全文获取类型
收费全文 | 5190篇 |
免费 | 541篇 |
国内免费 | 157篇 |
专业分类
测绘学 | 235篇 |
大气科学 | 587篇 |
地球物理 | 1928篇 |
地质学 | 2105篇 |
海洋学 | 265篇 |
天文学 | 337篇 |
综合类 | 187篇 |
自然地理 | 244篇 |
出版年
2023年 | 2篇 |
2022年 | 6篇 |
2021年 | 20篇 |
2020年 | 5篇 |
2019年 | 9篇 |
2018年 | 433篇 |
2017年 | 372篇 |
2016年 | 248篇 |
2015年 | 149篇 |
2014年 | 112篇 |
2013年 | 116篇 |
2012年 | 647篇 |
2011年 | 420篇 |
2010年 | 113篇 |
2009年 | 130篇 |
2008年 | 116篇 |
2007年 | 109篇 |
2006年 | 124篇 |
2005年 | 830篇 |
2004年 | 871篇 |
2003年 | 651篇 |
2002年 | 174篇 |
2001年 | 69篇 |
2000年 | 43篇 |
1999年 | 14篇 |
1998年 | 5篇 |
1997年 | 17篇 |
1996年 | 10篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1987年 | 4篇 |
1980年 | 3篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1973年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1966年 | 1篇 |
1965年 | 3篇 |
1963年 | 2篇 |
1962年 | 1篇 |
1961年 | 2篇 |
1959年 | 2篇 |
1956年 | 1篇 |
1955年 | 2篇 |
1954年 | 2篇 |
1951年 | 2篇 |
1948年 | 2篇 |
1925年 | 1篇 |
排序方式: 共有5888条查询结果,搜索用时 0 毫秒
131.
132.
Strontium isotope ratios and rare-earth element abundances have been measured in acid, intermediate and basic rocks from three late to postglacial volcanic complexes, and several other postglacial basalts in Iceland. Late and postglacial basalts in Iceland have been generated from a source region which is essentially homogeneous with respect to87Sr/86Sr. The mean87Sr/86Sr ratio for the basalts analysed is 0.70328 and the range is from 0.70317 ± 6to0.70334 ± 5 (2σ).Acid rocks from the Kerlinganfjöll and Namafjall volcanic complexes have87Sr/86Sr ratios which are indistinguishable from analysed basalts from the same complexes. However, intermediate and acid rocks from the Torfajökull complex have significantly higher87Sr/86Sr ratios and could not have been derived by fractional crystallization from basaltic magmas similar to those found in the same complex. These latter rocks have most probably been produced by remelting of Tertiary gabbroic rocks in Layer 3. Most of the basalts analysed have higher total rare-earth element abundances than typical dredged ocean-ridge tholeiites, and show less light rare-earth depletion. Intermediate and acid compositions show overall higher abundances and light rare-earth enrichments. The measured rare-earth abundances are compared with abundances generated by differential partial melting of various model source regions.It is shown that both the tholeiitic and alkali basalt compositions could be generated from the same source material by different degrees of partial melting. Variable partial melting of gabbroic material may account for the rare-earth element abundances of both the rhyolitic rocks (small degrees of melting) and the intermediate rocks (more extensive melting). 相似文献
133.
LlPingxiang WANGZhijun 《地球空间信息科学学报》2003,6(2):31-34
On the basis of a thorough understanding of the physical characteristics of remote sensing image,this paper employs the theories of wavelet transform and signal sampling to develop a new image fusion algorithm.The algorithm has been successfully applied to the image fusion of SPOT PAN and TM of Guangdong province, China The experimental results show that a perfect image fusion can be built up by using the image analytical solution and reconstruction in the image frequency domain based on the physical characteristics of the image formation.The method has demonstrated that the results of the image fusion do not change spectral characteristics of the original image. 相似文献
134.
135.
WEIWenzhan ZHONGYexun PENGYueying ZHENGHongbo 《地球空间信息科学学报》2003,6(3):62-65
136.
SAR image classification based on its texture features 总被引:2,自引:2,他引:0
LIPingxiang FANGShenghui 《地球空间信息科学学报》2003,6(3):16-19
SAR images not only have the characteristics of all-ay, all-eather, but also provide object infor-mation which is different from visible and infrared sensors. However, SAR images have some faults, such as more speckles and fewer bands. The au-thors conducted the experiments of texture statistics analysis on SAR im-age features in order to improve the accuracy of SAR image interpretation.It is found that the texture analysis is an effective method for improving the accuracy of the SAR image interpreta-tion. 相似文献
137.
PANHeping ZHANGChunsen 《地球空间信息科学学报》2003,6(2):48-54
This paper describes the structure,geometric model and geo-metric calibration of Photogrammetron I-the first type of photogrammetron which is designed to be a coherent stereo photogrammetric system in which two cameras are mounted on a physical base but driven by an intelligent agent architecture.The system calibration is divided into two parts:the in-lab calibration determines the fixed parameters in advance of system operation,and the insitu calibration keeps tracking the free parameters in real-time during the system operation.In a video surveillance set-up, prepared control points are tracked in stereo image sequences,so that the free parameters of the system can be continuously updated through iterative bundle adjustment and kalman filtering. 相似文献
138.
Metaheuristic techniques, which are based on ideas of Artificial Intelligence, are among the best methods for solving computationally the GPS surveying network problem. In this paper, the ant colony optimization metaheuristic, which is inspired by the behavior of real ant colonies, is developed to efficiently provide a general framework for optimizing GPS surveying networks. In this framework, a set of ants co-operate together using an indirect communication procedure to find good GPS observation schedules. A GPS surveying network can be defined as a set of stations, co-ordinated by a series of sessions formed by placing receivers on the stations. The problem is to search for the best order in which to observe these sessions to give the best schedule at minimum cost. Computational results obtained by applying the proposed technique on several networks, with known and unknown optimal schedules, prove the effectiveness of the proposed metaheuristic technique to solve the GPS surveying network problem. 相似文献
139.
JPL's BlackJack receiver currently represents the most widely used geodetic grade GPS receiver for space applications. Using data from the CHAMP science mission, the in-flight performance of the BlackJack receiver has been assessed and the impact of various software updates performed during the 2.5 years since launch is described. Key aspects of the study comprise the channel allocation, anomalous data points, and the noise level of the code and carrier data. In addition, it has been demonstrated that the code measurements collected onboard the CHAMP satellite are notably affected by multipath errors in the aft-looking hemisphere, which can be attributed to cross-talk between the occultation antenna string and the primary precise orbit determination antenna. For carrier smoothed 10 s normal points, the code noise itself varies between a minimum of 5 cm at high elevations and 0.5 m (C/A) to 1.0 m (P1, P2) at 10° elevation. Carrier-phase data exhibit representative errors of 0.2 to 2.5 mm. The results of the CHAMP GPS data analysis contribute to a better understanding and possible improvement of the BlackJack receiver and support the design of optimal data editing and weighting strategies in precise orbit determination applications. 相似文献
140.