全文获取类型
收费全文 | 317篇 |
免费 | 49篇 |
国内免费 | 173篇 |
专业分类
航空 | 226篇 |
航天技术 | 91篇 |
综合类 | 33篇 |
航天 | 189篇 |
出版年
2024年 | 2篇 |
2023年 | 9篇 |
2022年 | 7篇 |
2021年 | 23篇 |
2020年 | 12篇 |
2019年 | 13篇 |
2018年 | 19篇 |
2017年 | 26篇 |
2016年 | 20篇 |
2015年 | 21篇 |
2014年 | 27篇 |
2013年 | 32篇 |
2012年 | 25篇 |
2011年 | 38篇 |
2010年 | 36篇 |
2009年 | 32篇 |
2008年 | 28篇 |
2007年 | 22篇 |
2006年 | 26篇 |
2005年 | 20篇 |
2004年 | 12篇 |
2003年 | 15篇 |
2002年 | 25篇 |
2001年 | 9篇 |
2000年 | 5篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 4篇 |
排序方式: 共有539条查询结果,搜索用时 15 毫秒
71.
G.J. Nagy D.N. Severov V.A. Pshennikov M. De los Santos J.J. Lagomarsino K. Sans E.G. Morozov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(11):1876-1881
This paper deals with the application of SeaWIFS images to characterize spatial and temporal variability of fronts in the Rio de la Plata estuarine system over the period 2000–2003. We aim to depict the relationship between river outflow and variability of fronts’ loci on monthly to ENSO-related timescales and the influence of the winds along Rio de la Plata (axial winds) on the abrupt changes in frontal dynamics over synoptic timescales. During the studied period both La Niña (July 1999–June 2000) and El Niño (April 2002–May 2003) events induced significant displacements of fronts. Three distinct fronts were analyzed between river, estuarine, coastal and marine waters of the Rio de la Plata: Main Turbidity Front, Main Marine Front, and Secondary Marine Front. Their number, location and separation seem to be mainly related to river outflow and second, to fresh (>8 m/s) axial winds. During low discharge periods (i.e. summer time and/or La Niña events) these winds induce abrupt changes in the location of fronts (100–200 km) and greater separation between them over synoptic timescale, whereas during high river discharge or ENSO years some of the variability of fronts location is explained by the river’s outflow fluctuations, especially by the high variability of the River Uruguay discharge. 相似文献
72.
S.P. Teng Y.K. Chen K.S. Cheng H.C. Lo 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(11):1744-1754
Remote sensing images and technologies have been widely applied to environmental monitoring, in particular landuse/landcover classification and change detection. However, the uncertainties involved in such applications have not been fully addressed. In this paper two hypothesis-test-based change detection methods, namely the bivariate joint distribution method and the conditional distribution method, are proposed to tackle the uncertainties in change detection by making decisions based on the desired level of significance. Both methods require a data set of class-dependent no-change pixels to form the basis for class-dependent hypothesis test. Using an exemplar study area in central Taiwan, performance of the proposed methods are shown to be significantly superior to two other commonly applied methods (the post-classification comparison and the image differencing methods) in terms of the overall change detection accuracies. The conditional distribution method takes into consideration the correlation between digital numbers of the pre- and post-images and the effect of the known pre-image digital number on the range of the post-image digital number, and therefore yields the highest change detection accuracy. It is also demonstrated that the class-dependent change detection is crucial for accurate landuse/landcover change detection. 相似文献
73.
74.
运用Malmquist生产率指数方法,对中美15个航空公司2000~2006年期间的全要素生产率的变化作了测算与对比。分析结果显示,2000—2006年这些公司的生产效率总体上都有所提高,中国航空公司的全要素生产率的平均增长率为4.3%,美国的平均增长率为6.1%;综合效率改善与技术进步对中美两国航空公司生产率的影响程度并不相同,美国航空公司的主要驱动因素为技术变革与创新,中国航空公司的发展动力主要为对已有技术的运用以及组织管理效率的提升。中国航空公司全要素生产率水平的提高,除继续强调综合效率的重要性之外,也要进一步加强对技术进步的重视。 相似文献
75.
76.
高性能军用机环境控制系统研究发展趋势的探讨 总被引:11,自引:1,他引:10
讨论了未来高性能军用机环境控制系统发展可能采用的技术,例如:闭式空气循环技术;综合数字控制技术;蒸发循环技术;先进驱动的空气循环机和压气机技术;以及从属于综合机载机电系统的综合环境控制系统技术。 相似文献
77.
碳基复合相变装置以高导热碳基复合相变材料为主要储能和导热载体,利用高导热膨胀石墨强化导热,运用相变材料的潜热实现热量的削峰填谷,可有效抑制短时大热耗单机的温升,减少单机非工作时段所需的补偿功耗,节省卫星的重量资源和功耗资源。主要介绍了一种高导热碳基复合相变装置,结合复合相变装置与卫星结构板优化的综合散热系统,用于解决星载短时工作的大热耗单机温度控制问题。通过理论计算与热仿真分析相结合,对复合相变装置进行了优化设计;通过开展专项试验对复合相变装置的热性能和空间环境适应性进行了充分验证;并在整星真空热平衡试验中,验证了复合相变装置对大热耗单机的温度控制效果。 相似文献
传统可靠性评估方法只考虑产品处于恒定环境应力下的情况,然而工程实际中产品往往直接暴露于外界的自然环境中,所遭受的工作环境应力或贮存环境应力都是随时间变化的。针对此情况,引入典型地理位置的真实自然环境应力,使用六参数多项式拟合方法得到其环境应力变化趋势,并假定2种时变环境应力模式,在Nelson累积损伤模型的基础上,研究真实时变环境应力下基于加速寿命试验数据的产品可靠性评估方法。结果表明:产品在不同地理位置的可靠寿命差别较大,引入产品所处地理位置的真实环境应力,可更精确地评估产品的可靠性。 相似文献
79.
P.S. Sawyer H. Stephen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Recent variations in normal meteorological conditions indicate the earth’s climate is changing in ways that may impact delicate ecological balances in sensitive regions. Identifying how those changes are affecting the biosphere is essential if we are going to be able to adapt to those changes and to potentially mitigate their harmful consequences. This paper presents a time series study of an alpine ecosystem in the Big Pine Creek watershed in California’s Eastern Sierra Nevada Mountain’s. Raw Landsat data covering the years 1984 through 2011 is converted to observed surface reflectance and analyzed for trends that would indicate a change in the ecosystem. We found that over the time period of the study, observed surface reflectance shows a general decline across the spectrum while our analysis of environmental data demonstrates statistically significant increases in temperatures. While declining reflectance in the visible and short wave bands are indicators of increased surface cover, the fact that the IR band also shows declines is consistent with a decline in tree density. This study provides a useful insight into the ecological response of the Big Pine Creek watershed to recent climate change. These findings suggest that alpine ecosystems are particularly sensitive to increasing temperatures. If these results are replicated in other alpine watersheds it will demonstrate that the biosphere is already showing the effects of a warmer environment. 相似文献
80.