全文获取类型
收费全文 | 1149篇 |
免费 | 256篇 |
国内免费 | 371篇 |
专业分类
航空 | 741篇 |
航天技术 | 358篇 |
综合类 | 124篇 |
航天 | 553篇 |
出版年
2024年 | 6篇 |
2023年 | 34篇 |
2022年 | 43篇 |
2021年 | 46篇 |
2020年 | 63篇 |
2019年 | 60篇 |
2018年 | 56篇 |
2017年 | 52篇 |
2016年 | 80篇 |
2015年 | 79篇 |
2014年 | 94篇 |
2013年 | 86篇 |
2012年 | 87篇 |
2011年 | 101篇 |
2010年 | 86篇 |
2009年 | 81篇 |
2008年 | 100篇 |
2007年 | 61篇 |
2006年 | 72篇 |
2005年 | 50篇 |
2004年 | 52篇 |
2003年 | 44篇 |
2002年 | 57篇 |
2001年 | 39篇 |
2000年 | 29篇 |
1999年 | 36篇 |
1998年 | 28篇 |
1997年 | 27篇 |
1996年 | 28篇 |
1995年 | 12篇 |
1994年 | 16篇 |
1993年 | 9篇 |
1992年 | 16篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 11篇 |
1988年 | 13篇 |
1987年 | 1篇 |
1985年 | 3篇 |
排序方式: 共有1776条查询结果,搜索用时 671 毫秒
731.
C.L. Stokely E.G. Stansbery R.M. Goldstein 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
The continual monitoring of the low Earth orbit (LEO) debris environment using highly sensitive radars is essential for an accurate characterization of these dynamic populations. Debris populations are continually evolving since there are new debris sources, previously unrecognized debris sources, and debris loss mechanisms that are dependent on the dynamic space environment. Such radar data are used to supplement, update, and validate existing orbital debris models. NASA has been utilizing radar observations of the debris environment for over a decade from three complementary radars: the NASA JPL Goldstone radar, the MIT Lincoln Laboratory (MIT/LL) Long Range Imaging Radar (known as the Haystack radar), and the MIT/LL Haystack Auxiliary radar (HAX). All of these systems are highly sensitive radars that operate in a fixed staring mode to statistically sample orbital debris in the LEO environment. Each of these radars is ideally suited to measure debris within a specific size region. The Goldstone radar generally observes objects with sizes from 2 mm to 1 cm. The Haystack radar generally measures from 5 mm to several meters. The HAX radar generally measures from 2 cm to several meters. These overlapping size regions allow a continuous measurement of cumulative debris flux versus diameter from 2 mm to several meters for a given altitude window. This is demonstrated for all three radars by comparing the debris flux versus diameter over 200 km altitude windows for 3 nonconsecutive years from 1998 to 2003. These years correspond to periods before, during, and after the peak of the last solar cycle. Comparing the year to year flux from Haystack for each of these altitude regions indicate statistically significant changes in subsets of the debris populations. Potential causes of these changes are discussed. These analysis results include error bars that represent statistical sampling errors. 相似文献
732.
A.R. Qian P.F. Yang L.F. Hu W. Zhang S.M. Di Z. Wang J. Han X. Gao P. Shang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
The effects of a high magnetic gradient environment (HMGE) on the cytoskeletal architecture and genes associated with the cytoskeleton in osteoblasts (MC3T3-E1 and MG-63 cells) were investigated using confocal microscopy, real-time polymerase chain reaction (PCR) and atomic force microscopy (AFM). The findings showed that, under diamagnetic levitation conditions, the architecture and average height of the cytoskeleton and surface roughness in osteoblasts were dramatically altered. HMGE affects cytoskeleton arrangement and cytoskeleton-associated gene expression. 相似文献
733.
The 2014 session of the Legal Subcommittee (LSC) of the United Nations Committee on the Peaceful Uses of Outer Space (UNCOPUOS) saw a particularly extensive and thorough debate on the working methods of this forum, which has the status of the highest body in space law making. By discussing the working methods it also became apparent how delegations actually regard the LSC and what expectations they have from it. In a time of considerable changes in space activities and space diplomacy alike, it was only a matter of time that such a comprehensive discussion arose. And while it did not immediately lead to decisions, it provided numerous signals for the future of the LSC. 相似文献
734.
735.
分析了敏捷性与飞控系统控制律之间的关系.以某典型三代机为例,根据飞行品质要求对其控制律进行了重新设计,并在地面仿真系统上对与控制律相关的敏捷性尺度进行了试验.通过比较原飞机和采用新控制律飞机的敏捷性,验证了分析结果,并对控制律提出了改进意见. 相似文献
736.
737.
针对发动机数字电子控制系统中传感器易于出现故障的特点,以某型发动机为例,采用基于控制律容错重组方法在飞行包线内的不同点上设计出了发动机容错控制器。仿真结果表明:该控制器具有很好的容错能力,故障情况下系统的动态品质与正常情况下系统的动态品质具有良好的一致性。 相似文献
738.
739.
航天器动力学环境试验的发展概况和趋势 总被引:7,自引:3,他引:4
文章介绍了20世纪航天器动力学环境试验技术的发展概况和趋势,特别就试验技术的发展作了较详细的阐述.文章指出,试验工作对解决实际的问题仍然是主要的和基础的手段. 相似文献
740.
月球地质形貌及其环境概述 总被引:6,自引:1,他引:5
文章重点介绍了月球的地质形貌和月球环境.针对在月球上建立生产基地和居住基地,提出了需要重点开展的一些研究工作. 相似文献