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   2686篇
  免费   4篇
  国内免费   6篇
航空   1321篇
航天技术   923篇
综合类   9篇
航天   443篇
  2021年   18篇
  2019年   16篇
  2018年   110篇
  2017年   63篇
  2016年   40篇
  2015年   14篇
  2014年   56篇
  2013年   94篇
  2012年   56篇
  2011年   96篇
  2010年   76篇
  2009年   123篇
  2008年   142篇
  2007年   66篇
  2006年   63篇
  2005年   65篇
  2004年   67篇
  2003年   85篇
  2002年   44篇
  2001年   99篇
  2000年   49篇
  1999年   66篇
  1998年   70篇
  1997年   68篇
  1996年   65篇
  1995年   87篇
  1994年   89篇
  1993年   46篇
  1992年   52篇
  1991年   15篇
  1990年   24篇
  1989年   51篇
  1988年   32篇
  1987年   20篇
  1986年   29篇
  1985年   74篇
  1984年   70篇
  1983年   43篇
  1982年   70篇
  1981年   67篇
  1980年   17篇
  1979年   10篇
  1978年   32篇
  1977年   12篇
  1975年   25篇
  1974年   18篇
  1973年   16篇
  1972年   17篇
  1970年   9篇
  1969年   12篇
排序方式: 共有2696条查询结果,搜索用时 651 毫秒
581.
An optimized ambiguity search method is described in this paper focusing on speed and reliability. The method has incorporated several existing fast ambiguity search concepts and the ambiguity search speed can be further improved. In addition to the improvement of the ambiguity search speed, special attention has been paid to the improvement of the reliability of the ambiguity search process. Several theoretical and practical aspects involved in an ambiguity search algorithm are also addressed which have not been explored in previous literatures  相似文献   
582.
The UV telescope of 30 cm diameter equipped with a three channel scanner spectrometer aboard the highly eccentric orbit space station “ASTRON” is describedAbsolute calibration of the whole device is performed using spectra of standard stars, of which absolute values were obtained by TDI or IUE experiments. Several photometric controls have been made on the stability of the equipment after one year of operation which demonstrate reliability of some first results obtained by the UFT.The sensitivity curve of the spectrophotometric system has been observed with a reproducibility of 2σ = 10% or better.The repeated observation of one star (139 Tau) indicates that the overall performance exhibits the same quality after one year in orbit. The low resolution mode (Δ λ = 28 A?) leads to a powerful new investigation of faint extended sources.  相似文献   
583.
Phase accuracy and simplicity advantages of a direct-sampling coherent detection (DCD) system were demonstrated by computer simulation in earlier correspondence. This correspondence reviews the DCD approach and reports the results of experiments involving digitized samples of analog signals taken by a high-quality, off-the-shelf A/Dconverter. IF signals near 10, 20, and 30 MHz were sampled at 5 MHzresulting in rms DCD phase errors generally below 0.50 compared witherrors less than 0.20 computed in the earlier simulations.  相似文献   
584.
Noise subspace techniques in non-gaussian noise using cumulants   总被引:1,自引:0,他引:1  
We consider noise subspace methods for narrowband direction-of-arrival or harmonic retrieval in colored linear non-gaussian noise of unknown covariance and unknown distribution. The non-gaussian noise covariance is estimated via higher order cumulants and combined with correlation information to solve a generalized eigenvalue problem. The estimated eigenvectors are used in a variety of noise subspace methods such as multiple signal classification (MUSIC), MVDR and eigenvector. The noise covariance estimates are obtained in the presence of the harmonic signals, obviating the need for noise-only training records. The covariance estimates may be obtained nonparametrically via cumulant projections, or parametrically using autoregressive moving average (ARMA) models. An information theoretic criterion using higher order cumulants is presented which may be used to simultaneously estimate the ARMA model order and parameters. Third- and fourth-order cumulants are employed for asymmetric and symmetric probability density function (pdf) cases, respectively. Simulation results show considerable improvement over conventional methods with no prewhitening. The effects of prewhitening are particularly evident in the dominant eigenvalues, as revealed by singular value decomposition (SVD) analysis  相似文献   
585.
After more than two years of successful in-orbit operations, the γ-ray coded aperture SIGMA telescope has accumulated 800 hours of live-time observations of the Galactic Center region, including the remarkable hard source identified with the X-ray source 1E 1740.7–2942. The long-term behavior of the soft γ-ray emission of 1E 1740.7–2942, as determined from the SIGMA survey, supplemented with previously available soft γ-ray data, leads to its identification with a singular radio source, which consists of a double sided radio jet emanating from a compact variable core whose variability is correlated with that of the soft γ-ray source. The compact radio core, which lies well inside the improved soft γ-ray (40–150 keV) error circle (27″ radius) derived from the high-resolution SIGMA survey, is also inside the ROSAT and TTM error circle derived respectively in the soft and hard X-ray bands.  相似文献   
586.
Automatic target recognition (ATR) is an important capability for defense applications. Many aspects of image understanding (IU) research are traditionally used to solve ATR problems. The authors discuss ATR applications and problems in developing real-world ATR systems and present the status of technology for these systems. They identify several IU problems that need to be resolved in order to enhance the effectiveness of ATR-based weapon systems. They conclude that technological gains in developing robust ATR systems will lead to significant advances in many other areas of applications of image understanding  相似文献   
587.
The possibilities of using spectrographic observations of microwave radio emission as a solar flare plasma diagnostic are discussed. The spectral fine structure of the emission is interpreted in the context of plasma emission mechanisms. The balance equations for particles and plasma turbulence together with the transfer equations for electromagnetic waves in a plasma are solved for a model containing a diverging magnetic loop. As a result of the analysis of the blip-type spectral feature, the structure of energy release region and the unperturbed plasma concentration in the preflare loop are evaluated. The number of accelerated electrons and the intensity of the plasma turbulence in the source region are estimated using the properties of the weak continuum emission following the blip. Based on the degree of circular polarization of both the narrow band and the continuum emission, estimates for the external magnetic field strength and the angular width of the radiating plasma turbulence have been obtained.  相似文献   
588.
The paper reviews the main elements and major issues affecting human productivity and discusses some selected factors which might not be sufficiently identified and/or considered from the Bioastronautics point of view at the present stage of planning Space Station design and operation.  相似文献   
589.
Time dependent evolutions of the profile of free surface (bubble shapes) for a cylindrical container partially filled with a Newtonian fluid of constant density, rotating about its axis of symmetry, have been studied. Numerical computations of the dynamics of bubble shapes have been carried out with the following situations: (1) linear functions of spin-up and spin-down in low and microgravity environments, (2) step functions of spin-up and spin-down in a low gravity environment, and (3) sinusoidal function oscillation of gravity environment in high and low rotating cylinder speeds. The initial condition of bubble profiles was adopted from the steady-state formulations in which the computer algorithms have been developed by Hung and Leslie/1/, and Hung et al./2/  相似文献   
590.
Directional solidification of eutectic Al-Cu alloys has been studied to demonstrate the influence of both impurity concentration and convective conditions. The λ/R relations show that higher iron concentration and intensive stirring coarsen the microstructure. The formation and growth of new iron-containing phases at the solidification front restrict impurity influence. Kinetic data follow a dependence λ.Rn=const with n-value close to 0,5.  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