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   5418篇
  免费   13篇
  国内免费   11篇
航空   2836篇
航天技术   1920篇
综合类   23篇
航天   663篇
  2021年   30篇
  2019年   32篇
  2018年   57篇
  2017年   31篇
  2014年   82篇
  2013年   132篇
  2012年   107篇
  2011年   155篇
  2010年   114篇
  2009年   177篇
  2008年   245篇
  2007年   136篇
  2006年   140篇
  2005年   143篇
  2004年   114篇
  2003年   170篇
  2002年   101篇
  2001年   176篇
  2000年   109篇
  1999年   140篇
  1998年   164篇
  1997年   123篇
  1996年   169篇
  1995年   215篇
  1994年   182篇
  1993年   120篇
  1992年   127篇
  1991年   76篇
  1990年   62篇
  1989年   138篇
  1988年   62篇
  1987年   66篇
  1986年   60篇
  1985年   195篇
  1984年   150篇
  1983年   130篇
  1982年   133篇
  1981年   176篇
  1980年   58篇
  1979年   41篇
  1978年   50篇
  1977年   53篇
  1976年   38篇
  1975年   58篇
  1974年   38篇
  1973年   42篇
  1972年   51篇
  1971年   42篇
  1970年   44篇
  1969年   37篇
排序方式: 共有5442条查询结果,搜索用时 15 毫秒
761.
The author describes a simplified derivation of the representation of the circular error probability (CEP) integral, which is the integral over a disk centered at the origin of a zero mean two-dimensional Gaussian random variable, as a one-dimensional integral. In addition, two series are presented which can be used to compute efficiently the CEP integral. The domain of applicability of the series and methods for acceleration of the convergence of these series are discussed. The integral occurs in the evaluation of communication and radar signals, and in other statistical applications.<>  相似文献   
762.
A brief discussion is given of the physical processes that may lead to a differentiation of the nucleus of short period comets. It is concluded that samples from the near-surface layers of such comets may give us important information on the initial state of cometary organics and refractories. Cometary ices are more likely to be indicators of the recent evolution of the comet.  相似文献   
763.
The nonlinear system concepts of nonlinear transfer functions are extended to nonlinear cross sections which may be used to quantitatively describe the characteristics of a nonlinear scatter. The concept of nonlinear cross sections is used to generalize the radar equation for nonlinear scattering objects.  相似文献   
764.
Neugebauer  M.  Steinberg  J.T.  Tokar  R.L.  Barraclough  B.L.  Dors  E.E.  Wiens  R.C.  Gingerich  D.E.  Luckey  D.  Whiteaker  D.B. 《Space Science Reviews》2003,105(3-4):661-679
Some of the objectives of the Genesis mission require the separate collection of solar wind originating in different types of solar sources. Measurements of the solar wind protons, alpha particles, and electrons are used on-board the spacecraft to determine whether the solar-wind source is most likely a coronal hole, interstream flow, or a coronal mass ejection. A simple fuzzy logic scheme operating on measurements of the proton temperature, the alpha-particle abundance, and the presence of bidirectional streaming of suprathermal electrons was developed for this purpose. Additional requirements on the algorithm include the ability to identify the passage of forward shocks, reasonable levels of hysteresis and persistence, and the ability to modify the algorithm by changes in stored constants rather than changes in the software. After a few minor adjustments, the algorithm performed well during the initial portion of the mission. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
765.
Closed-loop control has been successfully applied to a microelectromechanical systems (MEMS) lateral comb resonator device in real-time to perform impulse disturbance damping and sinusoidal position control, enabled by the use of a through-wafer optical microprobe to obtain position feedback. This result leverages the application of lifetime, in-situ control of MEMS in order to provide quality assurance of microsystems in safety critical applications. A position feedback signal produced by a through-wafer optical microprobe has been used for comb resonator system model identification by two independent methods to accurately determine the effective mass, damping, and spring constant values of the device. After accurate determination of system parameters, closed-loop impulse disturbance damping and proportional-integral-differential (PID) translational control were applied. Closed-loop control results presented indicate controllability of such microstructures and response times on the order of the natural frequency of the device.  相似文献   
766.
Variable-geometry truss structures are likely to be used extensively in the future for in-orbit space construction. This paper considers dynamics formulation and vibration control of such structures. The truss system is modelled as a collection of sub-structures consisting of truss booms, prismatic actuator elements, and in some cases a manipulator at the end. Each truss boom is treated as a separate ‘link’ and its flexibility is modelled using the finite element method. Equations of motion for individual sub-structures are obtained which are then assembled. The non-working constraint forces are eliminated to obtain the equations governing the constrained dynamics of the entire system. For vibration control, the singular perturbation method is employed to construct two reduced-order models, for quasi-static motion and for modal co-ordinates, respectively. Computed torque with PD control is applied to maintain the quasi-static motion, while an optimal LQR method is used for vibration control. Typical simulation results are presented for the planar case.  相似文献   
767.
This viewpoint argues that the barriers preventing human expansion into space are not scientific but political and legal - an inability to secure funding; artificially high costs created by punitive insurance premiums and excessive bureaucracy; and uncertainty and disagreement about the extent and implications of legal regulation of space activities. Construction on Earth of a ‘metanation’ to oversee the governance of outer space is advocated as a possible solution.  相似文献   
768.
Muller AW 《Astrobiology》2003,3(3):555-564
During thermal cycling, organisms could live on thermosynthesis, a theoretical mechanism applicable to the origin of life and the early evolution of biological energy conversion. All extraterrestrial ice may be a repository for frozen dead or dormant organisms from earlier stages of evolution. In the presence of a thermal gradient within the ice, organisms might still be harvesting energy from thermosynthesis. Possible habitats for thermosynthesizers can be found throughout the Solar System, particularly in the cold traps on Mercury and the Moon, convecting waters on Mars, the oceans on moons in the outer Solar System, and smaller bodies rotating in the sunlight such as cosmic dust, meteorites, asteroids, and comets. A general strategy for detecting thermosynthetic organisms on Earth is offered, and highlights of current and upcoming robotic exploratory missions relevant to the detection of thermosynthesis are reviewed.  相似文献   
769.
A design procedure is outlined for optimizing stiffened, thin circular, cylindrical shells subject to a given set of destabilizing loads, in the presence of a specified initial geometric imperfection. The procedure incorporates two distinct steps (a) optimization based on linear buckling analyses and (b) imperfection sensitivity studies of the optimum design point from (a) as well as of the surrounding design space. A comparison of all these designs yield the true optimum in the presence of the specified geometric imperfection. The present methodology is fully demonstrated through two illustrative examples, one dealing with an axially loaded stiffened cylinder and one with a torsionally loaded one.  相似文献   
770.
Experimental drop tube of the metallurgy department of Grenoble   总被引:1,自引:0,他引:1  
The drop tube which will be available in the “Centre d'Etudes Nucléaires de Grenoble” is described. Its main features are the following: - Dimensions : Drop height : 47.1 m Drop time : 3.1 s Tube inside diameter : 0.2 m - Experimental atmosphere : 1 Ultra-vacuum : 10−6 to 10−7 Pa - Residual gravity level : 10−8 to 10−9 g according to the vacuum level and drop diameter.

This facility is unique insofar as it enables experiments to be performed under ultra-vacuum conditions which, by delaying the formation of surface oxides, should contribute to improving maximum undercooling values.

The techniques used for obtaining small metallic drops (0.5 to 3 mm) are described. The availability of this instrument for the scientific community is also foreseen by the french sponsoring organizations (CEA, CNES, CNRS) ; some practicle informations will be given to potential experimenters.  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