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   9913篇
  免费   2327篇
  国内免费   1002篇
航空   5714篇
航天技术   3500篇
综合类   1307篇
航天   2721篇
  2023年   28篇
  2022年   97篇
  2021年   110篇
  2020年   139篇
  2019年   108篇
  2018年   210篇
  2017年   167篇
  2016年   186篇
  2015年   217篇
  2014年   484篇
  2013年   507篇
  2012年   531篇
  2011年   606篇
  2010年   643篇
  2009年   743篇
  2008年   757篇
  2007年   573篇
  2006年   484篇
  2005年   450篇
  2004年   336篇
  2003年   324篇
  2002年   257篇
  2001年   386篇
  2000年   324篇
  1999年   443篇
  1998年   450篇
  1997年   349篇
  1996年   361篇
  1995年   327篇
  1994年   328篇
  1993年   256篇
  1992年   268篇
  1991年   191篇
  1990年   164篇
  1989年   194篇
  1988年   120篇
  1987年   119篇
  1986年   78篇
  1985年   122篇
  1984年   104篇
  1983年   90篇
  1982年   97篇
  1981年   126篇
  1980年   43篇
  1979年   37篇
  1978年   32篇
  1977年   25篇
  1976年   28篇
  1974年   25篇
  1973年   26篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.
针对某直升机专用交流地面电源车系统,建立了PSPICE软件的仿真模型,且进行了系统仿真,研究了基于独立源及受控源的SPWM波产生方法,分析了滤波器参数分布对于系统性能的影响。仿真结果表明该电源系统对于雷达、吊声类冲击性负载有较好的适应性。  相似文献   
282.
    
The cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of ({sim}25) experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions.  相似文献   
283.
Adaptive pulse compression via MMSE estimation   总被引:2,自引:0,他引:2  
Radar pulse compression involves the extraction of an estimate of the range profile illuminated by a radar in the presence of noise. A problem inherent to pulse compression is the masking of small targets by large nearby targets due to the range sidelobes that result from standard matched filtering. This paper presents a new approach based upon a minimum mean-square error (MMSE) formulation in which the pulse compression filter for each individual range cell is adaptively estimated from the received signal in order to mitigate the masking interference resulting from matched filtering in the vicinity of large targets. The proposed method is compared with the standard matched filter and least-squares (LS) estimation and is shown to be superior over a variety of stressing scenarios.  相似文献   
284.
A high-accuracy numerical scheme is proposed for vortex methods of flow simulation around airfoils of arbitrary shape including airfoils with sharp edges, because it does not require the solution continuity on the airfoil.  相似文献   
285.
The Galileo Dust Detector is intended to provide direct observations of dust grains with masses between 10-19 and 10-9 kg in interplanetary space and in the Jovian system, to investigate their physical and dynamical properties as functions of the distances to the Sun, to Jupiter and to its satellites, to study its interaction with the Galilean satellites and the Jovian magnetosphere. Surface phenomena of the satellites (like albedo variations), which might be effects of meteoroid impacts will be compared with the dust environment. Electric charges of particulate matter in the magnetosphere and its consequences will be studied; e.g., the effects of the magnetic field on the trajectories of dust particles and fragmentation of particles due to electrostatic disruption. The investigation is performed with an instrument that measures the mass, speed, flight direction and electric charge of individual dust particles. It is a multicoincidence detector with a mass sensitivity 106 times higher than that of previous in-situ experiments which measured dust in the outer solar system. The instrument weighs 4.2 kg, consumes 2.4 W, and has a normal data transmission rate of 24 bits s-1 in nominal spacecraft tracking mode. On December 29, 1989 the instrument was switched-on. After the instrument had been configured to flight conditions cruise science data collection started immediately. In the period to May 18, 1990 at least 168 dust impacts have been recorded. For 81 of these dust grains masses and impact speeds have been determined. First flux values are given.  相似文献   
286.
飞行控制中机电作动系统的发展是全电飞机的重要技术之一,而高速大功率密度的无刷直流电动机是其中的关键部件.为解决无刷直流电动机设计中的非线性优化问题,提出了使用遗传算法进行优化设计.为解决无刷直流电动机多目标优化问题,采用了多目标函数变量加权法,将多目标函数变成单目标函数进行优化计算.在计算中,为达到无刷直流电动机全局优化的目的,采用随机产生离散初始种群及随机交叉操作和变异操作方法.但由于采用随机交叉和变异操作,优化结果始终是发散的.为此采用优秀个体保护策略,加快了优化设计的收敛.在满足技术要求的前提下,通过计算达到了减小电机的体积重量、减小转子转动惯量及机电时间常数、提高无刷直流电动机功率密度的目的.   相似文献   
287.
In this paper, identification of dynamic stall effect of rotor blade is considered. Recurrent Neural Networks have the ability to identify the nonlinear dynamical systems from training data. This paper describes the use of recurrent neural networks for predicting the coefficient of lift (CZ) at high angle of attack. In our approach, the coefficient of lift (CZ) obtained from the experimental results (wind tunnel data) at different mean angle of attack θmean is used to train the recurrent neural network. Then the recurrent neural network prediction is compared with experimental ONERA OA212 airfoil data. The time and space complexity required to predict CZ in the proposed method is less and it is easy to incorporate in any commercially available rotor code.  相似文献   
288.
Based on the results of plasma and magnetic measurements at three different points of the heliosphere and telescopic observations of the Sun from these points we study simultaneously high-speed streams (HSS) of the solar wind (SW) near the Earths’s orbit and coronal holes (CH) that have generated them. The data from spacecraft STEREO-A, STEREO-B, ACE, and SOHO are used together with ground-based observations from March 2007 to May 2008. In this period there existed HSS whose sources represented CH of various polarity, geometry, and location relative to the heliographic and heliomagnetic equators. Dependence of SW parameters on mutual positions of spacecraft with respect to CH and heliospheric current sheet, and also on heliolatitude and geometry of the CH is revealed. A difference of more than 5° in locations of spacecraft with respect to the heliospheric current sheet in November 2007 allowed us to discover a heliolatitude velocity gradient of the SW streams between the STEREO-A and STEREO-B spacecraft. On the average this gradient at that time was equal to 20 km/s per degree. Substantial variations in SW streams associated with variations of the HSS SW sources during a few hours or days were also observed. This variability makes it difficult to use the data of spacecraft STEREO-B for sufficiently accurate prediction of SW properties in the near-Earth space by the method of simple advanced ti me shift due to heliolongitude difference between a spacecraft and the Earth even in solar activity minimum.  相似文献   
289.
  总被引:6,自引:0,他引:6  
The paper summarizes the data on proliferation and gravity-related gene expression of osteoblasts that were obtained from an experiment conducted under simulated and real microgravity conditions. Simulated microgravity conditions obtained in a clinostat depress proliferation of both osteoblast-like MC3T3-E1 and HeLa carcinoma cells. This depression of proliferation occurs in a collagen gel culture in which the flow of culture medium by rotation may be reduced. Interestingly, MC3T3-E1 cells which are probably one of target cells to microgravity are more sensitive than the HeLa cells. Simulated microgravity inhibited the epidermal growth factor (EGF)-induced c-fos gene expression in the MC3T3-El cells. To examine in detail the effect of real microgravity on the EGF signal transduction cascade in osteoblasts, MC3T3-E1 cells were cultured in the Cell Culture Experiment Module of the sounding rocket TR-1A6. The EGF-induced c-fos expression in cells was depressed under short-term microgravity conditions in the sounding rocket, while the phosphorylation of mitogen-activated protein kinase (MAPK) was not affected compared with the controls grown on the ground. These results suggest that an action site of microgravity in the signal transduction pathway may be downstream of MAPK.  相似文献   
290.
Transient performance of a single-axis rate gyroscope mounted in a spacecraft which is spinning about the spin axis of the gyro is presented. Analytical expressions for various time-domain and frequency-domain specifications as functions of the spin rate of the vehicle are obtained. Numerical results are presented which are useful in selecting the gyro parameters if it is to be used for the measurements of the angular velocity in spinning space vehicles.  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