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   120178篇
  免费   1696篇
  国内免费   1311篇
测绘学   3433篇
大气科学   8975篇
地球物理   24062篇
地质学   42108篇
海洋学   10426篇
天文学   27186篇
综合类   369篇
自然地理   6626篇
  2021年   1043篇
  2020年   1170篇
  2019年   1319篇
  2018年   2877篇
  2017年   2702篇
  2016年   3483篇
  2015年   1962篇
  2014年   3302篇
  2013年   6009篇
  2012年   3549篇
  2011年   4638篇
  2010年   4190篇
  2009年   5548篇
  2008年   4756篇
  2007年   4757篇
  2006年   4559篇
  2005年   3444篇
  2004年   3527篇
  2003年   3365篇
  2002年   3244篇
  2001年   2857篇
  2000年   2809篇
  1999年   2388篇
  1998年   2397篇
  1997年   2329篇
  1996年   1991篇
  1995年   1925篇
  1994年   1760篇
  1993年   1536篇
  1992年   1463篇
  1991年   1423篇
  1990年   1508篇
  1989年   1359篇
  1988年   1315篇
  1987年   1479篇
  1986年   1350篇
  1985年   1603篇
  1984年   1859篇
  1983年   1695篇
  1982年   1627篇
  1981年   1461篇
  1980年   1366篇
  1979年   1315篇
  1978年   1329篇
  1977年   1161篇
  1976年   1076篇
  1975年   1081篇
  1974年   1050篇
  1973年   1107篇
  1971年   714篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
162.
The possible correlation noted between the intrinsic quasar luminosity and the absorption line expulsion velocity is re-examined using homogeneous data sets for metal and L only line systems. The method of analysis is chosen to enable any reasonable form of correlation to be found. No correlation is detected at a confidence level >10%.  相似文献   
163.
164.
F.A. Gifford  R.P. Hosker  K.S. Rao 《Icarus》1978,36(1):133-146
The spreading angle of a number of light and dark Martian streaks is determined from selected Mariner 9 images. The resulting frequency distributions of spreading half-angles have maxima at ~5° for light, and ~7° for dark streaks; however the dark streaks have a secondary maximum spreading angle at ~14°. The smaller values, which include most streaks, are interpreted as crater-wake spreading phenomena. The larger value, found in only a few dark streaks or “tails,” may result from atmospheric diffusion and subsequent deposition of material from isolated sources such as vents or blowouts. An atmospheric diffusion-deposition analysis is presented, assuming this streak origin, from which it is possible to deduce the eddy diffusivity, K, in Mars' boudary layer. Calculated K values are found to agree with various theoretical estimates. They lie in the range 107 and 109 cm2 sec?1 and exhibit the proper scale dependence. Thus it appears that, in addition to streak-derived wind direction patterns and speed information, it is possible in a few cases to derive information on Mars' boundary-layer turbulence from streak-spreading measurements.  相似文献   
165.
The orbital state of a satellite in a central force field can be uniquely described by its velocity hodograph, a circle, rather than the Keplerian conic. Also, its coordinate-frame rotation about the attracting center is definable, without singularity, by the four-parameter set of Euler parameters. A unified state model of orbital trajectory and attitude dynamics has previously been developed by use of state variables of the orbital velocity hodograph and Euler parameters. The dynamical constraint equations of this orbital state model are especially effective in advanced techniques of state estimation, used for orbit determination and prediction. External observations of orbital vehicles, such as provided by optical and radar sensors of tracking systems, are transformable into corresponding velocity state maps, as presented in this paper. These transformations and the consequent state maps are essential for development of the orbit observation matrix used with the unified state matrix, in recursive estimators such as the Kalman filters. Line-of-sight rays and range spheres (or hemispheres) of observations map conformally into orthogonal spherical surfaces in velocity space, as the result of the point-contact transformations. In bispherical coordinates, the field of observation maps for a ground-based tracking system site is shown to be a reduced (or degenerate) form of the general field of observation maps for a satellite-based tracking site. These orbital state maps and transforms are directly useful in development of observation matrices for candidate observation sets, such as range only, angle only, or range plus range-rate tracking schemes. Also, surface coverage patterns can be generated for proposed new tracking systems, in mission analysis and system synthesis studies.  相似文献   
166.
167.
168.
169.
IAG Newsletter     
  相似文献   
170.
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