全文获取类型
收费全文 | 8728篇 |
免费 | 645篇 |
国内免费 | 506篇 |
专业分类
航空 | 4500篇 |
航天技术 | 2871篇 |
综合类 | 444篇 |
航天 | 2064篇 |
出版年
2023年 | 42篇 |
2022年 | 113篇 |
2021年 | 195篇 |
2020年 | 131篇 |
2019年 | 144篇 |
2018年 | 244篇 |
2017年 | 214篇 |
2016年 | 190篇 |
2015年 | 180篇 |
2014年 | 299篇 |
2013年 | 328篇 |
2012年 | 338篇 |
2011年 | 434篇 |
2010年 | 402篇 |
2009年 | 512篇 |
2008年 | 503篇 |
2007年 | 347篇 |
2006年 | 294篇 |
2005年 | 313篇 |
2004年 | 263篇 |
2003年 | 300篇 |
2002年 | 269篇 |
2001年 | 283篇 |
2000年 | 228篇 |
1999年 | 238篇 |
1998年 | 224篇 |
1997年 | 203篇 |
1996年 | 214篇 |
1995年 | 264篇 |
1994年 | 231篇 |
1993年 | 128篇 |
1992年 | 187篇 |
1991年 | 84篇 |
1990年 | 77篇 |
1989年 | 144篇 |
1988年 | 58篇 |
1987年 | 48篇 |
1986年 | 68篇 |
1985年 | 180篇 |
1984年 | 136篇 |
1983年 | 107篇 |
1982年 | 134篇 |
1981年 | 165篇 |
1980年 | 47篇 |
1979年 | 32篇 |
1978年 | 40篇 |
1977年 | 37篇 |
1976年 | 31篇 |
1974年 | 35篇 |
1970年 | 27篇 |
排序方式: 共有9879条查询结果,搜索用时 453 毫秒
971.
一种基于SIFT和KLT相结合的特征点跟踪方法研究 总被引:2,自引:0,他引:2
在目标发生明显姿态和大小变化条件下,为了利用基于特征点的跟踪算法实现对目标可靠、稳定跟踪,提出了一种SIFT算法和KLT(Kanade-Lucas-Tomasi)匹配算法相结合的特征点跟踪方法。通过对SIFT算法进行优化,使得到特征点分布相对均匀,同时不存在聚集现象;通过对KLT匹配算法进行分层迭代设计,提高了目标作快速运动时的匹配精度;最后根据特征点匹配结果,结合Greedy算法得到目标的准确位置。实验结果表明:该算法能够很好地适应目标姿态和大小的变化,实现对结构复杂目标的稳定跟踪;比KLT跟踪算法具有更好的鲁棒性和稳定性,能得到更加准确的目标位置。 相似文献
972.
973.
974.
The results of numerical solution of the wave equations for the oblique incidence of whistling electromagnetic waves upon the night ionosphere from above have been obtained and analyzed. In the studied region of altitudes, within the wavelength scale, charged particle concentration varies drastically, and damping caused by collisions between the charged and neutral particles decreases considerably. Below, the sharp lower boundary of the ionosphere, the refractive index of the whistler wave approaches unity, and plasma turbulence transform into atmospheric electromagnetic waves. The dependences of the whistler reflection factor are found in terms of energy and horizontal magnetic component of the electromagnetic wave near the Earth’s surface on the frequency and the wave vector transverse component for the plain-layered medium model at two values of latitude. Strong dependences have been found on the wave angle of incidence and frequency. At rather small angles of incidence, the wave disturbances reach the Earth’s surface, and the module of reflection coefficient logarithm is in the range of 0.4–1. At large angles of incidence, the reflection coefficient module varies over a wide range depending on specific conditions. The obtained results explain the absence of oscillation modes of plasma magnetosphere maser in the night magnetosphere. 相似文献
975.
A.?I.?EfimovEmail author L.?A.?Lukanina I.?V.?Chashei S.?F.?Kolomiets M.?K.?Bird M.?P?tzold 《Cosmic Research》2018,56(1):1-10
In 2013 and 2015, investigations of the internal solar wind were carried out using the method of two-frequency radio sounding by signals from the Mars Express European spacecraft. The values of the S- and X-bands’ frequency and the differential frequency were registered with a sampling rate of 1s at the American and European networks of ground-based tracking stations. The spatial distribution of the frequency fluctuation’s level has been studied. It has been shown that the intensity of frequency fluctuation considerably decreases at high heliolatitudes. In some radio sounding sessions, quasiperiodic oscillations of sub-mHz band have been observed in the temporal spectra of frequency fluctuations; they are supposed to be associated with the density inhomogeneities, the sizes of which are close to the turbulence outer scale. 相似文献
976.
M.?Yu.?Belyaev T.?V.?Matveeva M.?I.?Monakhov D.?N.?Rulev V.?V.?SazonovEmail author 《Cosmic Research》2018,56(1):54-67
We have reconstructed the uncontrolled rotational motion of the Progress M-29M transport cargo spacecraft in the single-axis solar orientation mode (the so-called sunward spin) and in the mode of the gravitational orientation of a rotating satellite. The modes were implemented on April 3–7, 2016 as a part of preparation for experiments with the DAKON convection sensor onboard the Progress spacecraft. The reconstruction was performed by integral statistical techniques using the measurements of the spacecraft’s angular velocity and electric current from its solar arrays. The measurement data obtained in a certain time interval have been jointly processed using the least-squares method by integrating the equations of the spacecraft’s motion relative to the center of mass. As a result of processing, the initial conditions of motion and parameters of the mathematical model have been estimated. The motion in the sunward spin mode is the rotation of the spacecraft with an angular velocity of 2.2 deg/s about the normal to the plane of solar arrays; the normal is oriented toward the Sun or forms a small angle with this direction. The duration of the mode is several orbit passes. The reconstruction has been performed over time intervals of up to 1 h. As a result, the actual rotational motion of the spacecraft relative to the Earth–Sun direction was obtained. In the gravitational orientation mode, the spacecraft was rotated about its longitudinal axis with an angular velocity of 0.1–0.2 deg/s; the longitudinal axis executed small oscillated relative to the local vertical. The reconstruction of motion relative to the orbital coordinate system was performed in time intervals of up to 7 h using only the angularvelocity measurements. The measurements of the electric current from solar arrays were used for verification. 相似文献
977.
978.
The problem of control of the on-board microgravity environment in order to extend the service life of the long-term space station has been discussed. Software developed for the ISS and the results of identifying dynamic models and external impacts based on telemetry data have been presented. Proposals for controlling the onboard microgravity environment for future long-term space stations have been formulated. 相似文献
979.
980.
The results of numerical calculation of the dependences of the electron density, the eigenfrequency and the dielectric plasma permeability on the geometric parameters and the altitude of body motion in the near and far wake behind a thin conical body with a spherical nose blunting have been presented. The electron density maximum has been shown to be located in the region of the neck of the near wake behind the body, which determines the type of this region (supercritical or subcritical). This in turn affects the propagation of radio waves through this plasma region. A comparative analysis was performed for two different bodies with the same ballistic coefficient values. No characteristic distinctions were revealed in the values of electron density or the plasma eigenfrequency in the near and far wake behind these bodies. However, it has been shown that there are differences in the values of the distance from the bottom cross section to the neck of the near wake behind these bodies. 相似文献