全文获取类型
收费全文 | 10063篇 |
免费 | 9399篇 |
国内免费 | 1677篇 |
专业分类
航空 | 17745篇 |
航天技术 | 1121篇 |
综合类 | 298篇 |
航天 | 1975篇 |
出版年
2024年 | 91篇 |
2023年 | 208篇 |
2022年 | 332篇 |
2021年 | 397篇 |
2020年 | 505篇 |
2019年 | 1191篇 |
2018年 | 1371篇 |
2017年 | 1275篇 |
2016年 | 1327篇 |
2015年 | 1243篇 |
2014年 | 1080篇 |
2013年 | 1097篇 |
2012年 | 1049篇 |
2011年 | 967篇 |
2010年 | 950篇 |
2009年 | 999篇 |
2008年 | 891篇 |
2007年 | 734篇 |
2006年 | 575篇 |
2005年 | 404篇 |
2004年 | 379篇 |
2003年 | 347篇 |
2002年 | 411篇 |
2001年 | 336篇 |
2000年 | 260篇 |
1999年 | 295篇 |
1998年 | 235篇 |
1997年 | 208篇 |
1996年 | 162篇 |
1995年 | 138篇 |
1994年 | 143篇 |
1993年 | 193篇 |
1992年 | 137篇 |
1991年 | 314篇 |
1990年 | 263篇 |
1989年 | 296篇 |
1988年 | 209篇 |
1987年 | 84篇 |
1986年 | 32篇 |
1983年 | 4篇 |
1982年 | 2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 31 毫秒
11.
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
12.
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
13.
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
14.
介绍了一种在CAD/CAPP集成系统中工序图自动生成的较通用的方法,提出了两种不同实现途径。并以实例表明在Pro/Engineer系统中得到了成功应用。 相似文献
15.
16.
本文介绍了一种简易的u型弯曲模具的设计方法,具有结构简单、制造容易、节约材料、重量轻的特点,在小批量生产中有借鉴意义. 相似文献
17.
区域极点及方差约束下的航天器拦截控制:静态输出反馈情形 总被引:2,自引:0,他引:2
本文提出并讨论了具有选通窗约束的航天器拦截控制问题,指出航天器拦截控制的稳暂态性能指标可表现为系统稳态状态方差及闭环区域极点的形式。然后,基于配置控制的思想,本文完整地给出了期望静态反馈控制器的存在条件及其解析表达式,并举例说明所提设计方法的直接性与有效性 相似文献
18.
19.
本文针对目前市场上对营销人员的需求旺盛情况,主要谈了从非专业人员中培养营销人员的方法。强调了学习创新的重要性。用通俗的引例,解释了复杂的心理学有关术语及认识过程与消费者购买行为的关系。培养营销人员准确把握消费者心理活动的能力,提高营销人员自身的生存能力和企业的竞争能力。 相似文献
20.