全文获取类型
收费全文 | 1771篇 |
免费 | 238篇 |
国内免费 | 372篇 |
专业分类
航空 | 994篇 |
航天技术 | 526篇 |
综合类 | 390篇 |
航天 | 471篇 |
出版年
2024年 | 5篇 |
2023年 | 6篇 |
2022年 | 13篇 |
2021年 | 25篇 |
2020年 | 20篇 |
2019年 | 22篇 |
2018年 | 23篇 |
2017年 | 19篇 |
2016年 | 24篇 |
2015年 | 69篇 |
2014年 | 101篇 |
2013年 | 102篇 |
2012年 | 107篇 |
2011年 | 113篇 |
2010年 | 143篇 |
2009年 | 121篇 |
2008年 | 128篇 |
2007年 | 99篇 |
2006年 | 86篇 |
2005年 | 71篇 |
2004年 | 57篇 |
2003年 | 57篇 |
2002年 | 82篇 |
2001年 | 73篇 |
2000年 | 83篇 |
1999年 | 95篇 |
1998年 | 85篇 |
1997年 | 78篇 |
1996年 | 67篇 |
1995年 | 54篇 |
1994年 | 63篇 |
1993年 | 52篇 |
1992年 | 52篇 |
1991年 | 48篇 |
1990年 | 24篇 |
1989年 | 38篇 |
1988年 | 33篇 |
1987年 | 18篇 |
1986年 | 9篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有2381条查询结果,搜索用时 328 毫秒
71.
文章介绍了以373型精密露点仪为主标准器的湿度标准装置的原理及构成,对该装置的重复性和稳定性进行了测试。结果表明温度重复性实验的标准偏差为0.00℃,相对湿度重复性实验的标准偏差为0.07%RH,露点温度重复性实验的标准偏差为0.01℃。温度稳定性实验的标准偏差为0.02℃,相对湿度稳定性实验的标准偏差为0.33%RH,露点温度稳定性实验的标准偏差为0.066℃。基于美国NI公司的Labview软件开发平台,开发了湿度仪器仪表计量软件。软件以模块化思想为基础,可实现数据自动处理,报告自动生成。 相似文献
72.
Xingyuan Yan Guanwen Huang Qin Zhang Chenchen Liu Le Wang Zhiwei Qin 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(3):1270-1279
Eight new-generation BeiDou satellites (BeiDou-3) have been launched into Medium Earth Orbit (MEO), allowing for global coverage since March 2018, and they are equipped with new hydrogen atomic clocks and updated rubidium clocks. Firstly, we analyzed the signals for the carrier-to-noise-density ratio (C/N0) and pseudorange multipath (MP) by using international GNSS (Global Navigation Satellite System) Monitoring and Assessment System (iGMAS) station data, and found that B1C has a lower C/N0, and B2a has the same level of C/N0 as the B1I and B3I signals. For pseudorange multipath, compared with the BeiDou-2 satellites, the obvious systematic variation of MP scatters related to the elevation angle is greatly improved for the BeiDou-3 and BeiDou-3e satellites signals. For the signals of the BeiDou-3 satellites, the order of the Root Mean Square (RMS) values of multipath and noise is B3I?<?B1I?<?B2a?<?B1C. Then, the comparison of the precise orbit determination and clock offset determination for the BeiDou-2, BeiDou-3, and BeiDou-3 experimental (BeiDou-3e) satellites was done by using 10 stations from iGMAS. The 3D precision of the 24?h orbit overlap is 24.55, 25.61, and 23.35?cm for the BeiDou-3, BeiDou-3e, and BeiDou-2 satellites, respectively. BeiDou-3 satellite has a comparable precision to that of the BeiDou-2 satellite. For the precision of clock offset estimation, the Standard Deviation (STD) of the BeiDou-3 MEO satellite is 0.350?ns, which is an improvement of 0.042?ns over that of the BeiDou-2 MEO satellite. The stabilities of the BeiDou-3 and BeiDou-3e onboard clocks are better than those of BeiDou-2 by factors of 2.84 and 1.61 at an averaging time of 1000 and 10,000?s, respectively. 相似文献
73.
针对复合壳阻尼结构的拓扑减振优化问题,以约束阻尼层的有限单元为设计变量,采用体积比、模态频率和振型为优化约束条件,构建以多模态权重系数的结构模态损耗因子数值关系为优化目标函数的拓扑减振优化模型。为了拓展优化目标灵敏度具有不局限于某一变密度法插值模型的形式,推导了数值表达式的一般函数式。动力学优化中优化目标灵敏度正、负数集共存,使得非凸性的目标函数设计变量出现负值或优化函数寻优于局部极值点。为此,推导出复合壳阻尼结构的全域灵敏度改进优化准则法迭代格,以确保每次迭代域均为全域设计变量集。结合有限单元法编程实现了复合壳阻尼结构改进准则法,并对复合壳结构进行拓扑减振优化分析。结果表明:在敷设体积减为全覆盖的50%时,复合壳结构的模态损耗因子增减偏差为10%,具有提升减振的轻量化设计目的;各阶目标函数和拓扑构型所需的迭代次数少,中间密度区域较小,多阶优于单阶模态优化函数,易于获得全域寻优的有效减振。 相似文献
74.
Rui Tu Jinhai Liu Rui Zhang Pengfei Zhang Xiaodong Huang Xiaochun Lu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(2):913-926
This paper proposes a real-time kinematic (RTK) model that uses one common reference satellite for the Galileo system with four frequency observations. In the proposed model, the double-differenced (DD) pseudorange and carrier phase biases among the different frequencies are estimated as unknown parameters to recover the integer features of the DD ambiguities among the different frequencies for ambiguity resolution and precise positioning. Analysis results show that the E5a, E5b, and E5 frequencies have virtually the same performance in terms of the positioning accuracy, observation residuals, and ratio values of ambiguity resolution. However, the E1 frequency performs worse than the E5a, E5b, and E5 frequencies. The RTK results for the combination of multiple frequencies are much better than those for a single-frequency observation, the coordinates’ standard deviation is improved about 20–30%, and the ambiguity fix time is improved about 10%. 相似文献
75.
Limin Zhou Brian Tinsley Jing Huang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Indices of the North Atlantic Oscillation and the Arctic Oscillation show correlations on the day-to-day timescale with the solar wind speed (SWS). Minima in the indices were found on days of SWS minima during years of high stratospheric aerosol loading. The spatial distribution of surface pressure changes during 1963–2011 with day-to-day changes in SWS shows a pattern resembling the NAO. Such a pattern was noted for year-to-year variations by Boberg and Lundstedt (2002), who compared NAO variations with the geo-effective solar wind electric field (the monthly average SWS multiplied by the average southward component, i.e., negative Bz component, of the interplanetary magnetic field). The spatial distribution of the correlations of geopotential height changes in the troposphere and stratosphere with the SWS; the geo-effective electric field (SWS∗Bz); and the solar 10.7 cm flux suggests that solar wind inputs connected to the troposphere via the global electric circuit, together with solar ultraviolet irradiance acting on the stratosphere, affect regional atmospheric dynamics. 相似文献
76.
现代飞行员培训普遍采用综合训练系统(TTS),训练模拟器是其中的重要组成部分。如何使训练模拟器能够快速、有效地培训飞行员,且具有一定的可持续发展能力,已成为训练模拟器的发展必需解决的问题。本文通过总结国内外训练模拟器的发展情况及分析高级教练机训练模拟器的特点,提出了高级教练机训练模拟器发展的相关建议和思路。 相似文献
77.
78.
79.
80.