全文获取类型
收费全文 | 2193篇 |
免费 | 409篇 |
国内免费 | 528篇 |
专业分类
航空 | 1708篇 |
航天技术 | 588篇 |
综合类 | 317篇 |
航天 | 517篇 |
出版年
2024年 | 10篇 |
2023年 | 38篇 |
2022年 | 66篇 |
2021年 | 99篇 |
2020年 | 87篇 |
2019年 | 95篇 |
2018年 | 127篇 |
2017年 | 104篇 |
2016年 | 131篇 |
2015年 | 120篇 |
2014年 | 159篇 |
2013年 | 132篇 |
2012年 | 175篇 |
2011年 | 191篇 |
2010年 | 140篇 |
2009年 | 159篇 |
2008年 | 135篇 |
2007年 | 140篇 |
2006年 | 141篇 |
2005年 | 100篇 |
2004年 | 91篇 |
2003年 | 73篇 |
2002年 | 72篇 |
2001年 | 62篇 |
2000年 | 49篇 |
1999年 | 54篇 |
1998年 | 53篇 |
1997年 | 41篇 |
1996年 | 40篇 |
1995年 | 38篇 |
1994年 | 38篇 |
1993年 | 26篇 |
1992年 | 33篇 |
1991年 | 42篇 |
1990年 | 22篇 |
1989年 | 24篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1984年 | 3篇 |
排序方式: 共有3130条查询结果,搜索用时 31 毫秒
991.
以虹桥国际机场飞行区扩建工程为依托,通过对拼接区域进行跑道表面沉降及周边土体竖向位移、水平位移、孔隙水压力和地下水位等项目的监测.对新建联络道与既有跑道的拼接工程不停航施工对跑道道面性能的影响进行全面监测与分析。监测结果表明,拼接工程引起的道面沉降量较小,超过60%的测点竖向位移小于3mm;拼接工程对周边土体地下水位没有影响:拼接工程会引起周边土体产生超孔隙水压力。最大可达到正常孔隙水压力的2倍,工后1年基本消散;拼接工程会引起周边土体产生竖向和水平向位移.影响程度随深度增加而减少,影响深度不超过7m。研究表明,通过采取合适的施工方案及必要的防范措施.拼接段施工不会影响跑道的正常使用。 相似文献
992.
993.
基于功率谱密度简化公式,通过数据拟合提取出PSD曲线的两个重要评价参数.研究PSD曲线变化与超光滑表面加工工艺相关性,提出基于超光滑表面工艺过程的PSD评价技术,以指导工艺改进和新工艺的开发. 相似文献
994.
依据经典的磁探仪应召搜潜方法,以某型反潜巡逻机为背景,建立了反潜巡逻机利用磁探仪执行平行航线、扩展矩形和螺旋形搜潜模型.对3种方法的搜潜概率进行分析,仿真了相同潜艇运动模型条件下,潜艇初始位置散布、初始距离、潜艇经济航速和海洋环境磁噪声对搜潜概率的影响.实验结果表明:相同条件下,螺旋形搜潜效能最高;潜艇只进行航速机动时... 相似文献
995.
目前航空公司对航空备件的需求计算主要针对单个备件进行,这产生一定的误差.因此,本文利用NSGA-Ⅱ多目标遗传优化算法来解决在一定成本约束条件下可用度、可靠度等性能指标均能得到满足.为了便于算法的应用,论文对罚函数进行了改进并进行实例分析.结果表明本方法适于处理航空公司复杂的备件需求决策,并起到一定的作用. 相似文献
996.
直升机旋转桨叶的固有频率是影响旋翼动响应以及气弹稳定性的重要因素之一,频率配置是旋翼桨叶设计的重要内容。受旋翼这种旋转系统信号谐波成分多、气动干扰大、信噪比低、模态密集等特点的影响,一直以来,旋转桨叶的固有频率识别都是直升机行业的一个难点,也是型号研制中必须进行测试的内容之一。通过将固定坐标系下的激励位移信号与旋转坐标系下的桨叶响应信号同步采集、激励位移信号坐标系转换、激励响应信号重采样处理等步骤,发展了基于传递函数分析的旋转桨叶固有频率识别方法,并通过试验验证。试验结果表明该方法能综合运用幅频曲线、相频曲线、相干系数等信息识别旋转桨叶固有频率,物理概念清晰,测试结果易于判断、稳定可靠。 相似文献
997.
Igor Kulikov Anthony J. Mannucci Xiaoqing Pi Carol Raymond George A. Hajj 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
In the coming years, opportunities for remote sensing of electron density in the Earth’s ionosphere will expand with the advent of Galileo, which will become part of the global navigation satellite system (GNSS). Methods for accurate electron density retrieval from radio occultation data continue to improve. We describe a new method of electron density retrieval using total electron content measurements obtained in low Earth orbit. This method can be applied to data from dual-frequency receivers tracking the GPS or Galileo transmitters. This simulation study demonstrates that the method significantly improves retrieval accuracy compared to the standard Abel inversion approach that assumes a spherically symmetric ionosphere. Our method incorporates horizontal gradient information available from global maps of Total Electron Content (TEC), which are available from the International GNSS Service (IGS) on a routine basis. The combination of ground and space measurements allows us to improve the accuracy of electron density profiles near the occultation tangent point in the E and F regions of the ionosphere. 相似文献
998.
Olga Pirog Marat Deminov Galina Deminova Gelii Zherebtsov Nelya Polekh 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
An analysis of properties and peculiarities of the nighttime winter foF2 increases (NWI) in the East Siberia is made on data of ionospheric station Irkutsk in the periods 1958–1992 and 2002–2009 and the empirical model of the F2 layer critical frequency under the geomagnetic quiet conditions deduced from these data (model Q-F2). It is revealed, that the NWI is the stable regularity of the quiet ionosphere over Irkutsk. The amplitude of the NWI (the difference between maximum and minimum foF2 values at night hours) is the greatest in December–January and nearly the same at low and middle solar activity. It is a peculiarity of the quiet ionosphere in the East Siberia. Maximum in night foF2 under quiet geomagnetic conditions is observed mainly after midnight (02-04 LT) and is shifted to predawn hours as solar activity increases. At low solar activity the quiet ionosphere at ∼02–04 LT shows the following properties: (a) the fluctuations of foF2 and hmF2 are in the reverse correlation but this dependence is weak; (b) very strong fluctuations of foF2 (|δfoF2| > 30%) occur seldom (∼4% of events) and almost all of them are positive; an example of very strong fluctuations of foF2 up to 60% can be an extreme increase in the foF2 on 19.12.2008; (c) the very strong enhancements of foF2 in the NWI maximum can be observed at the low geomagnetic activity, they occur more often during substorms but very seldom during geomagnetic storms. Possible reasons of these properties of NWI are discussed. 相似文献
999.
Richa Trivedi Amit Jain Sudhir Jain A.K. Gwal 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
GPS satellites data obtained at Bhopal (23.16° N, 77.36° E, geomagnetic latitude 14.23° N) India were analyzed to study the TEC changes during several geomagnetic storms (−300 nT < Dst < −50 nT) occurred in 2005–2007. We had segregated the storms according to the Dst value, i.e. moderate storms (−100 nT < Dst ? −50 nT), strong storms (−150 nT < Dst < −100 nT), and severe storms (Dst less than −150 nT). Total of 21 geomagnetic storms (10 moderate, 9 strong, 2 severe) are considered for the present study. Deviation in vertical total electron content (VTEC) during the main phase of the storm was found to be associated with the prompt penetration of electric field originated due to the under-shielding and over-shielding conditions for almost all geomagnetic storms discussed in this paper. For most of the storms VTEC shows the positive percentage deviation during the main phase while it shows positive as well as the negative deviation during the recovery phase of the storms. The −80% deviation in VTEC was found for geomagnetic storm occurred on July 17, 2005 and the negative trend continued for recovery phase of the storm. This was mainly due to the thermospheric composition changes by Joule heating effect at auroral latitudes that generate electric field disturbance at low latitudes. Traveling ionospheric disturbances (TIDs) were responsible for the formation of wave like nature in VTEC for the storms occurred on May 15, 2005, whereas it was not observed for storm occurred on August 24, 2005. 相似文献
1000.