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   3511篇
  免费   23篇
  国内免费   11篇
航空   1791篇
航天技术   1199篇
综合类   18篇
航天   537篇
  2021年   30篇
  2019年   29篇
  2018年   63篇
  2017年   47篇
  2016年   48篇
  2015年   23篇
  2014年   67篇
  2013年   84篇
  2012年   81篇
  2011年   118篇
  2010年   91篇
  2009年   137篇
  2008年   185篇
  2007年   91篇
  2006年   81篇
  2005年   95篇
  2004年   86篇
  2003年   113篇
  2002年   70篇
  2001年   122篇
  2000年   73篇
  1999年   88篇
  1998年   108篇
  1997年   70篇
  1996年   95篇
  1995年   131篇
  1994年   102篇
  1993年   62篇
  1992年   91篇
  1991年   34篇
  1990年   39篇
  1989年   86篇
  1988年   43篇
  1987年   37篇
  1986年   36篇
  1985年   96篇
  1984年   92篇
  1983年   82篇
  1982年   82篇
  1981年   90篇
  1980年   27篇
  1979年   27篇
  1978年   29篇
  1977年   27篇
  1976年   19篇
  1975年   35篇
  1974年   25篇
  1973年   18篇
  1972年   33篇
  1971年   18篇
排序方式: 共有3545条查询结果,搜索用时 93 毫秒
11.
This paper discusses and evaluates two innovative SETI education programs conducted at the University of Western Sydney, viz: the SETI Pathways Program and the Life in the Universe Curriculum Project.  相似文献   
12.
13.
A major technology barrier to the application of pulse compression for the meteorological functions required by a next generation ATC radar is range/time sidelobes which mask and corrupt observations of weak phenomena occurring near areas of strong extended meteorological scatterers. Techniques for suppressing range sidelobes are well known but without prior knowledge of the scattering medium's velocity distribution their performance degrades rapidly in the presence of Doppler. Recent investigations have presented a “doppler tolerant” range sidelobe suppression technique. The thrust of the work described herein is the extension of previous simulations to actual transmitted dispersed/coded waveforms using the S-band surveillance radar located at Rome Laboratory Surveillance Facility. The objectives of the experiment are: 1) to extend the verification of the simulation of the Doppler tolerant technique; and 2) to demonstrate that the radar transmitter, waveform generator, and receiver imperfections do not significantly degrade resolution, performance or reliability of meteorological spectral moment estimates  相似文献   
14.
The first estimations of the aerosol optical thickness (AOT) using Langley Method at Southern Space Observatory (SSO) at Southern Brazil (29.4°S, 53.8°W) are presented. In addition to ozone and sulphur dioxide columns, AOT can be obtained using Brewer Spectrophotometer at specific wavelengths: 306.3, 310.1, 313.5, 316.8 and 320.1 nm. The AOT was obtained for the period from November/2002 to May/2003. Very low AOT averages were obtained, whose values were about 0.21 ± 0.03 at 306.3 nm, 0.21 ± 0.02 at 310.1 nm, 0.19 ± 0.02 at 313.5 nm, 0.20 ± 0.02 at 316.8 nm and 0.20 ± 0.02 at 320.0 nm for all period analysed. Different behaviour of AOT were obtained at two daily specific periods of aerosol accumulation, one in the afternoons from November/2002 to February/2003, caused mainly by a mild biomass burning season’s in the region and other in the mornings from March to May/2003, due the high relative humidity presented in the region studied.  相似文献   
15.
The latest generation of computer vision technology is revolutionizing concepts, applications, and products in video surveillance and CCTV. This is of prime relevance to security for large outdoor facilities such as commercial airfields, refineries, power plants, and office/industrial campuses. Most airfields, for example, have open (unfenced) perimeters, high volume heterogeneous traffic, are easily accessed on foot or by water, and exist in areas where regulations providing a safety buffer are difficult to legislate or enforce. And all airfields require 24/7 outdoor monitoring - snow, fog, rain, or shine. Likewise, most high-value facilities appealing to criminals and terrorists are in close proximity to public areas (roads, residences, city, etc.). The appeal of automated real-time surveillance is obvious $maximizing efficiency and effectiveness of security personnel and resources while increasing the probability of preventing a serious security breach. Computer vision based solutions have the potential for very discriminating detection and very low false alarms. The bottom line is that applied computer vision has the potential for the greatest return on investment (ROI), both short-term and long-term.  相似文献   
16.
研究了表贴式永磁电机的两种充磁方式。首先利用已有文献电机参数验证了有限元模型的正确性,对比分析了两种充磁方式下电机磁密和磁钢磁势的分布特点,研究表明内转子结构平行充磁方式具有提供更大的气隙磁通潜力,而外转子结构则相反。接着研究了两种充磁方式,不同气隙下气隙总磁通随极对数和磁钢厚度的变化规律。同时明确了两种充磁方式各自使用场合,给出了最佳磁钢厚度的取值范围。本文工作有助于正确选择表贴式永磁电机的充磁方式和磁钢厚度,以发挥磁钢潜力,改善电机性能。  相似文献   
17.
Deep-space manned flight as a reality depends on a viable solution to the radiation problem. Both acute and chronic radiation health threats are known to exist, with solar particle events as an example of the former and galactic cosmic rays (GCR) of the latter. In this experiment Iron ions of 1A GeV are used to simulate GCR and to determine the secondary radiation field created as the GCR-like particles interact with a thick target. A NASA prepared food pantry locker was subjected to the iron beam and the secondary fluence recorded. A modified version of the Monte Carlo heavy ion transport code developed by Zeitlin at LBNL is compared with experimental fluence. The foodstuff is modeled as mixed nuts as defined by the 71st edition of the Chemical Rubber Company (CRC) Handbook of Physics and Chemistry. The results indicate a good agreement between the experimental data and the model. The agreement between model and experiment is determined using a linear fit to ordered pairs of data. The intercept is forced to zero. The slope fit is 0.825 and the R2 value is 0.429 over the resolved fluence region. The removal of an outlier, Z=14, gives values of 0.888 and 0.705 for slope and R2 respectively.  相似文献   
18.
The work studies the application of the morphological (structural) synthesis method of search for the new technical approaches. The method is based on the application of the theory of system, morphological and cluster analysis, methods of mathematics simulation and calculation experiments made by computer. The work shows how this method is used to synthesize the landing spacecrafts, hypersonic spacecrafts and medical equipment and gives the comparison of the present and synthesized technical systems (TS). The conclusion is that this method increases the degree of the validity decision made and broadens the number of possible variants which allows an increase in the quality (technical level) of the TS under development. The work also points out that the method allows the system to search for promising inventions to solve the problems of prognostication and to single out the trends of TS evolution. It also allows the system to single out these systems which are able to respond flexibly to the changing external conditions with the lowest expense, and therefore, increases the compatibility of the chosen TS  相似文献   
19.
The problem of vibration suppression in large flexible space structure (LFSS) is investigated. Based on the structure property of LFSS, an active damping strategy is proposed to effectively attenuate the critical vibrations of the structure subject to modeling uncertainty and external disturbance. Control algorithms are derived with the aim of suppressing both the vibrating magnitude and vibrating rate to an acceptable level. It is shown that the strategy exhibits robust and adaptive properties and is truly model-independent. The novelty of the proposed approach lies in the fact that it is fairly easy to set up the strategy and the overall computation involved is much less than any other strategies available to date. A two-bay truss is used to verify the validity of the proposed approach  相似文献   
20.
A large fraction of ISO observing time was used to study the late stages of stellar evolution. Many molecular and solid state features, including crystalline silicates and the rotational lines of water vapour, were detected for the first time in the spectra of (post-)Asymptotic Giant Branch (AGB) stars. Their analysis has greatly improved our knowledge of stellar atmospheres and circumstellar environments. A surprising number of objects, particularly young planetary nebulae with Wolf-Rayet (WR) central stars, were found to exhibit emission features in their ISO spectra that are characteristic of both oxygen-rich and carbon-rich dust species, while far-IR observations of the PDR around NGC 7027 led to the first detections of the rotational line spectra of CH and CH+. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.  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