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   5576篇
  免费   72篇
  国内免费   40篇
航空   2546篇
航天技术   1968篇
综合类   81篇
航天   1093篇
  2021年   58篇
  2019年   34篇
  2018年   109篇
  2017年   104篇
  2016年   67篇
  2015年   49篇
  2014年   134篇
  2013年   170篇
  2012年   147篇
  2011年   263篇
  2010年   199篇
  2009年   259篇
  2008年   302篇
  2007年   169篇
  2006年   154篇
  2005年   171篇
  2004年   141篇
  2003年   176篇
  2002年   130篇
  2001年   161篇
  2000年   122篇
  1999年   141篇
  1998年   145篇
  1997年   135篇
  1996年   114篇
  1995年   146篇
  1994年   137篇
  1993年   91篇
  1992年   117篇
  1991年   50篇
  1990年   58篇
  1989年   107篇
  1988年   54篇
  1987年   44篇
  1986年   46篇
  1985年   150篇
  1984年   121篇
  1983年   107篇
  1982年   91篇
  1981年   147篇
  1980年   56篇
  1979年   42篇
  1978年   47篇
  1977年   32篇
  1976年   38篇
  1975年   53篇
  1974年   30篇
  1972年   39篇
  1971年   31篇
  1967年   28篇
排序方式: 共有5688条查询结果,搜索用时 31 毫秒
941.
Due to the great increase in Information Technology (IT) systems where user authentication is needed, security in those systems relies on using PINs or passwords. During recent years, the scientific community has been trying to improve biometric techniques to be accepted as an alternative to other user authentication schemes. One of the sectors where user identity must be verified, is the identification cards sector. In fact, if great security wants to be achieved, smart cards should be used. But cardholder verification is performed using Card Holder Verification Keys (CHV-Keys), which are PIN-based. The authors are trying to integrate fingerprint verification inside a smart card, following their works in the past. The fingerprint scheme used is shown, and the work to achieve the integration inside a commercial smart card is detailed.  相似文献   
942.
943.
We outline an analytical method for studying the motion of charged dust particles that orbit an oblate planet having a tilted, offset, dipolar magnetic field. Our computed trajectories closely mimic previous numerical results; equilibrium dust potentials must be less then 10 volts or the Jovian ring would be thicker than observed. We identify several Lorentz resonances, where the periods of components of the Lorentz force, as seen by a reference particle moving in the equatorial plane, match the particle's orbital period; several seem to be near observed features of the Jovian ring system.  相似文献   
944.
The principle of determination of wind fields by a tomographic method is described. The airborne stereoscopic radiometer JANUS has been built to assess the feasibility of such measurements. Results of preliminary flights over isolated cumulus compare favourably with direct measurements. New flights with improved auxiliary parameter determinations are ongoing.  相似文献   
945.
A small coronagraph has been placed in orbit to monitor the sun's outer corona from 2.5 to 10.0 solar radii, and five years of nearly continuous synoptic observations have now been completed. Rapid and sensitive image processing techniques have been developed to screen the data for transient phenomena, particularly coronal mass ejections (CMEs). About 50,000 coronal images have been examined, out of a five-year total of 68,000, and a standardized listing of more than 1,200 coronal transients for the period 1979–1982 has been prepared. These data have been analysed in the light of other available information, particularly on conditions in the interplanetary plasma. The dynamical characteristics of the active corona, as they are beginning to emerge from the data, are presented. We find that coronal mass ejections exercise significant influence on the interplanetary solar wind. They are the source of disturbances that are frequent and energetic, that tend to be somewhat focussed, that often reach shock intensity, and that propagate to large heliocentric distances, sometimes causing major geomagnetic storms.  相似文献   
946.
With major emphasis on simulation, a university laboratory telerobotics facility permits problems to be approached by groups of graduate students. Helmet-mounded displays provide realism; the slaving of the display to the human operator's viewpoint gives a sense of `telepresence' that may be useful for prolonged tasks. Using top-down 3-D model control of distant images allows distant images to be reduced to a few parameters to update the model used for display to the human operator in a preview model to circumvent, in part, the communication delay. Also, the model can be used as a format for supervisory control and permit short-term local autonomous operations. Image processing algorithms can be made simpler and faster without trying to construct sensible images from the bottom. Control studies of telerobots lead to preferential manual control modes and, in this university environment, to basic paradigms for human motion and thence, perhaps, to redesign of robotic control, trajectory path planning, and rehabilitation prosthetics. Speculation as to future industrial drives for this telerobotic field suggests efficient roles for government agencies such as NASA  相似文献   
947.
948.
Long-term integration is defined as integration, perhaps interrupted, over time periods long enough for targets to move through volumes in space resolvable by the radar. Because the motion of the target is unknown prior to detection, long-term integration must be performed along multiple paths representing plausible target paths. The geometry of such a set of integration paths affects detection performance in several ways. The simplest implementation of long-term integration, using constant radial velocity paths, is investigated. The effects of path geometry on detection is quantified and optimized for a target whose motion is nearly radial but otherwise unknown  相似文献   
949.
950.
The instantaneous frequency measurement (IFM) receiver is capable of measuring the center frequency of single frequency pulses over a wide range (bandwidth) of center frequencies. Because of various constraints, the frequency resolution requirement results in long correlator delay times that reduce the single correlator bandwidth. A large bandwidth can be achieved only if two or more correlators are used. The problem of estimating frequency is then reduced to the simultaneous congruence problem of number theory. A design procedure is presented for solving the congruence problem for a given amount of noise protection, a stated frequency resolution, a minimum bandwidth, and a fixed level of precision (bits) in the IFM receiver  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