全文获取类型
收费全文 | 24125篇 |
免费 | 1239篇 |
国内免费 | 1402篇 |
专业分类
航空 | 13847篇 |
航天技术 | 6934篇 |
综合类 | 1337篇 |
航天 | 4648篇 |
出版年
2022年 | 157篇 |
2021年 | 309篇 |
2020年 | 204篇 |
2019年 | 227篇 |
2018年 | 302篇 |
2017年 | 184篇 |
2016年 | 283篇 |
2015年 | 314篇 |
2014年 | 802篇 |
2013年 | 901篇 |
2012年 | 875篇 |
2011年 | 1067篇 |
2010年 | 949篇 |
2009年 | 1314篇 |
2008年 | 1286篇 |
2007年 | 831篇 |
2006年 | 816篇 |
2005年 | 710篇 |
2004年 | 665篇 |
2003年 | 717篇 |
2002年 | 701篇 |
2001年 | 773篇 |
2000年 | 603篇 |
1999年 | 736篇 |
1998年 | 675篇 |
1997年 | 533篇 |
1996年 | 522篇 |
1995年 | 570篇 |
1994年 | 563篇 |
1993年 | 492篇 |
1992年 | 418篇 |
1991年 | 371篇 |
1990年 | 322篇 |
1989年 | 464篇 |
1988年 | 266篇 |
1987年 | 284篇 |
1986年 | 252篇 |
1985年 | 646篇 |
1984年 | 527篇 |
1983年 | 405篇 |
1982年 | 497篇 |
1981年 | 615篇 |
1980年 | 245篇 |
1979年 | 184篇 |
1978年 | 189篇 |
1976年 | 156篇 |
1975年 | 184篇 |
1974年 | 181篇 |
1973年 | 161篇 |
1972年 | 188篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
141.
142.
P. B. Price 《Space Science Reviews》1973,15(1):69-88
The composition of cosmic rays and solar particles is reviewed with emphasis on the question of whether they are representative samples of Galactic and solar matter. The composition of solar particles changes with energy and from flare to flare. A strong excess of heavy elements at energies below a few MeV/nuc decreases with energy, and at energies above 15 MeV/nuc the composition of solar particles resembles that of galactic cosmic rays somewhat better than that of the solar atmosphere. The elements Ne through Pb have remarkably similar abundances in cosmic ray sources and in the matter of the solar system. The lighter elements are depleted in cosmic rays, whereas U and Th may be enriched or not, depending on whether the meteoritic or solar abundance of Th is used. Two prototype sources of cosmic rays are considered: gas with solar system composition but enriched in elements with Z > 8 during acceleration and emission (by analogy with solar particle emission), and highly evolved matter enriched in r-process elements such as U, Th and transuranic elements. The energy-dependence of cosmic ray composition suggests that both sources may contribute at different energies.Miller Institute Professor, 1972–73. 相似文献
143.
针对电传控制增稳飞机进行了进场着陆的飞行品质地面模拟试验研究,验证了适合于进场着陆的控制律,并得出一些重要的有关进场着陆的驾驶员评定意见。此外,还计算了轨迹角超调值,验证了该参数数可以作为一个重要的飞行品质准则。 相似文献
144.
空间环境试验与观测是航天工程的重要组成部分,体现了国家航天技术的发展水平,是国际航天界关注的焦点,一直作为独立的专业领域开展工作.今天,我们齐聚一堂,共同研讨空间环境试验与观测技术的发展方向和重点,必将对提高我国航天工程水平、实现空间技术的跨越式发展起到十分重要的推动作用. 相似文献
145.
我们用BASIC语言设计了根据频响数据辨识传递函数模型(以下简称模型)的应用程序。在计算机上对八个模型的频响数据进行了辨识,结果较吻合。 一、原理和误差判定准则 1.原理 应用最小二乘原理,使试验结果C(jω1)和辨识结果Ga(jω1)的误差平方和为最小。 相似文献
146.
H.-E. Frhlich V. Müller H. Oleak 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,3(10-12)
Dusty pregalactic Population III objects may provide a mechanism for an effective thermalization of the star radiation. They may generate the observed microwave background and so the high cosmic photon entropy. Assuming a tepid universe a smaller primordial entropy contribution results in reasonable mass scales and amplification factors of pregalactic density fluctuations. 相似文献
147.
Loehr J. Siskaninetz W. Wiemeri J. Feld S. 《Aerospace and Electronic Systems Magazine, IEEE》1998,13(4):9-12
Avionic information processing and transmission requirements are increasing geometrically, with no end in sight, The only feasible way to meet them is to incorporate fiber-optic communication systems into avionic platforms, Such systems can be employed to replace standard electronic components, to augment existing systems, or to enable new technologies. In this paper, we survey the field of low-power communication systems from the avionics engineering perspective. We review the fundamental merits of optical fiber for information transmission and discuss various information modulation schemes. These modulation techniques determine the performance requirements for laser transmitters, We describe the horizontal- and vertical-cavity semiconductor lasers currently available for these systems, and measure their performance against the additional environmental constraints of avionics platforms. Finally, we outline expected near- and far-term trends in system and device development 相似文献
148.
A power generation scheme using a synchronous generator with an excitation control system driven by a variable-speed wind turbine is described. The proposed scheme has been analyzed over a wide speed range and under wind gusting conditions. Performance characteristics show that it is feasible to use variable-speed fixed-pitch wind turbines to generate electric power. 相似文献
149.
Recursive state equation estimation algorithms are derived to determine optimal estimation error covariance and state estimate for a linear dynamic system, driven by time-varying and positionverying (or angle-varying) functions whose a priori covariance are described. Retracing the same trajectory with the system measuring device causes the position varying function to repeat and can significantly reduce estimation errors. Applications for these algorithms include improving accuracy of a position dependent quantity to be mapped, or recursively processing radar or sonar data from repeating scans over the same area. Three types of return path patterns are considered: 1) multiple independent returns, 2) reverse returns, and 3) cyclical returns. 相似文献
150.