全文获取类型
收费全文 | 26366篇 |
免费 | 365篇 |
国内免费 | 213篇 |
专业分类
测绘学 | 522篇 |
大气科学 | 1966篇 |
地球物理 | 5250篇 |
地质学 | 9175篇 |
海洋学 | 2164篇 |
天文学 | 6128篇 |
综合类 | 47篇 |
自然地理 | 1692篇 |
出版年
2021年 | 172篇 |
2020年 | 207篇 |
2019年 | 218篇 |
2018年 | 507篇 |
2017年 | 486篇 |
2016年 | 587篇 |
2015年 | 439篇 |
2014年 | 626篇 |
2013年 | 1239篇 |
2012年 | 743篇 |
2011年 | 1024篇 |
2010年 | 896篇 |
2009年 | 1235篇 |
2008年 | 1072篇 |
2007年 | 1075篇 |
2006年 | 1000篇 |
2005年 | 824篇 |
2004年 | 841篇 |
2003年 | 805篇 |
2002年 | 744篇 |
2001年 | 692篇 |
2000年 | 638篇 |
1999年 | 573篇 |
1998年 | 578篇 |
1997年 | 578篇 |
1996年 | 442篇 |
1995年 | 425篇 |
1994年 | 385篇 |
1993年 | 344篇 |
1992年 | 314篇 |
1991年 | 282篇 |
1990年 | 311篇 |
1989年 | 281篇 |
1988年 | 238篇 |
1987年 | 313篇 |
1986年 | 262篇 |
1985年 | 353篇 |
1984年 | 385篇 |
1983年 | 373篇 |
1982年 | 336篇 |
1981年 | 311篇 |
1980年 | 315篇 |
1979年 | 283篇 |
1978年 | 312篇 |
1977年 | 264篇 |
1976年 | 269篇 |
1975年 | 276篇 |
1974年 | 239篇 |
1973年 | 244篇 |
1972年 | 159篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.
282.
Bovolo F. Bruzzone L. Capobianco L. Garzelli A. Marchesi S. Nencini S. 《Geoscience and Remote Sensing Letters, IEEE》2010,7(1):53-57
283.
Automated GPS Data Analysis Service 总被引:4,自引:1,他引:4
James F. Zumberge 《GPS Solutions》1999,2(3):76-78
Automatic analysis of geodetic-quality GPS data is available with the use of e-mail and ftp (file transfer program) as an
interface to a computer at the Jet Propulsion Laboratory (JPL), where precise transmitter parameters – GPS ephemerides and
clock errors – are computed regularly. The interface is such that e-mail from an external user causes the JPL computer to
fetch the user's data. The computer than analyzes the data, and places the results in an area accessible to the user. An e-mail
to the user gives information on the location of the analysis results, which the user can subsequently fetch. Operations on
the JPL computer are entirely automatic, and require essentially no labor. ? 1999 John Wiley & Sons, Inc. 相似文献
284.
A GPS-Supported Gravity Survey in the Amazon of Ecuador 总被引:1,自引:0,他引:1
Mamadou Balde Jeremy Fishman Carlos L. V. Aiken Mohamed Abdel-Salam Mauricio F. de la Fuente 《GPS Solutions》1999,2(3):3-17
The growing importance and effectiveness of the Global Positioning System (GPS) in positioning geophysical surveys is now
well established. In Ecuador's Amazon basin the use of GPS was the determining factor in the successful completion of a gravity
survey carried out in 1994. Over 500 gravity stations, spaced from 1 to 5 km apart and positioned by GPS, were collected in
the jungle environment in the country's southeast, where the preexisting data, of variable and unknown quality, were mostly
confined to areas along the major rivers. This station densification, which includes the reoccupation of selected points,
contributed to increase the gravity coverage; it also allowed us to evaluate and correct 1100 preexisting data points in the
area, so that the different surveys can be merged. The composite data set is used for a geologic analysis of the study area,
where previously unknown structures are discovered. ? 1999 John Wiley & Sons, Inc. 相似文献
285.
Paul F. McMillan Brent T. Poe Thomas R. Stanton Richard L. Remmele 《Physics and Chemistry of Minerals》1993,19(7):454-459
We have obtained high quality Raman spectra for two H/D isotopically substituted hydrous aluminosilicate glasses with compositions along the NaAlSi3O8-SiO2 join. Consistent with the results of previous studies, the isotope shift for the band near 900 cm–1, whose intensity grows with increasing water content, is extremely small: v
h
/v
d
= 1.004 ± 0.004. The lack of a definite H/D isotope shift for this band does not, however, preclude its association with a vibration of a hydrous species in the glass, because of likely strong coupling between different vibrational modes of hydrated framework species. The 900 cm–1 band could well be due to a T — OH (T = Si, Al) stretching or bending vibration in the hydrous glass, as required by the presence of a combination band near 4500 cm–1 in near-infrared spectra. 相似文献
286.
L. A. Nolan J. S. Dunlop R. Jimenez A. F. Heavens 《Monthly notices of the Royal Astronomical Society》2003,341(2):464-476
We explore whether the rest-frame near-ultraviolet spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by first testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-ultraviolet spectra of F stars of known (subsolar and supersolar) metallicity. F stars are of particular interest because the rest-frame near-ultraviolet spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turn-off.
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3–5 . 相似文献
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3–5 . 相似文献
287.
We present an automated extraction method based on the continuous wavelet transform (CWT) for analyzing solar magnetic loops. The aim of the work is to extract, from the images taken from solar EUV telescopes, the traces of bright loops presumably shaped by the magnetic field of the solar corona. The technique is that of wavelet analysis, using the two-dimensional Morlet wavelet, because of its efficiency in detecting oriented features, which allows us to follow closely the curvature of the loops. Next, we segment the wavelet modulus image and we threshold it, both globally and locally (i.e., adaptively), in order to eliminate the remaining noise. Altogether, our method performs well, it is robust and fast, and could be used as a standard tool for analyzing large data sets expected from missions like SDO. 相似文献
288.
Corridor location: the multi-gateway shortest path model 总被引:1,自引:1,他引:1
Maria P. Scaparra Richard L. Church F. Antonio Medrano 《Journal of Geographical Systems》2014,16(3):287-309
The problem of corridor location can be found in a number of fields including power transmission, highways, and pipelines. It involves the placement of a corridor or rights-of-way that traverses a landscape starting at an origin and ending at a destination. Since most systems are subject to environmental review, it is important to generate competitive, but different alternatives. This paper addresses the problem of generating efficient, spatially different alternatives to the corridor location problem. We discuss the weaknesses in current models and propose a new approach which is designed to overcome many of these problems. We present an application of this model to a real landscape and compare the results to past work. Overall, the new model called the multi-gateway shortest path problem can generate a wide variety of efficient alignments, which eclipse what could be generated by past work. 相似文献
289.
Josef Gochermann Walter F. Wargau Claus Tappert Theodor Schmidt-Kaler Robert S. Stobie Fred Marang Greg Roberts Francois G. van Wyk Peter Rucks 《Experimental Astronomy》1999,9(1):1-15
In order to decide whether the seeing conditions at SAAO/Sutherland justify the erection of a 3.5 m telescope and also to compare Sutherland with the Gamsberg/Namibia site, a seeing campaign covering 15 months has been carried out. For direct comparison with the results of the seeing campaign at Gamsberg twenty years before the same QUESTAR telescope was employed. The seeing is determined by the scattering of the star-trail exposed on a film in the focal plane of the telescope. The campaign commenced in February 1992. Up to May 1993, data for 204 nights, that is 47.3% of the total number of nights, were collected. Due to wind speeds above 30 km h-1, 25 out of the 204 nights were not considered in the final reduction. The useful 179 nights are evenly distributed over the campaign period. The median seeing value for the whole period is = 0.52. There are differences during the year: the best season gives = 0.42, the worst = 0.67. Each night was divided into three intervals, although data for each of the three intervals were not always available. Generally, there is an improvement in the seeing during the course of a night. The results are compared to the seeing values of Gamsberg/Namibia and ESO/La Silla. 相似文献
290.
Unattenuated solar photo rate coefficients and excess energies for dissociation, ionization, and dissociative ionization are presented for atomic and molecular species that have been identified or are suspected to exist in the atmospheres of planets, satellites (moons), comets, or as pollutants in the Earth atmosphere. The branching ratios and cross sections with resonances have been tabulated to the greatest detail possible and the rate coefficients and excess energies have been calculated from them on a grid of small wavelength bins for the quiet and the active Sun at 1 AU heliocentric distance. 相似文献