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   3266篇
  免费   22篇
  国内免费   10篇
航空   1577篇
航天技术   1151篇
综合类   4篇
航天   566篇
  2021年   26篇
  2019年   23篇
  2018年   55篇
  2017年   51篇
  2016年   46篇
  2015年   26篇
  2014年   71篇
  2013年   91篇
  2012年   82篇
  2011年   119篇
  2010年   79篇
  2009年   137篇
  2008年   153篇
  2007年   99篇
  2006年   71篇
  2005年   92篇
  2004年   90篇
  2003年   99篇
  2002年   72篇
  2001年   115篇
  2000年   65篇
  1999年   68篇
  1998年   87篇
  1997年   58篇
  1996年   93篇
  1995年   112篇
  1994年   111篇
  1993年   48篇
  1992年   71篇
  1991年   34篇
  1990年   32篇
  1989年   78篇
  1988年   28篇
  1987年   24篇
  1986年   35篇
  1985年   90篇
  1984年   91篇
  1983年   65篇
  1982年   66篇
  1981年   99篇
  1980年   19篇
  1979年   25篇
  1978年   29篇
  1977年   27篇
  1975年   29篇
  1974年   23篇
  1973年   21篇
  1972年   20篇
  1969年   20篇
  1968年   18篇
排序方式: 共有3298条查询结果,搜索用时 0 毫秒
291.
292.
293.
    
A statistical technique for amplitude calibration of radar systems is presented. As distinct from the input-output crosscorrelation measurement of a linear system impulse response [1], this technique measures the amplitude transfer function of a memoryless, nonlinear system. A generalized theory is developed and calibration accuracy bounds are derived. Used in the calibration of a modern pulsed radar, the technique is compared with conventional reference pulse calibration. The relative merits of the deterministic and statistical approaches are compared.  相似文献   
294.
Kamide  Y.  Kihn  E.A.  Ridley  A.J.  Cliver  E.W.  Kadowaki  Y. 《Space Science Reviews》2003,107(1-2):307-316
We report the recent progress in our joint program of real-time mapping of ionospheric electric fields and currents and field-aligned currents through the Geospace Environment Data Analysis System (GEDAS) at the Solar-Terrestrial Environment Laboratory and similar computer systems in the world. Data from individual ground magnetometers as well as from the solar wind are collected by these systems and are used as input for the KRM and AMIE magnetogram-inversion algorithms, which calculate the two-dimensional distribution of the ionospheric parameters. One of the goals of this program is to specify the solar-terrestrial environment in terms of ionospheric processes, providing the scientific community with more than what geomagnetic activity indices and statistical models provide. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
295.
We present an efficient two-scan data association method (TSDA) based on an interior point linear programming (LP) approach. In this approach, the TSDA problem is first formulated as a 3-dimensional assignment problem, and then relaxed to a linear program; the latter is subsequently solved by the highly efficient homogeneous, self-dual interior point LP algorithm. When the LP algorithm generates a fractional optimal solution, we use a technique similar to the joint probabilistic data association method (JPDA) to compute a weighted average of the resulting fractional assignments, and use it to update the states of the existing tracks generated by Kalman filters. Unlike the traditional single scan JPDA method, our TSDA method provides an explicit mechanism for track initiation. Extensive computer simulations have demonstrated that the new TSDA method is not only far more efficient in terms of low computational complexity, but also considerably more accurate than the existing single-scan JPDA method  相似文献   
296.
    
The main goal of this paper is to get physically informative comprehensive data about dynamics of the solar magnetic field, geomagnetic field, and interplanetary magnetic field over large time scales. The total sunspot magnetic flux, aa and IDV indices of geomagnetic activity, the IMF strength, the dipole-octopole index of the large-scale magnetic field of the Sun, and the open magnetic flux are reconstructed for 400 years. The reconstruction of the π index of the large-scale polar magnetic field of the Sun is performed for 150 years.  相似文献   
297.
    
LET variation with the absorber thickness was measured by plastic detectors exposed on COSMOS-1129 satellite.  相似文献   
298.
    
An algorithm is described for initial synchronization in a communication system with a digital adaptive array. This algorithm can also be used for message extraction. A set of consecutive complex video samples of the array output is processed to obtain optimum adaptive array weights, based on a least mean square (LMS) error criterion. This computation is performed for each of the possible alternative signals which may be present during an observation interval. The correct synchronization time or message symbol is selected as the one which yields the minimum LMS error. Assuming orthogonality of the alternative codes, a probability distribution for the output of this processor has been derived.  相似文献   
299.
    
We analyzed high-angular rate streaks first recorded by OSIRIS-REx’s MapCam during a 2017 search for Earth Trojan asteroids. We interpret them as water-ice particles that translated across the imager’s field of view, originating from the spacecraft itself. Their translation velocities approximated 0.1–1?m/s based on reasonable conclusions about their range. Pursuing several lines of investigation to seek a coherent hypothesis, we conclude that the episodic releases of the water ice particles are associated with spacecraft attitudes that resulted in solar illumination of previously shadowed regions. This correlation suggests that the OSIRIS-REx spacecraft itself possesses micro-climatic zones consisting of hot regions and cold traps that may temporarily potentially pass volatiles back and forth before losing most of them.  相似文献   
300.
This paper is intended as a critical review of current ideas concerning the mechanisms responsible for the geomagnetic storm.The dynamical theory of the geomagnetic storm phenomenon is formulated as a problem in elasticity. The observed variations in the field are the strains produced by particle stresses exerted by gases in interplanetary space, by gases enmeshed in the field, and by the gases in the ionosphere. The stresses exerted by interplanetary gases are principally inward, resulting in the initial phase increase of the horizontal component. The stresses exerted by gases enmeshed in the field are principally outward, resulting in the main phase decrease of the horizontal component. The transient sudden commencement is a hydromagnetic wave phenomenon.The main phase is most simply explained by the shock heating of the ions to kev energies at 3 – 5 R E during the active phase of the storm. The recovery follows then from charge exchange with the ambient neutral hydrogen. The predicted more rapid recovery at sunspot minimum has been verified observationally.This work was supported by the National Aeronautics and Space Administration under grant NASA-NsG-96-60.  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