全文获取类型
收费全文 | 257986篇 |
免费 | 4327篇 |
国内免费 | 3373篇 |
专业分类
测绘学 | 6957篇 |
大气科学 | 19089篇 |
地球物理 | 54215篇 |
地质学 | 88975篇 |
海洋学 | 21332篇 |
天文学 | 56097篇 |
综合类 | 1012篇 |
自然地理 | 18009篇 |
出版年
2021年 | 2257篇 |
2020年 | 2604篇 |
2019年 | 2855篇 |
2018年 | 3289篇 |
2017年 | 2983篇 |
2016年 | 5606篇 |
2015年 | 4229篇 |
2014年 | 6936篇 |
2013年 | 14245篇 |
2012年 | 6439篇 |
2011年 | 7760篇 |
2010年 | 6682篇 |
2009年 | 9392篇 |
2008年 | 8318篇 |
2007年 | 7691篇 |
2006年 | 9703篇 |
2005年 | 7714篇 |
2004年 | 7658篇 |
2003年 | 7154篇 |
2002年 | 6773篇 |
2001年 | 6041篇 |
2000年 | 5969篇 |
1999年 | 5213篇 |
1998年 | 5237篇 |
1997年 | 5047篇 |
1996年 | 4696篇 |
1995年 | 4443篇 |
1994年 | 4121篇 |
1993年 | 3870篇 |
1992年 | 3661篇 |
1991年 | 3598篇 |
1990年 | 3771篇 |
1989年 | 3523篇 |
1988年 | 3308篇 |
1987年 | 3859篇 |
1986年 | 3407篇 |
1985年 | 4242篇 |
1984年 | 4748篇 |
1983年 | 4419篇 |
1982年 | 4329篇 |
1981年 | 3937篇 |
1980年 | 3646篇 |
1979年 | 3511篇 |
1978年 | 3483篇 |
1977年 | 3281篇 |
1976年 | 3046篇 |
1975年 | 2960篇 |
1974年 | 2922篇 |
1973年 | 3078篇 |
1972年 | 2029篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
112.
This paper presents an on-line trained neural net work controller for ship track-keeping problems. Following a brief review of the ship track-keeping control development since the 1980's, an analysis of various existing backpropagation-based neural controllers is provided. We then propose a single-input multioutput (SIMO) neural control strategy for situations where the exact mathematical dynamics of the ship are not available. The aim of this study is to build an autonomous neural controller which uses rudder to regulate both the tracking error and heading error. During the whole control process, the proposed SIMO neural controller adapts itself on-line from a direct evaluation of the control accuracy, and hence the need for a “teacher” or an off-line training process can be removed. With a relatively modest amount of quantitative knowledge of the ship behavior, the design philosophy enables real time control of a nonlinear ship model under random wind disturbances and measurement noise. Three different track-keeping tasks have been simulated to demonstrate the effectiveness of the training method and the robust performance of the proposed neural control strategy 相似文献
113.
114.
The emergence of artificial neural network (ANN) technology has provided many promising results in the field of hydrology and water resources simulation. However, one of the major criticisms of ANN hydrologic models is that they do not consider/explain the underlying physical processes in a watershed, resulting in them being labelled as black‐box models. This paper discusses a research study conducted in order to examine whether or not the physical processes in a watershed are inherent in a trained ANN rainfall‐runoff model. The investigation is based on analysing definite statistical measures of strength of relationship between the disintegrated hidden neuron responses of an ANN model and its input variables, as well as various deterministic components of a conceptual rainfall‐runoff model. The approach is illustrated by presenting a case study for the Kentucky River watershed. The results suggest that the distributed structure of the ANN is able to capture certain physical behaviour of the rainfall‐runoff process. The results demonstrate that the hidden neurons in the ANN rainfall‐runoff model approximate various components of the hydrologic system, such as infiltration, base flow, and delayed and quick surface flow, etc., and represent the rising limb and different portions of the falling limb of a flow hydrograph. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
115.
The dynamics of clumps observed in planetary nebulae are considered. The possibility that SiO maser spots in evolved stars and the planetary nebula clumps are formed by the Parker instability behind shocks in pulsating stars' atmospheres is raised. Molecular observations of the clumps are suggested. The effects of the ablation of clumps on the global flow structure of a more tenuous plasma in which they are embedded are reviewed. 相似文献
116.
117.
118.
119.
F. J. Ormeling Sr. 《GeoJournal》1986,13(4):413-416
Conclusion The above account is just a selection of Perthes' multifarious activities. As such it is fragmentary and incomplete. Many other publications and collaborators could have been mentioned. Even in its incompleteness the report may justify the statement that the success of Justus Perthes Geographische Verlagsanstalt, was apart from the managing qualities of its governors, undoubtedly due to the long series of scholarly competent collaborators whom they succeeded in engaging. As illustrated above most of theme were among the avant garde of the profession. Apparently they were at ease with Justus Perthes where they were granted enough freedom of action to realise their ambitious projects under the vigilant eye of the management. In fact the history of the firm is a continuous story of their achievements. Their combined effort initiated modern scientific atlas-making, laid down the foundation of methodic school cartography and accelerated the growth of thematic cartography. The Justus Perthes maps and atlasses stand for intellectual honestly and have an authoritative appearance of truth and exactness. For a long time to come the name of the firm will be considered as a classical example of the promotion of geography and cartography by private enterprise. 相似文献
120.
N.J.P. Owens 《Estuarine, Coastal and Shelf Science》1985,20(4):505-510
In this study, the 14N:15N ratio of suspended particulate material collected from the Tamar river estuary, south-west England, is described. Three populations of particles, distinguishable by their 15N content, were observed. This investigation has shown that populations of estuarine particles are generated by biological transformations in situ and that the 15N content of estuarine particles does not merely reflect hydrodynamic mixing of the freshwater and seawater source particulate material. 相似文献