全文获取类型
收费全文 | 5466篇 |
免费 | 665篇 |
国内免费 | 488篇 |
专业分类
航空 | 2903篇 |
航天技术 | 1597篇 |
综合类 | 439篇 |
航天 | 1680篇 |
出版年
2024年 | 12篇 |
2023年 | 46篇 |
2022年 | 115篇 |
2021年 | 171篇 |
2020年 | 133篇 |
2019年 | 128篇 |
2018年 | 187篇 |
2017年 | 187篇 |
2016年 | 166篇 |
2015年 | 171篇 |
2014年 | 251篇 |
2013年 | 258篇 |
2012年 | 265篇 |
2011年 | 347篇 |
2010年 | 359篇 |
2009年 | 394篇 |
2008年 | 337篇 |
2007年 | 252篇 |
2006年 | 210篇 |
2005年 | 217篇 |
2004年 | 157篇 |
2003年 | 180篇 |
2002年 | 204篇 |
2001年 | 173篇 |
2000年 | 139篇 |
1999年 | 146篇 |
1998年 | 120篇 |
1997年 | 124篇 |
1996年 | 103篇 |
1995年 | 121篇 |
1994年 | 87篇 |
1993年 | 73篇 |
1992年 | 88篇 |
1991年 | 47篇 |
1990年 | 42篇 |
1989年 | 57篇 |
1988年 | 37篇 |
1987年 | 24篇 |
1986年 | 23篇 |
1985年 | 69篇 |
1984年 | 56篇 |
1983年 | 44篇 |
1982年 | 45篇 |
1981年 | 67篇 |
1980年 | 35篇 |
1979年 | 18篇 |
1978年 | 20篇 |
1976年 | 18篇 |
1975年 | 15篇 |
1974年 | 14篇 |
排序方式: 共有6619条查询结果,搜索用时 390 毫秒
151.
We find the forms of the orbits in a self-consistent galactic model generated by a N-body simulation of the collapse of a protogalaxy. The model represents a stationary elliptical galaxy of type E5, which is approximately axisymmetric around its longest axis. The orbits are of three main types, box orbits (including box-like orbits), tube orbits and chaotic orbits. The box or box-like and tube orbits are represented by closed invariant curves on a Poincaré surface of section. The forms of the orbits and of the invariant curves can be explained by a third integral of motion I, that is given by the Giorgilli (1979) computer program. The nonresonant form of the third integral explains the box orbits, while a resonant form of this integral explains both the box orbits and the 1:1 tube orbits. The N-body model gives the distribution of velocities F, which is an exponential of the third integral. 相似文献
152.
Spectral-domain covariance estimation with a priori knowledge 总被引:2,自引:0,他引:2
Gurram P.R. Goodman N.A. 《IEEE transactions on aerospace and electronic systems》2006,42(3):1010-1020
A knowledge-aided spectral-domain approach to estimating the interference covariance matrix used in space-time adaptive processing (STAP) is proposed. Prior knowledge of the range-Doppler clutter scene is used to identify geographic regions with homogeneous scattering statistics. Then, minimum-variance spectral estimation is used to arrive at a spectral-domain clutter estimate. Finally, space-time steering vectors are used to transform the spectral-domain estimate into a data-domain estimate of the clutter covariance matrix. The proposed technique is compared with ideal performance and to the fast maximum likelihood technique using simulated results. An investigation of the performance degradation that can occur due to various inaccurate knowledge assumptions is also presented 相似文献
153.
三角翼低速动态大攻角气动特性试验研究 总被引:5,自引:7,他引:5
对后掠角分别为X=60°、70°和80°尖前缘平板三角翼模型和一个前缘后掠角为(76° 40°)的双三角翼模型在低速风洞中作大攻角俯仰摩波运动,振幅am=30°、60°和90°,缩减频率K=0.01~0.12,基于根弦长雷诺数Re=2.76×10^5~8.23×10^5。进行了六分量动态气动载荷测量,动态流动显示和70°三角翼上翼面非定常压力测量,并分别与对应的静态试验结果比较。分析了运动参数包括缩减频率、振幅和Re数、后掠角对气流动态迟滞特性的影响。 相似文献
154.
Yoshikawa M. Morinaga N. Namekawa T. 《IEEE transactions on aerospace and electronic systems》1978,(4):623-629
By the term "m-distributed optical signal" we mean a noise-like optical signal whose envelope (or intensity) fluctuation probability is modeled by Nakagami's "m-distribution." Using the m-distribution which has been widely used as an analytical model of the fading envelope in radio communications, it is shown that one can generally analyze the statistical properties such as the photoelectron count probabilities and error probabilities for the wider class of noise-like optical signals; some numerical results are also given. 相似文献
155.
环境专业教学体系结构改革探索——以就业市场需求为导向 总被引:4,自引:0,他引:4
通过对西安航专2006与2007年环境监测与治理技术专业毕业生就业情况的对比分析,认为目前环境类专业的就业形势发生良性转变,需求范围增加,同时就业单位对毕业生的专业技术能力、技术管理能力等也提出了更高的要求。据此,我们从专业发展方向、课程体系结构以及实验实训三个方面进行建设与调整来满足就业单位的需求。 相似文献
156.
157.
从中国的国情入笔,论速了农业航空在发展国民经济中的重要作用;农业航空采用农业专用飞机是必然趋势;指出农业航空滑坡问题亟待解决。 相似文献
158.
A relative navigation system for formation flight 总被引:1,自引:0,他引:1
A relative navigation system based on both the Inertial Navigation System (INS) and the Global Positioning System (GPS) is developed to support situational awareness during formation flight. The architecture of the system requires an INS/GPS integration across two aircraft via a data link. A fault-tolerant federated filter is used to estimate the relative INS errors based on relative GPS measurements and a range measurement obtained from the data link. The filter is constructed based on a reduced-order model of the relative INS error process. A method for analyzing the filter performance is presented. A case involving two helicopters in formation flight is studied under three different night trajectories to account for the effect of vehicle motion on the INS state transition matrix. The results of the covariance analysis are compared with actual night results over an instrumented test range. 相似文献
159.
160.
Instrument failure detection using the dedicated observer scheme (DOS) depends on partial state observability through each instrument which is monitored. For instrument fault detection by the DOS technique, a quantitative measure of partial state observability is established for each instrument and used to determine a necessary condition on the output structure of the system. This measure, called the internal redundancy of the instrument, indicates the complexity of the logic required for failure detection, and it also indicates where some hardware redundancy can be introduced into the system to improve the fault detection capability of the DOS. The principles developed are applied to a simulation of the pitch axis autopilot of the A7 jet aircraft. 相似文献