全文获取类型
收费全文 | 1648篇 |
免费 | 108篇 |
国内免费 | 47篇 |
专业分类
测绘学 | 92篇 |
大气科学 | 47篇 |
地球物理 | 96篇 |
地质学 | 48篇 |
海洋学 | 16篇 |
天文学 | 1469篇 |
综合类 | 25篇 |
自然地理 | 10篇 |
出版年
2024年 | 7篇 |
2023年 | 19篇 |
2022年 | 19篇 |
2021年 | 20篇 |
2020年 | 13篇 |
2019年 | 22篇 |
2018年 | 10篇 |
2017年 | 13篇 |
2016年 | 15篇 |
2015年 | 14篇 |
2014年 | 32篇 |
2013年 | 20篇 |
2012年 | 26篇 |
2011年 | 29篇 |
2010年 | 22篇 |
2009年 | 108篇 |
2008年 | 90篇 |
2007年 | 134篇 |
2006年 | 108篇 |
2005年 | 123篇 |
2004年 | 160篇 |
2003年 | 104篇 |
2002年 | 105篇 |
2001年 | 100篇 |
2000年 | 90篇 |
1999年 | 86篇 |
1998年 | 114篇 |
1997年 | 32篇 |
1996年 | 22篇 |
1995年 | 28篇 |
1994年 | 24篇 |
1993年 | 8篇 |
1992年 | 23篇 |
1991年 | 7篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有1803条查询结果,搜索用时 62 毫秒
311.
312.
313.
A lunar occultation (LO) technique in the near-infrared (NIR) provides angular resolution down to milliarcseconds for an occulted source, even with ground- based 1 m class telescopes. LO observations are limited to brighter objects because they require a high signal-to-noise ratio (S/N ~40) for proper extraction of angular diameter values. Hence, methods to improve the S/N ratio by reducing noise using Fourier and wavelet transforms have been explored in this study. A sample of 54 NIR LO light curves observed with the IR camera at Mt Abu Observatory has been used. It is seen that both Fourier and wavelet methods have shown an improvement in S/N compared to the original data. However, the application of wavelet transforms causes a slight smoothing of the fringes and results in a higher value for angular diameter. Fourier transforms which reduce discrete noise frequencies do not distort the fringe. The Fourier transform method seems to be effective in improving the S/N, as well as improving the model fit, particularly in the fainter regime of our sample. These methods also provide a better model fit for brighter sources in some cases, though there may not be a significant imorovement in S/N. 相似文献
314.
We present new results of heliographic observations of quiet‐Sun radio emission fulfilled by the UTR‐2 radio telescope. The solar corona investigations have been made close to the last solar minimum (Cycle 23) in the late August and early September of 2010 by means of the two‐dimensional heliograph within 16.5–33 MHz. Moreover, the UTR‐2 radio telescope was used also as an 1‐D heliograph for one‐dimensional scanning of the Sun at the beginning of September 2010 as well as in short‐time observational campaigns in April and August of 2012. The average values of integral flux density of the undisturbed Sun continuum emission at different frequencies have been found. Using the data, we have determined the spectral index of quiet‐Sun radio emission in the range 16.5–200 MHz. It is equal to –2.1±0.1. The brightness distribution maps of outer solar corona at frequencies 20.0 MHz and 26.0 MHz have been obtained. The angular sizes of radio Sun were estimated. It is found that the solar corona at these frequencies is stretched‐out along equatorial direction. The coefficient of corona ellipticity varies slightly during above period. Its mean magnitudes are equal to ≈ 0.75 and ≈ 0.73 at 20.0 MHz and 26.0 MHz, respectively. The presented results for continuum emission of solar corona conform with being ones at higher frequencies. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
315.
A bistatic space debris observation system using a radio telescope as the receiving part is introduced. The detection capability of the system at different working frequencies is analyzed based on real instruments. The detection range of targets with a fixed radar cross section and the detection ability of small space debris at a fixed range are discussed. The simulations of this particular observation system at different transmitting powers are also implemented and the detection capability is discussed. The simulated results approximately match the actual experiments. The analysis in this paper provides a theoretical basis for developing a space debris observation system that can be built in China. 相似文献
316.
317.
318.
319.
载波相位差分技术在馈源舱动态定位中的应用 总被引:1,自引:0,他引:1
针对FAST项目馈源舱扫描跟踪设计,作者根据GPS载波相位差分精密动态定位原理,讨论了馈源舱位置和姿态的GP动态检测系统的组成,按照我国贵州KARST地形的地理位置给出了动态检测时需要使用的馈源舱位姿转换矩阵和求解方程,与GPS动态定位系统相结合就可以完成对馈源舱位姿的跟踪检测,实现舱体的闭环控制。 相似文献
320.