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   3518篇
  免费   56篇
  国内免费   48篇
航空   1740篇
航天技术   1206篇
综合类   31篇
航天   645篇
  2021年   39篇
  2019年   29篇
  2018年   60篇
  2017年   58篇
  2016年   52篇
  2015年   35篇
  2014年   89篇
  2013年   103篇
  2012年   88篇
  2011年   139篇
  2010年   83篇
  2009年   149篇
  2008年   161篇
  2007年   103篇
  2006年   77篇
  2005年   105篇
  2004年   98篇
  2003年   129篇
  2002年   108篇
  2001年   127篇
  2000年   70篇
  1999年   77篇
  1998年   87篇
  1997年   61篇
  1996年   97篇
  1995年   115篇
  1994年   116篇
  1993年   55篇
  1992年   71篇
  1991年   36篇
  1990年   32篇
  1989年   82篇
  1988年   28篇
  1987年   24篇
  1986年   36篇
  1985年   93篇
  1984年   93篇
  1983年   66篇
  1982年   67篇
  1981年   100篇
  1980年   21篇
  1979年   26篇
  1978年   32篇
  1977年   27篇
  1975年   33篇
  1974年   24篇
  1973年   22篇
  1972年   25篇
  1970年   21篇
  1969年   21篇
排序方式: 共有3622条查询结果,搜索用时 15 毫秒
921.
The RADiatiOn Monitor (RADOM) is a miniature dosimeter-spectrometer that flew onboard the Chandrayaan-1 lunar mission in order to monitor the local radiation environment. Primary objective of the RADOM experiment was to measure the total absorbed dose, flux of surrounding energetic particles and spectrum of the deposited energy from high energy particles both en-route and in lunar orbit. RADOM was the first experiment to be switched on after the launch of Chandrayaan-1 and was operational until the end of the mission. This paper summarizes the observations carried out by RADOM during the entire life time (22 October 2008–31 August 2009) of the Chandrayaan-1 mission and compares the measurement by RADOM with the radiation belt models such as AP-8, AE-8 and CRRESS.  相似文献   
922.
The structure of Swerling's 1957 model for fluctuating radar cross section and signal amplitude is examined prinwrily in terms of mixture representations. The authors also show the relationship of Nakagami's m distrubution to Swerling's amplitude distribution, and Nakagami's q distribution (Hoyt vector density) is reduced to the Swerling distribution. Nonuniform phase distributions associated with Swerling's functions are also derived. Results are given when Swerling's amplitude density is used as a prior distribution with Bennett's (Rice's) distribution for the amplitude of a sine wave plus narrowband Gaussian noise.  相似文献   
923.
924.
925.
Knowledge of geomagnetism and its effects was recently much augmented during the IGY; when added to what is known of stellar fields, interplanetary fields, and meteoric magnetism, and combined with magnetic theory, this knowledge not only suggests that much may be learned of our neighbors in space by making an early determination of their magnetic fields, but also provides a foundation for many inferences and rough estimates on the magnetic fields of other solar-system bodies. Present estimates range from 660 cgs for Jupiter to 0.004 for the smaller satellites and the planetoids.  相似文献   
926.
927.
A family of 4-wire electronic equipments is being developed for use in a tactical switching system for the field army. The characteristics and design of two automatic switchboards to be used in the system are discussed, as well as the signaling methods and numbering plan that are employed.  相似文献   
928.
The design of an adaptive tracking filter control system and its application to large flexible booster vehicles is described. The problem of elastic vehicle stability is considered along with its solution by conventional and adaptive techniques. The improvement in vehicle stability, which may be achieved with the adaptive tracking filter technique as compared to conventional techniques, is shown. The control system evolved uses two adaptive tracking filters to phase stabilize the first and second bending modes, in addition to conventional compensation techniques. The application of this adaptive technique to a vehicle in which the modal frequencies are in close proximity (<2.5 percent separation) is discussed. The mechanization of the adaptive control system involves the selection of design techniques and components that are electrically and physically compatible with the intended airborne application. The development of a suitable frequency tracking technique and tracking filter is described, in addition to the electronic and mechanical design of the adaptive control system prototype.  相似文献   
929.
A prominent neuroscientist A. Damasio has observed that “More may have been learned about the brain and the mind in the 1990s... than during the entire previous history of psychology and neuroscience” (1999). This progress, which should continue at an exponentially increasing pace as we move into the 21st century, sets the stage for another revolution-in the scientific formulation of intentional behavior. Just as applied sciences such as fluid mechanics and solid-state electronics benefited from breakthroughs in physics, so it may be expected that social psychology and economics, for example, will similarly benefit from our expanding knowledge of psychosomatic and psychological function. Economics, however, with its basic concepts dating from the nineteenth century, must endure an extensive renewal. Toward this end, a new mathematical theory of behavior was formulated in the early 1990s based on the contributions of prominent psychologists and economists over the past two centuries. Presentation of the new approach at international conferences continued in 2000 with attention moving beyond the fundamentals into macroeconomic concerns including business cycles, productivity, and unemployment  相似文献   
930.
The Coronal Diagnostic Spectrometer (CDS) on SOHO is a grazing/normal incidence spectrograph, aimed to produce stigmatic spectra of selected regions of the solar surface in six spectral windows of the extreme ultraviolet from 150 Å to 785 Å (Harrison et al. 1995). In the present work, CDS, EIT, MDI and Yohkoh observations of active region lops have been analyzed. These observations are part of JOP 54. CDS monochromatic images from lines at different temperatures have been co-aligned with EIT and MDI images, and loop structures have been clearly identified using Fe XVI emission lines. Density sensitive lines and lines from adjacent stages of ionization of Fe ions have been used to measure electron density and temperature along the loop length; these measurements have been used to determine the electron pressure along the loop and test the constant pressure assumption commonly used in loop modeling. The observations have been compared with a static, isobaric loop model (Landini and Monsignori Fossi 1975) assuming a temperature-constant heating function in the energy balance equation. Good agreement is found for the temperature distribution along the loop at the coronal level. The model pressure is somewhat higher than obtained from density sensitive line ratios.  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