全文获取类型
收费全文 | 6131篇 |
免费 | 1156篇 |
国内免费 | 1103篇 |
专业分类
航空 | 4348篇 |
航天技术 | 1919篇 |
综合类 | 755篇 |
航天 | 1368篇 |
出版年
2024年 | 48篇 |
2023年 | 140篇 |
2022年 | 198篇 |
2021年 | 311篇 |
2020年 | 364篇 |
2019年 | 509篇 |
2018年 | 602篇 |
2017年 | 501篇 |
2016年 | 435篇 |
2015年 | 387篇 |
2014年 | 323篇 |
2013年 | 342篇 |
2012年 | 459篇 |
2011年 | 490篇 |
2010年 | 352篇 |
2009年 | 339篇 |
2008年 | 369篇 |
2007年 | 339篇 |
2006年 | 382篇 |
2005年 | 264篇 |
2004年 | 211篇 |
2003年 | 145篇 |
2002年 | 111篇 |
2001年 | 116篇 |
2000年 | 80篇 |
1999年 | 71篇 |
1998年 | 82篇 |
1997年 | 66篇 |
1996年 | 67篇 |
1995年 | 68篇 |
1994年 | 84篇 |
1993年 | 28篇 |
1992年 | 31篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 18篇 |
1988年 | 13篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1984年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有8390条查询结果,搜索用时 437 毫秒
1.
2.
Wenwu Ding Bingfeng Tan Yongchang Chen Felix Norman Teferle Yunbin Yuan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(3):951-961
The performance of real-time (RT) precise positioning can be improved by utilizing observations from multiple Global Navigation Satellite Systems (GNSS) instead of one particular system. Since the end of 2012, BeiDou, independently established by China, began to provide operational services for users in the Asia-Pacific regions. In this study, a regional RT precise positioning system is developed to evaluate the performance of GPS/BeiDou observations in Australia in providing high precision positioning services for users. Fixing three hourly updated satellite orbits, RT correction messages are generated and broadcasted by processing RT observation/navigation data streams from the national network of GNSS Continuously Operating Reference Stations in Australia (AUSCORS) at the server side. At the user side, RT PPP is realized by processing RT data streams and the RT correction messages received. RT clock offsets, for which the accuracy reached 0.07 and 0.28?ns for GPS and BeiDou, respectively, can be determined. Based on these corrections, an accuracy of 12.2, 30.0 and 45.6?cm in the North, East and Up directions was achieved for the BeiDou-only solution after 30 min while the GPS-only solution reached 5.1, 15.3 and 15.5?cm for the same components at the same time. A further improvement of 43.7, 36.9 and 45.0 percent in the three directions, respectively, was achieved for the combined GPS/BeiDou solution. After the initialization process, the North, East and Up positioning accuracies were 5.2, 8.1 and 17.8?cm, respectively, for the BeiDou-only solution, while 1.5, 3.0, and 4.7?cm for the GPS-only solution. However, we only noticed a 20.9% improvement in the East direction was obtained for the GPS/BeiDou solution, while no improvements in the other directions were detected. It is expected that such improvements may become bigger with the increasing accuracy of the BeiDou-only solution. 相似文献
3.
4.
本文首先分析了当前直升机在运用CAD/CAE/CAM技术进行研制中存在的一些问题,接着提供了解决的方法,介绍了PDM的概念、构成、主要功能。最后简述了PDM技术在直升机研制中的作用,以及企业在实施PDM当中应注意的一些问题。 相似文献
5.
经营机制创新是中小型外贸企业摆脱困境的必由之路。针对目前面临的主要问题,从外部环境及企业自身经营机制缺陷方面对中小型外贸企业生存环境进行分析,并从实现经营机制创新角度探讨改善现状的方法,力图找出中小型外贸企业经营机制创新的突破点。 相似文献
6.
张吴晔 《中国民航学院学报》1992,10(2):10-13
本文介绍了中外航空公司飞机发动机恒速传动装置几个故障典例及造成的危害,分析了填加滑油量不当对发动机恒速传动装置的影响,提出了预防措施。 相似文献
7.
8.
A numerical procedure for the calculation of the transonic dip of airfoils in the time domain is presented. A viscous-inviscid aerodynamic interaction method is taken to calculate the unsteady aerodynamic loads. In the present case the integral boundary layer equations are coupled with the Transonic Small Disturbance (TSD) Potential Equation. The coupling between structural motion and aerodynamic loads is carried out using State Space equation. It is solved by State Transition Matrix technique. Results are presented for NACA 64A010 and NLR 7301 airfoils with structural data from Isogai and DLR, respectively. Comparisons show good agreement with other numerical results. Certain deviations of experimental data taken from literature need more insight in the detailed test conditions. 相似文献
9.
ected g 《桂林航天工业高等专科学校学报》2006,(2)
with primary goat embryonic fibroblast showed no difference from traditional feederlayer-based culture method used in mouse and human. The putative primary EG colonies were multilayer clumps ofcompact cel 相似文献
10.
本文结合Protel 99 SE在高频PCB设计中的应用,着重探讨在电路原理图的绘制上需要重点注意的一些特殊问题,并提出解决问题的方法,尽可能地减小对高频PCB设计的影响,提高PCB设计的效率。 相似文献