全文获取类型
收费全文 | 1575篇 |
免费 | 195篇 |
国内免费 | 322篇 |
专业分类
航空 | 952篇 |
航天技术 | 404篇 |
综合类 | 299篇 |
航天 | 437篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 19篇 |
2021年 | 33篇 |
2020年 | 32篇 |
2019年 | 31篇 |
2018年 | 33篇 |
2017年 | 36篇 |
2016年 | 40篇 |
2015年 | 53篇 |
2014年 | 70篇 |
2013年 | 98篇 |
2012年 | 114篇 |
2011年 | 93篇 |
2010年 | 100篇 |
2009年 | 114篇 |
2008年 | 96篇 |
2007年 | 120篇 |
2006年 | 81篇 |
2005年 | 60篇 |
2004年 | 51篇 |
2003年 | 71篇 |
2002年 | 75篇 |
2001年 | 54篇 |
2000年 | 64篇 |
1999年 | 62篇 |
1998年 | 69篇 |
1997年 | 52篇 |
1996年 | 52篇 |
1995年 | 54篇 |
1994年 | 40篇 |
1993年 | 44篇 |
1992年 | 29篇 |
1991年 | 29篇 |
1990年 | 32篇 |
1989年 | 25篇 |
1988年 | 20篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1965年 | 1篇 |
排序方式: 共有2092条查询结果,搜索用时 0 毫秒
151.
孟氢钡 《南京航空航天大学学报》2012,44(3):405-408
采用双辉等离子渗金属技术在钨表面进行渗镍实验,并采用XRD,SEM,EDX等分析手段对渗镍试样的微观组织进行了表征。结果表明,镍改性层与基体结合良好,无明显缺陷。改性层与基体表面之间存在明显的扩散层,扩散层中存在少量NiW和Ni4W中间相。利用划痕法研究了渗镍层与基体间的结合强度。结果表明:持续加载100 N未发生改性层剥落现象。对钨表面双辉等离子渗镍改性层的形成机制进行了探讨。 相似文献
152.
Qile Zhao Jing Guo Zhigang Hu Chuang Shi Jingnan Liu Hua Cai Xianglin Liu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The GRACE (Gravity Recovery And Climate Experiment) monthly gravity models have been independently produced and published by several research institutions, such as Center for Space Research (CSR), GeoForschungsZentrum (GFZ), Jet Propulsion Laboratory (JPL), Centre National d’Etudes Spatiales (CNES) and Delft Institute of Earth Observation and Space Systems (DEOS). According to their processing standards, above institutions use the traditional variational approach except that the DEOS exploits the acceleration approach. The background force models employed are rather similar. The produced gravity field models generally agree with one another in the spatial pattern. However, there are some discrepancies in the gravity signal amplitude between solutions produced by different institutions. In particular, 10%–30% signal amplitude differences in some river basins can be observed. In this paper, we implemented a variant of the traditional variational approach and computed two sets of monthly gravity field solutions using the data from January 2005 to December 2006. The input data are K-band range-rates (KBRR) and kinematic orbits of GRACE satellites. The main difference in the production of our two types of models is how to deal with nuisance parameters. This type of parameters is necessary to absorb low-frequency errors in the data, which are mainly the aliasing and instrument errors. One way is to remove the nuisance parameters before estimating the geopotential coefficients, called NPARB approach in the paper. The other way is to estimate the nuisance parameters and geopotential coefficients simultaneously, called NPESS approach. These two types of solutions mainly differ in geopotential coefficients from degree 2 to 5. This can be explained by the fact that the nuisance parameters and the gravity field coefficients are highly correlated, particularly at low degrees. We compare these solutions with the official and published ones by means of spectral analysis. It is found that our solutions are, in general, consistent with others in the spatial pattern. The water storage variations of the Amazon, Chari and Ganges river basins have also been computed. The variations computed with the NPARB approach are closer to those produced by JPL and DEOS solutions, while the variations produced with the NPESS approach are in good agreement with those produced by the CSR and GFZ solutions. A simulation study is implemented with considering realistic noise and low-frequency error. The two approaches are used to recover the true model. The NPESS solution appears closer to the true one. Therefore we are inclined to estimate the nuisance parameters simultaneously with the geopential coefficients. 相似文献
153.
154.
155.
对发射端采用双平衡正交调制器产生线性调频造成载波泄漏的原因进行了详细分析,提出了采用直接数字合成(DDS)技术产生发射端线性调频信号的实现方法,从而很好地解决了载波泄漏的问题,并给出了试验结果。 相似文献
156.
157.
基于全解耦奇偶方程的动态系统执行器故障检测与识别 总被引:3,自引:0,他引:3
给出了基于全解耦奇偶方程的动态系统执行器故障的检测与识别方法。讨论了全解耦奇偶向量的产生方法,给出了全解耦奇偶向量的存在条件,并结合卡尔曼滤波方法得到了故障模型参数的估计方法。最后给出了仿真实例。结果表明,全解耦奇偶方程方法能估计出动态系统执行器故障模型。 相似文献
158.
159.
本文是在文献[1]的基础上,讨论遥控机器人追踪自由飞行目标物的动态性能问题。应用黄金分割法和元素交替极小化法对扩展的卡尔曼滤波初值条件进行搜索并获得优化解。仿真结果表明优化方法简单可行,优化解满足系统性能要求。 相似文献
160.