全文获取类型
收费全文 | 22474篇 |
免费 | 653篇 |
国内免费 | 1034篇 |
专业分类
航空 | 12239篇 |
航天技术 | 6744篇 |
综合类 | 1016篇 |
航天 | 4162篇 |
出版年
2021年 | 205篇 |
2019年 | 151篇 |
2018年 | 245篇 |
2016年 | 217篇 |
2015年 | 200篇 |
2014年 | 707篇 |
2013年 | 775篇 |
2012年 | 701篇 |
2011年 | 852篇 |
2010年 | 805篇 |
2009年 | 1067篇 |
2008年 | 1113篇 |
2007年 | 674篇 |
2006年 | 660篇 |
2005年 | 607篇 |
2004年 | 579篇 |
2003年 | 685篇 |
2002年 | 570篇 |
2001年 | 742篇 |
2000年 | 554篇 |
1999年 | 669篇 |
1998年 | 646篇 |
1997年 | 503篇 |
1996年 | 564篇 |
1995年 | 600篇 |
1994年 | 596篇 |
1993年 | 475篇 |
1992年 | 438篇 |
1991年 | 342篇 |
1990年 | 311篇 |
1989年 | 502篇 |
1988年 | 252篇 |
1987年 | 272篇 |
1986年 | 257篇 |
1985年 | 658篇 |
1984年 | 531篇 |
1983年 | 418篇 |
1982年 | 494篇 |
1981年 | 615篇 |
1980年 | 247篇 |
1979年 | 187篇 |
1978年 | 189篇 |
1977年 | 146篇 |
1976年 | 155篇 |
1975年 | 195篇 |
1974年 | 181篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1969年 | 147篇 |
排序方式: 共有10000条查询结果,搜索用时 193 毫秒
501.
结合双温度试验系统方案设计,介绍了可编程控制器在温度控制上的应用,有助于增进读者对PLC等的了解。 相似文献
502.
试验是在一个 WZ- 5燃气发生器上进行的 ,燃料用低热值气态燃料 (体积热值为 8172 k J/N· m3) ;水蒸汽可由燃烧室前部或从后部掺混孔处注入。试验结果表明 ,水蒸汽注入对燃烧效率影响不大而使总压损失增加。当水蒸汽流量与总空气流量之比为 10 % ,且水蒸汽从后部掺混孔处注入 ,则总压损失会增大 12 %左右 ;如果水蒸汽从前排注入 ,则总压损失会更大些。 相似文献
503.
We present models of the extragalactic background light (EBL) based on several scenarios of galaxy formation and evolution. We have treated galaxy formation with the Press-Schecter approximation for both cold dark matter (CDM) and cold+hot dark matter (CHDM) models, representing a moderate (z
f
3) and a late (z
f
1) era of galaxy formation respectively. Galaxy evolution has been treated by considering a variety of stellar types, different initial mass functions and star formation histories, and with an accounting of dust absorption and emission. We find that the dominant factor influencing the EBL is the epoch of galaxy formation. A recently proposed method for observing the EBL utilizing the absorption of 0.1 to 10 TeV gamma-rays from active galactic nuclei (AGN) is shown to be capable of discriminating between different galaxy formation epochs. The one AGN viewed in TeV light, Mrk 421, does show some evidence for a cutoff above 3 TeV; based on the EBL models presented here, we suggest that this is due to extinction in the source. The large absorption predicted at energies > 200 GeV for sources at z > 0.5 indicates that observations of TeV gamma-ray bursts (GRB) would constrain or eliminate models in which the GRB sources lie at cosmological distances.Now at University of Chicago, Dept. of Astronomy & Astrophysics. 相似文献
504.
Discrete-time observability and estimability analysis for bearings-only target motion analysis 总被引:2,自引:0,他引:2
Observability in the context of bearings-only tracking (BOT) is still the subject of important literature. Different from previous approaches, where continuous-time analysis was considered, our approach relies on discrete-time analysis. It is then shown that this allows us to use directly and efficiently the simple formalisms of linear algebra. Using the direct approach, observability analysis is essentially reduced to basic considerations about subspace dimensions. Even if this approach is conceptually quite direct, it becomes more and more complex as the source-encounter scenario complexity increases. For complex scenarios, the dual approach may present some advantages essentially due to the direct use of multilinear algebra. New results about BOT observability for maneuvering sources are thus obtained. Observability analysis is then extended to unknown instants of source velocity changes. Even if observability analysis provides thorough insights about the algebraic structure of the BOT problem, the optimization of the observer maneuvers is essentially a control problem. Basic algebraic considerations prove that a relevant cost functional for this control problem is the determinant of the Fisher information matrix (FIM). So, a large part of this work is devoted to the analysis of this cost functional. Using multilinear algebra, general approximations of this functional are given. In order to involve only directly estimable parameters, the source bearing-rates are examined. Using these approximations, a general framework for optimizing the observer trajectory is derived which allow us to approximate the optimal sequence of controls. It is worth stressing that our approach does not require the knowledge of the source trajectory parameters and is still valid for a maneuvering source. 相似文献
505.
Two energetic events in the Earth’s magnetotail detected by Geotail are examined with detailed analysis of three-dimensional
velocity phase space density. It is found that the occurrence of multiple ion components is high during these dynamic episodes.
Different populations evolve independently of each other, suggesting particles from multiple activity sites contributing to
the observed phase space density. The transport properties with consideration of multiple components are evaluated, with the
result showing significant differences from those based on a single fluid approach. This comparison indicates that precise
evaluation of the energy and magnetic flux transport of energetic events in the magnetotail requires resolving individual
populations in the phase space density. 相似文献
506.
507.
508.
L. Jacquin D. Fabre D. Sipp V. Theofilis H. Vollmers 《Aerospace Science and Technology》2003,7(8):94-593
This paper presents a review of theoretical and experimental results on stability and other unsteady properties of aircraft wakes. The basic mechanisms responsible for the propagation and the amplification of perturbation along vortices, namely the Kelvin waves and the cooperative instabilities, are first detailed. These two generic unsteady mechanisms are described by considering asymptotic linear stability analysis of model flows such as vortex filaments or Lamb–Oseen vortices. Extension of the linear analysis to more representative flows, using a biglobal stability approach, is also described. Experimental results obtained using LDV, hot wire and PIV in wind tunnels are presented and they are commented upon the light of theory. 相似文献
509.
PHD filters of higher order in target number 总被引:14,自引:0,他引:14
The multitarget recursive Bayes nonlinear filter is the theoretically optimal approach to multisensor-multitarget detection, tracking, and identification. For applications in which this filter is appropriate, it is likely to be tractable for only a small number of targets. In earlier papers we derived closed-form equations for an approximation of this filter based on propagation of a first-order multitarget moment called the probability hypothesis density (PHD). In a recent paper, Erdinc, Willett, and Bar-Shalom argued for the need for a PHD-type filter which remains first-order in the states of individual targets, but which is higher-order in target number. In this paper we show that this is indeed possible. We derive a closed-form cardinalized PHD (CPHD) filter, which propagates not only the PHD but also the entire probability distribution on target number. 相似文献
510.