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   7389篇
  免费   9篇
  国内免费   16篇
航空   3463篇
航天技术   2321篇
综合类   196篇
航天   1434篇
  2021年   61篇
  2019年   37篇
  2018年   177篇
  2017年   129篇
  2016年   141篇
  2015年   49篇
  2014年   163篇
  2013年   209篇
  2012年   202篇
  2011年   283篇
  2010年   206篇
  2009年   338篇
  2008年   335篇
  2007年   213篇
  2006年   147篇
  2005年   175篇
  2004年   179篇
  2003年   225篇
  2002年   248篇
  2001年   298篇
  2000年   124篇
  1999年   163篇
  1998年   199篇
  1997年   131篇
  1996年   169篇
  1995年   234篇
  1994年   197篇
  1993年   107篇
  1992年   153篇
  1991年   64篇
  1990年   81篇
  1989年   154篇
  1988年   64篇
  1987年   72篇
  1986年   63篇
  1985年   205篇
  1984年   162篇
  1983年   148篇
  1982年   150篇
  1981年   218篇
  1980年   78篇
  1979年   59篇
  1978年   68篇
  1977年   47篇
  1976年   51篇
  1975年   57篇
  1974年   49篇
  1972年   60篇
  1971年   56篇
  1969年   38篇
排序方式: 共有7414条查询结果,搜索用时 604 毫秒
811.
The paper is related to specific emissions at frequency <3 MHz observed by the low altitude satellite DEMETER in relation with the thunderstorm activity. At its altitude (~700 km), the phenomena observed on the E-field and B-field spectrograms recorded by the satellite are mainly dominated by whistlers. Particular observations performed by DEMETER are reported. It concerns multiple hop whistlers and interaction between whistlers and lower hybrid noise. Two new phenomena discovered by the satellite are discussed. First, V-shaped emissions up to 20 kHz are observed at mid-latitude during night time. They are centered at the locations of intense thunderstorm activity. By comparison with VLF saucers previously observed by other satellites in the auroral zones it is hypothesized that the source region is located below the satellite and that the triggering mechanism is due to energetic electrons accelerated during sprite events. Second, emissions at frequency ~2 MHz are observed at the time of intense whistlers. These emissions are produced in the lower ionosphere in probable relation with Transient Luminous Events (TLEs).  相似文献   
812.
This describes a NOAA-14 spacecraft loss of control event and the lessons learned. The lessons learned are in two categories: contingency recovery operations; and on-orbit propulsion system isolation methods. The work is motivated in recognition of the effort of the NOAA off-line engineering support team and their NOAA civil servant mission leads and operations support staff.  相似文献   
813.
814.
Congress authorized NASA's Prometheus Project in February 2003, with the first Prometheus mission slated to explore the icy moons of Jupiter. The project had two major objectives: 1) to develop a nuclear reactor that would provide unprecedented levels of power and show that it could be processed safely and operated reliably in space for long-duration, deep-space exploration; and 2) to explore the three icy moons of Jupiter - Callisto, Ganymede, and Europa - and return science data that would meet the scientific goals as set forth in the Decadal Survey Report of the National Academy of Sciences. Early in project planning, it was determined that the development of the Prometheus nuclear-powered spaceship would be complex and require the intellectual knowledge residing at numerous organizations across the country. In addition, because of the complex nature of the project and the multiple partners, approaches beyond those successfully used to manage a typical JPL project would be needed. This describes the key experiences in managing Prometheus, which should prove useful for future projects of similar scope and magnitude.  相似文献   
815.
This paper provides a brief summary on the current knowledge of the properties of the Circum-Heliospheric Interstellar Medium (CHISM). It discusses what can be learnt on the parameters of CHISM’s components from analysis of measurements performed inside the heliosphere. The analysis is based on the kinetic-gasdynamic models of the solar wind/interstellar medium interaction. We focus the analysis on three types of diagnostics: 1) interstellar H atom number density at the heliospheric termination shock inferred from pickup ion measurements, 2) the location and time of the Voyager 1 and 2 termination shock crossings, 3) the deflection of the interstellar H atom flow inside the heliosphere as been measured by SOHO/SWAN. From these results estimations of the unknown local interstellar parameters are deduced. The parameters are the number densities of interstellar H+ and H and the magnitude and direction of the interstellar magnetic field in the vicinity of the solar system.  相似文献   
816.
Network management is one of the key technologies needed to fulfill the requirements. In other words, a new paradigm for network management is also needed, in order to address the requirements arising both from the ATN side and the telecommunication side of the new telecommunication infrastructure. In particular, the new network management paradigm will incorporate the ATN management framework consisting of related standards, along with the telecommunication management framework, notably Telecommunication Management Network (TMN) standards from ITU-T.  相似文献   
817.
The results of a study of the characteristics of the second-order Type II phase-locked loop with a Gaussian noise input, and obtained by digital computer simulation, are presented. The digital simulation is described and the random state variables are defined such that their characteristics can be interpreted in terms of existing phase portraits of autonomous phase-locked loops. The statistics associated with the state variables, which are phase error and a measure of frequency error, and those associated with the number of cycles skipped and the mean time to unlock, are given.  相似文献   
818.
819.
820.
This rapporteur paper discusses the solar corona and the solar wind in the context of their chemical composition. The abundances of elements, both obtained by optical and by in situ observations, are used to infer the sources of the slow solar wind and of the fast streams. The first ionisation potential (FIP) fractionation effect is also discussed, in particular the agreed basics and the open questions. This revised version was published online in June 2006 with corrections to the Cover Date.  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