全文获取类型
收费全文 | 18705篇 |
免费 | 240篇 |
国内免费 | 440篇 |
专业分类
航空 | 10551篇 |
航天技术 | 5389篇 |
综合类 | 522篇 |
航天 | 2923篇 |
出版年
2021年 | 173篇 |
2018年 | 185篇 |
2016年 | 171篇 |
2014年 | 502篇 |
2013年 | 587篇 |
2012年 | 485篇 |
2011年 | 643篇 |
2010年 | 509篇 |
2009年 | 849篇 |
2008年 | 877篇 |
2007年 | 432篇 |
2006年 | 518篇 |
2005年 | 423篇 |
2004年 | 451篇 |
2003年 | 523篇 |
2002年 | 498篇 |
2001年 | 607篇 |
2000年 | 404篇 |
1999年 | 512篇 |
1998年 | 467篇 |
1997年 | 364篇 |
1996年 | 404篇 |
1995年 | 466篇 |
1994年 | 423篇 |
1993年 | 399篇 |
1992年 | 298篇 |
1991年 | 279篇 |
1990年 | 256篇 |
1989年 | 405篇 |
1988年 | 208篇 |
1987年 | 239篇 |
1986年 | 227篇 |
1985年 | 637篇 |
1984年 | 511篇 |
1983年 | 397篇 |
1982年 | 483篇 |
1981年 | 610篇 |
1980年 | 243篇 |
1979年 | 184篇 |
1978年 | 189篇 |
1977年 | 144篇 |
1976年 | 156篇 |
1975年 | 184篇 |
1974年 | 181篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Tracking multiple objects with particle filtering 总被引:8,自引:0,他引:8
Hue C. Le Cadre J.-P. Perez P. 《IEEE transactions on aerospace and electronic systems》2002,38(3):791-812
We address the problem of multitarget tracking (MTT) encountered in many situations in signal or image processing. We consider stochastic dynamic systems detected by observation processes. The difficulty lies in the fact that the estimation of the states requires the assignment of the observations to the multiple targets. We propose an extension of the classical particle filter where the stochastic vector of assignment is estimated by a Gibbs sampler. This algorithm is used to estimate the trajectories of multiple targets from their noisy bearings, thus showing its ability to solve the data association problem. Moreover this algorithm is easily extended to multireceiver observations where the receivers can produce measurements of various nature with different frequencies. 相似文献
993.
994.
995.
996.
Fahr H. J. Neutsch W. Grzedzielski S. Macek W. Ratkiewicz-Landowska R. 《Space Science Reviews》1986,43(3-4):329-381
Existing heliopause models are critically rediscussed under the new aspect of possible plasma mixing between the solar wind and the ambient ionized component of the local interstellar medium (LISM). Based on current kinetic plasma theories, effective diffusion rates across the heliopause are evaluated for several models with turbulence caused by electrostatic or electromagnetic interactions that could be envisaged in this context. Some specific cases that may lead to high diffusion rates are investigated, especially in regard to their LISM magnetic field dependence.For weak fields (less than 10–7 G), macroscopic hydrodynamic instabilities, such as of Rayleigh-Taylor or Kelvin-Helmholtz-types, can be excited. The resulting plasma mixing rates at the heliopause may amount to 20–30% of the impinging mass flow.Recently, an unconventional new approach to the problem for the case of tangential magnetic fields at the heliopause was published in which a continuous change of the plasma properties within an extended boundary layer is described by a complete set of two-fluid plasma equations including a hybrid MHD-formulation of wave-particle interaction effects. If a neutral sheet is assumed to exist within the boundary layer, the magnetic field direction is proven to be constant for a plane-parallel geometry. Considering the electric fields and currents in the layer, an interesting relationship between the field-reconnection probability and the electric conductivity can be derived, permitting a quantitative determination of either of these quantities.An actual value for the electrical conductivity is derived here on the basis of electron distribution functions given by a superposition of Maxwellians with different temperatures. Using two-stream instability theory and retaining only the most unstable modes, an exact solution for the density, velocity, and magnetic and electric fields can be obtained. The electrical conductivity is then shown to be six orders of magnitude lower than calculated by conventional formulas. Interestingly, this leads to an acceptable value of 0.1 for the reconnection coefficient.By analogy with the case of planetary magnetopauses, it is shown here for LISM magnetic fields of the order of 10–6 G or larger that field reconnection processes may also play an important role for the plasma mixing at the heliopause. The resulting plasma mixing rate is estimated to amount to an average value of 10% of the incident mass flow. It is suggested here that the dependence of the cosmic-ray penetration into the heliosphere on the distribution of reconnecting areas at the heliopause may provide a means of deriving the strength and orientation of the LISM field.A series of observational implications for the expected plasma mixing at the heliopause is discussed in the last part of the paper. In particular, consequences are discussed for the generation of radio noise at the heliopause, for the penetration of LISM neutrals into the heliosphere, for the propagation of cosmic rays towards the inner part of the solar system and for convective electric field mergings into the heliosphere during the course of the solar cycle, depending on the solar cycle variations. With concern to a recent detection of electrostatic plasma waves by plasma receivers on Voyagers 1 and 2, we come to an interesting alternate explanation: the heliopause, rather than the heliospheric shock front, could be responsible for the generation of these waves. 相似文献
997.
Time of arrival (TOA) estimation of narrowband signals is a problem of considerable practical interest in radar and sonar applications. A new technique is presented to analyze the mean square error (MSE) performance of TOA estimation schemes, based on recently developed lower bound. We obtain a complete characterization of the MSE as a function of the signal and noise parameters. The results are given in a simple closed-form analytical expression. 相似文献
998.
在桨扇技术发展过程中,桨扇的气动弹性力学与气动声学是主要技术关键中的两个很受重视的问题。本文对于上述二问题的历史发展过程,处理方法及己取得的主要成果作了阐述,对于其现状和发展趋势提出了一些看法。 相似文献
999.
1000.
Mr.WANG ZHENHUA 《航空动力学报》1987,2(4):384-384
The Journal of Aerospace Power is a leading publication of its kind in China devoted to the advancement of the science and technology of aerospace power.It provides a common medium through which academic and industrial researchers and designers can communicate results, review progress and identify requirements for further work in this increasingly significant field. 相似文献