全文获取类型
收费全文 | 29664篇 |
免费 | 403篇 |
国内免费 | 228篇 |
专业分类
测绘学 | 735篇 |
大气科学 | 2257篇 |
地球物理 | 5766篇 |
地质学 | 10863篇 |
海洋学 | 2557篇 |
天文学 | 6298篇 |
综合类 | 65篇 |
自然地理 | 1754篇 |
出版年
2021年 | 275篇 |
2020年 | 298篇 |
2019年 | 302篇 |
2018年 | 727篇 |
2017年 | 725篇 |
2016年 | 839篇 |
2015年 | 468篇 |
2014年 | 802篇 |
2013年 | 1560篇 |
2012年 | 926篇 |
2011年 | 1223篇 |
2010年 | 1055篇 |
2009年 | 1364篇 |
2008年 | 1184篇 |
2007年 | 1173篇 |
2006年 | 1161篇 |
2005年 | 840篇 |
2004年 | 839篇 |
2003年 | 767篇 |
2002年 | 768篇 |
2001年 | 713篇 |
2000年 | 702篇 |
1999年 | 550篇 |
1998年 | 519篇 |
1997年 | 599篇 |
1996年 | 468篇 |
1995年 | 475篇 |
1994年 | 471篇 |
1993年 | 381篇 |
1992年 | 387篇 |
1991年 | 358篇 |
1990年 | 373篇 |
1989年 | 342篇 |
1988年 | 344篇 |
1987年 | 368篇 |
1986年 | 325篇 |
1985年 | 425篇 |
1984年 | 414篇 |
1983年 | 446篇 |
1982年 | 415篇 |
1981年 | 365篇 |
1980年 | 403篇 |
1979年 | 324篇 |
1978年 | 302篇 |
1977年 | 294篇 |
1976年 | 266篇 |
1975年 | 264篇 |
1974年 | 269篇 |
1973年 | 254篇 |
1971年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
In this paper we compute the rate of solar EUV heating in the upper atmosphere by photo-dissociation and photo-ionization, taking care to include properly the effects of oblique incidence of solar flux, sphericity of the atmosphere and ellipticity of the Earth's orbit. The time and latitudinal variations of the computed heat function are revealed by numerical Fourier analysis of the heat function into harmonics of the yearly cycle. It is shown that EUV absorption contains a ‘latitude independent’ semi-annual component of heating with the ‘proper phase’ to account for the semi-annual density variations. Further, the annual component of the heat function predicts the existence of ‘summer polar’ density increases in the northern and southern hemispheres. 相似文献
102.
103.
Summary All of the barium and marginal barium stars in the 1972 MacConnell, Frey, and Upgren (1972: MFU) list of such objects which
are located south of the celestial equator were examined for visual duplicity. Over 200 objects were observed, and a total
of six possible candidates for wide pair visual binaries were found. Of these, four are most likely optical in nature. These
data suggest that the incidence of wide binaries among barium stars is of the order of 1%. 相似文献
104.
105.
In this paper an attempt has been made to search a new parameter for the prediction of the Indian summer monsoon rainfall.
For this purpose the relationship of the global surface-air temperature of four standard seasons viz., Winter (December-January-February),
Spring (March-April-May), Summer (June-July-August), Autumn (September-October-November) with the Indian summer monsoon rainfall
has been carried out. The same analysis is also carried out with surface-air temperature anomalies within the tropical belt
(30°S to 30°N) and Indian summer monsoon rainfall. For the present study data for 30 years period from 1958 to 1988 have been
used. The analysis reveals that there is a strong inverse relationship between the monsoon activity and the tropical belt
temperature. 相似文献
106.
107.
Tatenda Dalu Timothy Dube P. William Froneman Mwazvita T.B. Sachikonye Bruce W. Clegg Tamuka Nhiwatiwa 《国际地球制图》2015,30(10):1130-1143
Traditional approaches to monitoring aquatic systems are often limited by the need for data collection which often is time-consuming, expensive and non-continuous. The aim of the study was to map the spatio-temporal chlorophyll-a concentration changes in Malilangwe Reservoir, Zimbabwe as an indicator of phytoplankton biomass and trophic state when the reservoir was full (year 2000) and at its lowest capacity (year 2011), using readily available Landsat multispectral images. Medium-spatial resolution (30 m) Landsat multispectral Thematic Mapper TM 5 and ETM+ images for May to December 1999–2000 and 2010–2011 were used to derive chlorophyll-a concentrations. In situ measured chlorophyll-a and total suspended solids (TSS) concentrations for 2011 were employed to validate the Landsat chlorophyll-a and TSS estimates. The study results indicate that Landsat-derived chlorophyll-a and TSS estimates were comparable with field measurements. There was a considerable wet vs. dry season differences in total chlorophyll-a concentration, Secchi disc depth, TSS and turbidity within the reservoir. Using Permutational multivariate analyses of variance (PERMANOVA) analysis, there were significant differences (p < 0.0001) for chlorophyll-a concentration among sites, months and years whereas TSS was significant during the study months (p < 0.05). A strong positive significant correlation among both predicted TSS vs. chlorophyll-a and measured vs. predicted chlorophyll-a and TSS concentrations as well as an inverse relationship between reservoir chlorophyll-a concentrations and water level were found (p < 0.001 in all cases). In conclusion, total chlorophyll-a concentration in Malilangwe Reservoir was successfully derived from Landsat remote sensing data suggesting that the Landsat sensor is suitable for real-time monitoring over relatively short timescales and for small reservoirs. Satellite data can allow for surveying of chlorophyll-a concentration in aquatic ecosystems, thus, providing invaluable data in data scarce (limited on site ground measurements) environments. 相似文献
108.
An approach to improve orbital state vectors by orbit error estimates derived from residual phase patterns in synthetic aperture radar interferograms is presented. For individual interferograms, an error representation by two parameters is motivated: the baseline error in cross-range and the rate of change of the baseline error in range. For their estimation, two alternatives are proposed: a least squares approach that requires prior unwrapping and a less reliable gridsearch method handling the wrapped phase. In both cases, reliability is enhanced by mutual control of error estimates in an overdetermined network of linearly dependent interferometric combinations of images. Thus, systematic biases, e.g., due to unwrapping errors, can be detected and iteratively eliminated. Regularising the solution by a minimum-norm condition results in quasi-absolute orbit errors that refer to particular images. For the 31 images of a sample ENVISAT dataset, orbit corrections with a mutual consistency on the millimetre level have been inferred from 163 interferograms. The method itself qualifies by reliability and rigorous geometric modelling of the orbital error signal but does not consider interfering large scale deformation effects. However, a separation may be feasible in a combined processing with persistent scatterer approaches or by temporal filtering of the estimates. 相似文献
109.
Processing of data from global navigation satellite systems (GNSS), such as GPS, GLONASS and Galileo, can be considerably impeded by disturbances in the ionosphere. Cycle-slip detection and correction thus becomes a crucial component of robust software. Still, dealing with ionospheric cycle slips is not trivial due to scintillation effects in both the phase and the amplitude of the signals. In this contribution, a geometry-based approach with rigorous handling of the ionosphere is presented. A detailed analysis of the cycle-slip correction process is also tackled by examining its dependence on phase and code noise, non-dispersive effects and, of course, the ionosphere. The importance of stochastic modeling in validating the integer cycle-slip candidates is emphasized and illustrated through simulations. By examining the relationship between ionospheric bias and ionospheric constraint, it is shown that there is a limit in the magnitude of ionospheric delay variation that can be handled by the cycle-slip correction process. Those concepts are applied to GNSS data collected by stations in northern Canada, and show that enhanced cycle-slip detection can lead to decimeter-level improvements in the accuracy of kinematic PPP solutions with a 30-s sampling interval. Cycle-slip correction associated with ionospheric delay variations exceeding 50 cm is also demonstrated, although there are risks with such a procedure and these are pointed out. 相似文献
110.
Rimjhim Bhatnagar Singh S. S. Ray S. K. Bal B. S. Sekhon G. S. Gill Sushma Panigrahy 《Journal of the Indian Society of Remote Sensing》2013,41(2):301-308
Crop residue has become an increasingly important factor in agriculture management. It assists in the reduction of soil erosion and is an important source of soil organic carbon (soil carbon sequestration). In recent past, remote sensing, especially narrowband, data have been explored for crop residue assessment. In this context, a study was carried out to identify different narrow-bands and evaluate the performance of SWIR region based spectral indices for crop residue discrimination. Ground based hyperspectral data collected for wheat crop residue was analyzed using Stepwise Discriminant Analysis (SDA) technique to select significant bands for discrimination. Out of the seven best bands selected to discriminate between matured crop, straw heap, combine-harvested field with stubbles and soil, four bands were from SWIR (1980, 2030, 2200, 2440 nm) region. Six spectral indices were computed, namely CAI, LCA, SINDRI, NDSVI, NDI5 and hSINDRI for crop residue discrimination. LCA and CAI showed to be best (F?>?115) in discriminating above classes, while LCA and SINDRI were best (F?>?100) among all indices in discriminating crop residue under different harvesting methods. Comparison of different spectral resolution (from 1 nm to 150 nm) showed that for crop residue discrimination a resolution of 100 nm at 2100–2300 m region would be sufficient to discriminate crop residue from other co-existing classes. 相似文献