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   3326篇
  免费   44篇
  国内免费   15篇
航空   1406篇
航天技术   1265篇
综合类   27篇
航天   687篇
  2022年   18篇
  2021年   38篇
  2019年   30篇
  2018年   79篇
  2017年   62篇
  2016年   57篇
  2015年   23篇
  2014年   93篇
  2013年   104篇
  2012年   94篇
  2011年   168篇
  2010年   123篇
  2009年   183篇
  2008年   216篇
  2007年   97篇
  2006年   89篇
  2005年   117篇
  2004年   109篇
  2003年   107篇
  2002年   80篇
  2001年   117篇
  2000年   54篇
  1999年   73篇
  1998年   85篇
  1997年   59篇
  1996年   67篇
  1995年   74篇
  1994年   61篇
  1993年   57篇
  1992年   73篇
  1991年   21篇
  1990年   28篇
  1989年   66篇
  1988年   32篇
  1987年   22篇
  1986年   24篇
  1985年   79篇
  1984年   75篇
  1983年   59篇
  1982年   59篇
  1981年   79篇
  1980年   38篇
  1979年   22篇
  1978年   23篇
  1977年   16篇
  1976年   20篇
  1975年   21篇
  1974年   15篇
  1970年   9篇
  1969年   11篇
排序方式: 共有3385条查询结果,搜索用时 156 毫秒
991.
The ionic charge states of helium, carbon, oxygen, and iron have been determined for three solar particle enhancements by an electrostatic deflection analyzer, which is combined with a thin window proportional counter dE/dX vs. E system. The observations are obtained during the periods September 23 to 29, 1978, June 6 to 8, 1979, and September 15 to 26, 1979, with an instrument onboard the ISEE-3 spacecraft. The mean ionic charge states for He, C, and oxygen exhibit a high degree of ionization with values of Q = 2, 6, and 7.2, respectively. The charge state of iron is near 13 charge units. Variations from flare to flare and within the September 23, 1978 flare are small. The most surprising feature of the charge state measurement is the observation of a small (~10%) but finite contribution of singly ionized helium.  相似文献   
992.
Summary Photometric observations of A0538-66 in quiescence show that the optical star is an object of equivalent spectral type B7eII. Spectroscopic data taken simultaneously confirm this spectral classification and are consistent with the reported periodic variations seen in the emission lines. It is unlikely that the optical outburst is due to X-rays reprocessed in the atmosphere of the primary star or elsewhere.  相似文献   
993.
A precise calibration method for range and angle observation has been developed for eliminating the systematic error of tracking systems, thus improving the accuracy of orbit determination for geostationary satellites. The principle of calibration is based on an orbit determination employing a point of optical angle observation in addition to radio tracking observation, in which we estimate observation bias parameters simultaneously with orbital elements, including the effects of geodetic mismodelings. As shown by an actual calibration experiment in our ground station, orbit determinations is sufficiently accurate that the error of predicting satellite range falls within a few meters at four days after the day of orbit determination.  相似文献   
994.
The means for enhancing the efficiency of rocket propulsion of spacecraft is considered in terms of the control of propellant consumption, trajectory planning, and in-flight operations management. Particular attention is given to techniques by which all the fuel and oxidizer are consumed completely by mixture adjustment as an example of a terminal control system providing significant propulsion efficiency.  相似文献   
995.
The design of a high-accuracy digital VOR signal generator is described. The main part of the generator is formed of three digital sinewave generators, one of which is frequency modulated. The equations needed to design these generators are given and the practical realization based on read-only memories is described.  相似文献   
996.
Dynamics of the satellite angular momenta is modeled by ordinary differential equations with additive white noise. A feedback controller er is desired to maintain tolerable satellite momenta and to economize ze the fuel consumption. The stochastic optimal control problem is transformed into an equivalent deterministic optimization problem involving a parabolic partial differential equation. Necessary optimality y conditions are used to develop a computation algorithm. Results regarding optimal controls, expected costs, and confinement probabilities in different noisy environments are presented.  相似文献   
997.
There is a problem in deciding whether a radar observation of an object in outer space is of a known catalogued satellite or of a newly launched satellite. With some radar systems joint or dumbell radar images of two satellites cause occasional confusion, and appropriate decision theory for this situation is derived. Decision theory for the individual image is given in an appendix.  相似文献   
998.
999.
The prediction of energy detector performance requires a complicated calculation or a tedious manipulation of nomograms. For a large time-bandwidth product WT, however, it is commonplace to use the formula (E/No) = d?WT to anticipate the required average input energy-to-noise spectral density ratio for a wanted signal detectability parameter d and thus avoid the computational difficulty. This paper proposes a modified formula (E/No) = ?d?WT that is applicable for all range of WT, where ? is the modification factor derived on an empirical basis. The Van Trees measure of the signal detectability parameter of the energy detector also is derived analytically and compared to the modified equation.  相似文献   
1000.
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