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   7542篇
  免费   47篇
  国内免费   54篇
航空   3498篇
航天技术   2442篇
综合类   48篇
航天   1655篇
  2021年   86篇
  2019年   50篇
  2018年   245篇
  2017年   176篇
  2016年   132篇
  2015年   68篇
  2014年   195篇
  2013年   234篇
  2012年   235篇
  2011年   391篇
  2010年   283篇
  2009年   387篇
  2008年   403篇
  2007年   284篇
  2006年   159篇
  2005年   222篇
  2004年   192篇
  2003年   232篇
  2002年   181篇
  2001年   234篇
  2000年   131篇
  1999年   171篇
  1998年   190篇
  1997年   111篇
  1996年   174篇
  1995年   205篇
  1994年   192篇
  1993年   131篇
  1992年   151篇
  1991年   49篇
  1990年   48篇
  1989年   145篇
  1988年   59篇
  1987年   56篇
  1986年   65篇
  1985年   184篇
  1984年   147篇
  1983年   107篇
  1982年   116篇
  1981年   214篇
  1980年   51篇
  1979年   45篇
  1978年   50篇
  1977年   42篇
  1975年   54篇
  1974年   39篇
  1973年   32篇
  1972年   40篇
  1971年   35篇
  1970年   39篇
排序方式: 共有7643条查询结果,搜索用时 453 毫秒
711.
Krymskii  A.M.  Breus  T.K.  Ness  N. F  AcuÑa  M.H. 《Space Science Reviews》2000,92(3-4):535-564
The Mars Global Surveyor mission has revealed that localized crustal paleomagnetic anomalies are a common feature of the Southern Hemisphere of Mars. The magnetometer measured small-scale magnetic fields associated with many individual magnetic anomalies have magnitudes ranging from hundreds to thousands nT at altitude above 120 km. That makes Mars globally different from both Venus and Earth. The data collected by Lunar Prospector near the Moon were interpreted as evidence that above regions of inferred strong surface magnetic fields on the Moon the SW flow is deflected, and a small-scale mini-magnetosphere exists under some circumstances. With a factor of 100 stronger magnetic fields at Mars and a lower SW dynamic pressure, those conditions offer the opportunity for a larger size of small `magnetospheres' which can be formed by the crustal magnetic fields. Outside the regions of the magnetic anomalies, the SW/Mars interaction is Venus-like. Thus, at Mars the distinguishing feature of the magnetic field pile-up boundary most likely varies from Venus-like to Earth-like above the crustal magnetic field regions. The observational data regarding the IMF pile-up regions near Venus and the Earth are initially reviewed. As long as the SW/Mars interaction remains like that at Venus, the IMF penetrates deep into the Martian ionosphere under the `overpressure' conditions. Results of numerical simulations and theoretical expectations regarding the temporal evolution of the IMF inside the Venus ionosphere and appearance of superthermal electrons are also reviewed and assessed.  相似文献   
712.
The results of a preliminary analysis of microperturbations on the International Space Station during physical exercises of the crew are presented. The goal of this paper is to identify the parameters of perturbations when physical exercises are performed. The results of measurements by sensors of microaccelerations of both Russian and American segments during physical exercises in the service module of the Russian segment are analyzed.  相似文献   
713.
The paper describes the first results of all-sky polarization measurements of the twilight background started in central Russia in the very beginning of summer 2011. Time-frequency data of the sky intensity and polarization over a wide range of sky point zenith distances are used to separate single and multiple scattering and construct the altitude dependence of the scattering coefficient and polarization in the mesosphere (altitudes from 60 to 90 km) at different angles. The undisturbed structure of the mesosphere without noticeable aerosol stratification on observation days makes it possible to estimate the temperature of the atmosphere at these altitudes.  相似文献   
714.
Stern  S.A. 《Space Science Reviews》1999,90(1-2):355-361
The remote sensing of comets in the ultraviolet bandpass has been a valuable tool for studying the structure, composition, variability, and physical processes at work in cometary comae. By extension, these studies of comae have revealed key insights into the composition of cometary nuclei. Here we briefly review the ultraviolet studies of comets, and then take a look toward the future of such work as anticipated by the advent of several key new instruments. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
715.
A method is substantiated to ensure energy security for the satellite communication systems (SCS) at a close position of the radio interception receiver. This is done by lowering the carrier frequency down to f 0 = 60…80 MHz and by applying spaced measurements with n ≥ 4 receiving antennas.  相似文献   
716.
In order to observe exoplanets we propose a space-based achromatic stellar coronagraph combined with a 0.8–1.5 m telescope. We develop an achromatic common path interferometer for observing an exoplanet (a faint off-axis source) on the background of a hoste star (bright axial source). An image of the star and its copy acquire an achromatic phase shift by 180° and interfere in antiphase. The achromatic phase shift is caused by geometric phase in the scheme of a three-dimensional interferometer. The interference process divides spatially the dark and light fields of the star image redirecting them to the opposite sides of a beam splitter. The interference process does not weaken the image of a planet, with equal intensities it is redirected to both sides of a beam splitter. The suggested scheme of common path interferometer ensures mechanical stability. The background signal is experimentally demonstrated to be reduced by six orders of magnitude.  相似文献   
717.
An efficient self-contained trajectory optimization software is generated by making use of de Pontécoulant's analytic lunar theory removing the need for an outside third body ephemeris program to compute the lunar and solar position vectors at each integration step. The accelerations being further resolved along the rotating Euler–Hill frame after expansion to third order in the spacecraft radial distance, the adjoint differential equations are derived in a direct manner complementing the generation of the dynamic system of equations for full compatibility. Because the variation of parameters equations are cast in terms of the nonsingular equinoctial elements with the perturbation accelerations resolved in their analytic form along the rotating axes, the adjoint equations are also derived in the same manner providing a highly efficient and accurate system of equations for rapid computations in conjunction with Aerospace Corporation's NLP2 nonlinear programming codes to search for the initial values of the multipliers that steer the spacecraft towards its target orbit in minimum time. Numerical simulations show that the solutions obtained by the analysis developed in this paper are essentially identical to the more indirect approach based on the use of inertial accelerations obtained from a separate ephemeris generator and subsequent conversions to the thrust frame and equinoctial system.  相似文献   
718.
The constraints imposed on parameters of a force field and of a symmetrical gyrostat in it are determined, at which regular precession of this gyrostat demonstrate Lyapunov stability (or, respectively, instability).  相似文献   
719.
Future space exploration may involve communications between spacecraft moving at relativistic velocities. One of the significant problems associated with such communication is spectral distortion of signals which are propagated between relativistic frames. This distortion is generated by both changing propagation distances and purely relativistic electromagnetic field transformations. In this paper a linear integral transformation is formulated for relating the Fourier spectra of the source antenna excitation current and the resulting incident electric field at the receiving antenna. The kernel of the transformation is evaluated for the case of a steerable source antenna tracking on the advanced receiver position. The transformation is then applied to the case of an ideal thin-wire half-wave dipole source antenna excited by a narrowband, double-sideband modulated current. The specific distortions of spectral spreading and translation are then related to increased bandwidth and upper cutoff frequency requirements of receiving systems in relativistic applications.  相似文献   
720.
We address the problem of detection of targets obscured by a forest canopy using an ultrawideband (UWB) radar. The forest clutter observed in the radar imagery is a highly impulsive random process that is more accurately modeled with the recently proposed class of alpha-stable processes as compared with Gaussian, Weibull, and K-distribution models. With this more accurate model, segmentation is performed on the imagery into forest and clear regions. Further, a region-adaptive symmetric alpha stable (SαS) constant false-alarm rate (CFAR) detector is introduced and its performance is compared with the Weibull and Gaussian CFAR detectors. The results on real data show that the SαS CFAR performs better than the Weibull and Gaussian CFAR detectors in detecting obscured targets  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