全文获取类型
收费全文 | 256259篇 |
免费 | 5182篇 |
国内免费 | 3363篇 |
专业分类
测绘学 | 6853篇 |
大气科学 | 19020篇 |
地球物理 | 53695篇 |
地质学 | 88917篇 |
海洋学 | 21349篇 |
天文学 | 56152篇 |
综合类 | 1001篇 |
自然地理 | 17817篇 |
出版年
2021年 | 2235篇 |
2020年 | 2586篇 |
2019年 | 2848篇 |
2018年 | 3394篇 |
2017年 | 3096篇 |
2016年 | 5637篇 |
2015年 | 4193篇 |
2014年 | 6902篇 |
2013年 | 14204篇 |
2012年 | 6435篇 |
2011年 | 7712篇 |
2010年 | 6762篇 |
2009年 | 9421篇 |
2008年 | 8211篇 |
2007年 | 7670篇 |
2006年 | 9615篇 |
2005年 | 7662篇 |
2004年 | 7590篇 |
2003年 | 7082篇 |
2002年 | 6694篇 |
2001年 | 5982篇 |
2000年 | 5935篇 |
1999年 | 5199篇 |
1998年 | 5215篇 |
1997年 | 5022篇 |
1996年 | 4677篇 |
1995年 | 4410篇 |
1994年 | 4101篇 |
1993年 | 3852篇 |
1992年 | 3627篇 |
1991年 | 3586篇 |
1990年 | 3759篇 |
1989年 | 3507篇 |
1988年 | 3295篇 |
1987年 | 3844篇 |
1986年 | 3405篇 |
1985年 | 4218篇 |
1984年 | 4728篇 |
1983年 | 4401篇 |
1982年 | 4305篇 |
1981年 | 3922篇 |
1980年 | 3632篇 |
1979年 | 3503篇 |
1978年 | 3480篇 |
1977年 | 3280篇 |
1976年 | 3040篇 |
1975年 | 2954篇 |
1974年 | 2913篇 |
1973年 | 3080篇 |
1972年 | 2022篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
411.
To derive a matched filter for detecting a weak target signal in a hyperspectral image, an estimate of the band-to-band covariance of the target-free background scene is required. We investigate the effects of including some of the target signal in the background scene. Although the covariance is contaminated by the presence of a target signal (there is increased variance in the direction of the target signature), we find that the matched filter is not necessarily affected. In fact, if the variation in plume strength is strictly uncorrelated with the variation in background spectra, the matched filter and its signal-to-clutter ratio (SCR) performance will not be impaired. While there is little a priori reason to expect significant correlation between the plume and the background, there usually is some residual correlation, and this correlation leads to a suppressing effect that limits the SCR obtainable even for strong plumes. These effects are described and quantified analytically, and the crucial role of this correlation is illustrated with some numerical examples using simulated plumes superimposed on real hyperspectral imagery. In one example, we observe an order-of-magnitude loss in SCR for a matched filter based on the contaminated covariance. 相似文献
412.
413.
414.
415.
416.
417.
Radiation exposure and Mission Strategies
for Interplanetary Manned Missions (REMSIM) 总被引:1,自引:0,他引:1
C. Cougnet N. B. Crosby S. Eckersley C. Foullon V. Guarnieri S. Guatelli D. Heynderickx A. Holmes-Siedle C. Lobascio S. Masiello P. Nieminen G. Parisi P. Parodi M. A. Perino M. G. Pia R. Rampini P. Spillantini V. Tamburini E. Tracino 《Earth, Moon, and Planets》2004,94(3-4):279-285
Cosmic radiation is an important problem for human interplanetary missions. The “Radiation Exposure and Mission Strategies for Interplanetary Manned Missions–REMSIM” study is summarised here. They are related to current strategies and countermeasures to ensure the protection of astronauts from radiation during interplanetary missions, with specific reference to: radiation environment and its variability; radiation effects on the crew; transfer trajectories and associated fluences; vehicle and surface habitat concepts; passive and active shielding concepts; space weather monitoring and warning systems. 相似文献
418.
We examine the potential contamination of cometary nuclei through impacts from asteroidal origin meteoroids. The paper uses
a simple model and has the goal of determining whether asteroidal contamination is potentially significant. We assume a meteoroid
power law mass distribution with index values in the range from s=1.83 to s=2.09. We used maximum and minimum models which we believe will bracket the true meteoroid mass distribution. We identify
those comets which are expected to be most significantly contaminated, and find values of up to 3.6 kg of asteroidal meteoroid
impact per square meter of the cometary surface per orbital revolution. This is less than the expected mass loss per perihelion
passage for most comets. Therefore any remnant effects of the contamination will depend on the penetration depth of the meteoroids
in the cometary nucleus, and possibly on the distribution of active and inactive areas on cometary nuclei. We present a simple
model which suggests that even small meteoroids will embed relatively deeply into a cometary nucleus. 相似文献
419.
A. G. Nikoghossian 《Astrophysics》2004,47(3):412-421
The approach proposed in the previous parts of this series of papers is used to solve the radiative transfer problem in scattering and absorbing multicomponent atmospheres. Linear recurrence relations are obtained for both the reflectance and transmittance of these kinds of atmospheres, as well as for the emerging intensities when the atmosphere contains energy sources. Spectral line formation in a one-dimensional inhomogeneous atmosphere is examined as an illustration of the possibility of generalizing our approach to the matrix case. It is shown that, in this case as well, the question reduces to solving an initial value problem for linear differential equations. Some numerical calculations are presented. 相似文献
420.