全文获取类型
收费全文 | 19285篇 |
免费 | 11181篇 |
国内免费 | 3177篇 |
专业分类
航空 | 24356篇 |
航天技术 | 2954篇 |
综合类 | 1100篇 |
航天 | 5233篇 |
出版年
2024年 | 210篇 |
2023年 | 467篇 |
2022年 | 1022篇 |
2021年 | 1120篇 |
2020年 | 1149篇 |
2019年 | 1593篇 |
2018年 | 1736篇 |
2017年 | 1820篇 |
2016年 | 1632篇 |
2015年 | 1771篇 |
2014年 | 1590篇 |
2013年 | 1689篇 |
2012年 | 1781篇 |
2011年 | 1731篇 |
2010年 | 1708篇 |
2009年 | 1661篇 |
2008年 | 1553篇 |
2007年 | 1392篇 |
2006年 | 1168篇 |
2005年 | 900篇 |
2004年 | 790篇 |
2003年 | 599篇 |
2002年 | 644篇 |
2001年 | 496篇 |
2000年 | 462篇 |
1999年 | 390篇 |
1998年 | 267篇 |
1997年 | 239篇 |
1996年 | 184篇 |
1995年 | 155篇 |
1994年 | 158篇 |
1993年 | 203篇 |
1992年 | 147篇 |
1991年 | 321篇 |
1990年 | 266篇 |
1989年 | 296篇 |
1988年 | 208篇 |
1987年 | 83篇 |
1986年 | 32篇 |
1983年 | 5篇 |
1981年 | 3篇 |
1972年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
T. J. Ponman A. M. Read 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):361-364
Using data from the Wide Field Camera EUV all-sky survey, we have established upper limits to the EUV flux from a sample of 30 bright, nearby, non-active spiral galaxies. These galaxies were chosen to be those most likely to be detected in the EUV on the basis of (i) low interstellar absorption within our own galaxy, (ii) brightness in other wavebands, (iii) high star formation activity, and (iv) proximity. The derived EUV upper limits are restrictive, and establish for the first time that the EUV flux escaping from galaxies does not constitute a major component of their bolometric luminosity, and in particular that it cannot be the sink for the energy injected into the interstellar medium by supernova explosions, as had been suggested following the failure to detect this power in the X-ray band. 相似文献
22.
23.
The authors present a new scientific space mission consisting of a satellite carrying a receiving- only SAR which receives the signal transmitted by the ENVISAT-1 SAR. The integration of ENVISAT-1 SAR and bistatic radar data offers an improved potentiality of surface classification, three-dimensional observation, and the opportunity of advanced scientific experiments in the field of bistatic scattering. The small satellite nominal orbit and the attitude manoeuvres are designed in order to maintain an adequate overlap between the two radar swaths along the whole orbit, taking into account the ENVISAT-1 attitude and pointing. A preliminary satellite design (2-year lifetime) is then performed to evaluate the orbit decay and to determine the appropriate orbit manoeuvres (every 4 days) to control the satellites relative phase. The numerical simulation shows that a spacecraft of about 584kg is able to meet the mission requirements. 相似文献
24.
In extension to common applications such as groundtrack displays and antenna steering, the SGP4 orbit model is proposed for operational orbit determination in small satellite missions. SGP4 is an analytical orbit model for Low-Earth orbiting satellites that is widely used for the propagation of NORAD twoline elements. Twoline elements may hence be generated completely independent of NORAD. Their use as exclusive source of orbital information simplifies the operations concept and reduces mission costs through the extensive use of existing low-cost mission support software. Due to small computer resource requirements of 8–10kByte, the SGP4 model may also be applied for onboard orbit computations making use of e.g. a 80186 processor, thus ensuring full compatibility of ground-based and onboard operations. The proposed approach is particularly suited in combination with a space-borne GPS receiver, were the C/A-code navigation solutions are treated as measurements that are adjusted in a least-squares sense using the SGP4 model. As consequence, inherent drawbacks of the pure navigation solutions such as data gaps and scatter as well as limited velocity accuracy are avoided, while the operational navigation activities are kept at a minimum. The feasibility of the concept is illustrated based on real GPS navigation data from the TOPEX/Poseidon and the MIR space station with an inherent data quality of 50–100 m. It is shown that 3 hours of data within a 4 day period are sufficient to keep the position error within 4 km, that is considered sufficient for most applications. 相似文献
25.
26.
27.
近期远程监控系统在工业中逐步推广,针对工业系统的特点和工业网络的实际情况,在原有工业系统的基础上,增加现场设备区域的图像监控和数据传输功能,提出了一种工业远程监控系统实施方案。本文论述了该系统的结构、原理及实现方法,经具体实施证明,本文提出的方案已基本能够满足一个通用的基于公用网络的远程视频监控系统的要求,体现了较重要的理论价值和很强的工程实用价值。 相似文献
28.
29.
高校学报编辑与作者的关系定位 总被引:3,自引:0,他引:3
高校学报要出想精品、创特色,充分发挥学报这个文化传播的窗口作用,促进学报发展,应避免把“编辑工作是出版发行工作的中心环节”绝对化,充分认识编辑与作者之间互相尊重的平等关系,因共同的工作目标而相互合作的关系,因不同的出发点而相互制约的关系,和在合作过程中相互促进提高的关系,注重建立编辑与作者之间的良性互动关系。 相似文献
30.