entropy, in each EM scheme. A simulation case of aircraft penetration mission is given for the method validation where the best aircraft MSS design is obtained by GA according to the minimum entropy. Results from two schemes are compared at the end.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号