全文获取类型
收费全文 | 1472篇 |
免费 | 573篇 |
国内免费 | 243篇 |
专业分类
航空 | 1467篇 |
航天技术 | 209篇 |
综合类 | 178篇 |
航天 | 434篇 |
出版年
2024年 | 10篇 |
2023年 | 33篇 |
2022年 | 87篇 |
2021年 | 69篇 |
2020年 | 84篇 |
2019年 | 74篇 |
2018年 | 67篇 |
2017年 | 96篇 |
2016年 | 114篇 |
2015年 | 78篇 |
2014年 | 98篇 |
2013年 | 100篇 |
2012年 | 100篇 |
2011年 | 127篇 |
2010年 | 96篇 |
2009年 | 99篇 |
2008年 | 70篇 |
2007年 | 94篇 |
2006年 | 92篇 |
2005年 | 82篇 |
2004年 | 87篇 |
2003年 | 53篇 |
2002年 | 54篇 |
2001年 | 53篇 |
2000年 | 46篇 |
1999年 | 36篇 |
1998年 | 45篇 |
1997年 | 47篇 |
1996年 | 40篇 |
1995年 | 29篇 |
1994年 | 17篇 |
1993年 | 26篇 |
1992年 | 24篇 |
1991年 | 10篇 |
1990年 | 18篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 7篇 |
1986年 | 1篇 |
排序方式: 共有2288条查询结果,搜索用时 328 毫秒
971.
Hao Wang Ming Wei Guoping Li Shenghui Zhou Qingfeng Zeng 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
The rainfall process of Chengdu region in autumn has obvious regional features. Especially, the night-time rain rate of this region in this season is very high in China. Studying the spatial distribution and temporal variation of regional atmospheric precipitable water vapor (PWV) is important for our understanding of water vapor related processes, such as rainfall, evaporation, convective activity, among others in this area. Since GPS detection technology has the unique characteristics, such as all-weather, high accuracy, high spatial and temporal resolution as well as low cost, tracking and monitoring techniques on water vapor has achieved rapid developments in recent years. With GPS–PWV data at 30-min interval gathered from six GPS observational stations in Chengdu region in two autumns (September 2007–December 2007 and September 2008–December 2008), it is revealed that negative correlations exist between seasonally averaged value of GPS–PWV as well as its variation amplitude and local terrain altitude. The variation of PWV in the upper atmosphere of this region results from the water vapor variation from surface to 850 hPa. With the help of Fast Fourier Transform (FFT), it is found that the autumn PWV in Chengdu region has a multi-scale feature, which includes a seasonal cycle, 22.5 days period (quasi-tri-weekly oscillation). The variation of the GPS–PWV is related to periodical change in the transmitting of the water vapor caused by zonal and meridional wind strengths’ change and to the East Asian monsoon system. According to seasonal variation characteristics, we concluded that the middle October is the critical turning point in PWV content. On a shorter time scale, the relationship between autumn PWV and ground meteorological elements was obtained using the composite analysis approach. 相似文献
972.
电站典型大滞后过程难于控制的原因在于其大滞后本身导致系统不稳定及锅炉负荷频繁变化导致的参数时变特性。本文设计了一种模型参考自适应预估控制算法,引入二阶惯性与时间滞后两个补偿器,其主要参数如增益、时间常数和滞后时间均可在线自适应调整,致使过程与参考模型时刻匹配。该算法已应用于电站200 MW锅炉机组再热汽温过程控制,控制精度达到2℃以内。 相似文献
973.
974.
针对航班改航路径的优劣提出了三种评估模型.首先,从航线的安全性、经济性、管制员及飞行员的工作负荷、同计划航线的差异和航线飞行的舒适性角度出发,选取5个评估指标.然后,应用层次分析法分别确定这5个指标在飞行-空管评估模型、航空公司评估模型和综合评估模型下的权重.最后,将各评估模型应用于现有改航飞行路径的评估,研究结果表明... 相似文献
975.
日益增长的空中流量,对管制信息系统的信息整合提出了更高的要求。本文总结了成都管制中心的数据整合实践经验,提出整合管制信息系统的思路和建议,为未来的项目建设提供有意义的参考。 相似文献
976.
介绍了用PGC302气相工业色谱仪(FID检测器)直接测定空分装置液态氧中碳氢化合物的分析方法,该方法具有自动化程度高,灵敏度精密度好等优点. 相似文献
977.
针对传统飞行控制系统中的作动轴和位移传感器等执行部件,开展数字化、轻小型作动轴系统集成技术的研究进行了介绍。并介绍了该技术的设计原理、集成工艺、集成数字控制系统,成功在样机平台上实现了位移信号在结构件本体上的测量试验。 相似文献
978.
低空导航吊舱系统视频处理模块中MC6845的编程及外同步控制电路设计 总被引:1,自引:1,他引:0
详细介绍了低空导航吊舱系统视频处理模块中CRT控制器MC6 845的编程及导引画面与红外信息的叠加显示时外同步控制电路设计。 相似文献
979.
压力、差压变送器是过程变量变送器中最重要的一类,随着生产过程自动化程度的不断提高,在生产过程控制领域得到越来越广泛的应用,除了可用于压力、差压测量之外,还可用于流量、液位、比重等其他参数测量,以差压原理的流量测量技术更是在流量测量领域占有绝对的高的比例.差压变送器在各种应用领域中的安装及设置方法也多种多样,针对其在这些测量领域的测量方法及原理进行分类的阐述. 相似文献
980.
The Processing of Band-Limited Measurements; Filtering Techniques in the Least Squares Context and in the Presence of Data Gaps 总被引:2,自引:0,他引:2
This paper discusses the treatment of correlated measurements in the least squares context. We focus on the processing of
band-limited measurements and on long time series with a constant sampling interval. Time domain as well as frequency domain
approaches were discussed to offer different ways to integrate the filtering process into the optimization scheme as good
as possible. The focus was on long equispaced data sets. The application of discrete filters in the space domain makes it
possible to decorrelate the observations during data acquisition. This opens the way to a sequential adjustment procedure,
where the design matrix is treated row-by-row. Huge systems with millions of observations can be solved by direct or iterative
strategies, and both approaches benefit from well-tailored filter techniques. Because of the sequential access the computational
effort of this giant task can be easily distributed to a cluster of parallel processors and offers, in addition, the possibility
to treat data gaps in a straightforward way.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献