全文获取类型
收费全文 | 10091篇 |
免费 | 1854篇 |
国内免费 | 2073篇 |
专业分类
测绘学 | 928篇 |
大气科学 | 1420篇 |
地球物理 | 2250篇 |
地质学 | 4620篇 |
海洋学 | 1869篇 |
天文学 | 950篇 |
综合类 | 646篇 |
自然地理 | 1335篇 |
出版年
2024年 | 48篇 |
2023年 | 158篇 |
2022年 | 339篇 |
2021年 | 403篇 |
2020年 | 435篇 |
2019年 | 495篇 |
2018年 | 342篇 |
2017年 | 430篇 |
2016年 | 451篇 |
2015年 | 503篇 |
2014年 | 670篇 |
2013年 | 644篇 |
2012年 | 622篇 |
2011年 | 746篇 |
2010年 | 605篇 |
2009年 | 730篇 |
2008年 | 687篇 |
2007年 | 645篇 |
2006年 | 667篇 |
2005年 | 543篇 |
2004年 | 488篇 |
2003年 | 444篇 |
2002年 | 402篇 |
2001年 | 314篇 |
2000年 | 328篇 |
1999年 | 272篇 |
1998年 | 274篇 |
1997年 | 221篇 |
1996年 | 235篇 |
1995年 | 158篇 |
1994年 | 142篇 |
1993年 | 101篇 |
1992年 | 92篇 |
1991年 | 68篇 |
1990年 | 54篇 |
1989年 | 35篇 |
1988年 | 37篇 |
1987年 | 28篇 |
1986年 | 25篇 |
1985年 | 35篇 |
1984年 | 26篇 |
1983年 | 19篇 |
1982年 | 14篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 2篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1973年 | 1篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 303 毫秒
981.
本文给出了一个在观测中辨别光球横向磁场指向的一个判据。并证明了光球横向电流与横向磁场的指向无关,因而完全可以由向量磁象仪测定。 相似文献
982.
Valerio Iafolla David M. Lucchesi Sergio Nozzoli Francesco Santoli 《Celestial Mechanics and Dynamical Astronomy》2007,97(3):165-187
We have estimated a preliminary error budget for the Italian Spring Accelerometer (ISA) that will be allocated onboard the
Mercury Planetary Orbiter (MPO) of the European Space Agency (ESA) space mission to Mercury named BepiColombo. The role of
the accelerometer is to remove from the list of unknowns the non-gravitational accelerations that perturb the gravitational
trajectory followed by the MPO in the strong radiation environment that characterises the orbit of Mercury around the Sun.
Such a role is of fundamental importance in the context of the very ambitious goals of the Radio Science Experiments (RSE)
of the BepiColombo mission. We have subdivided the errors on the accelerometer measurements into two main families: (i) the
pseudo-sinusoidal errors and (ii) the random errors. The former are characterised by a periodic behaviour with the frequency
of the satellite mean anomaly and its higher order harmonic components, i.e., they are deterministic errors. The latter are
characterised by an unknown frequency distribution and we assumed for them a noise-like spectrum, i.e., they are stochastic
errors. Among the pseudo-sinusoidal errors, the main contribution is due to the effects of the gravity gradients and the inertial forces, while among the random-like errors the main disturbing effect is due to the MPO centre-of-mass displacements produced by the onboard High Gain Antenna (HGA) movements and by the fuel consumption and sloshing. Very subtle to be considered
are also the random errors produced by the MPO attitude corrections necessary to guarantee the nadir pointing of the spacecraft.
We have therefore formulated the ISA error budget and the requirements for the satellite in order to guarantee an orbit reconstruction
for the MPO spacecraft with an along-track accuracy of about 1 m over the orbital period of the satellite around Mercury in
such a way to satisfy the RSE requirements. 相似文献
983.
984.
The absorption spectrum and thermal radiation fluxes in the lower atmosphere of Venus are calculated using the theory of molecular state interference in the strong collision approximation. Comparison is made with the absorption and radiative transfer calculations in terms of the statistical theory of collisional line broadening and based on an empirical form factor. The calculations show that the line broadening mechanism does not affect the thermal regime of the atmosphere at heights above 60 km, but affects significantly the behavior of the greenhouse effect below the cloud layer. 相似文献
985.
交通大数据在人文与经济地理学的应用及学科影响 总被引:2,自引:0,他引:2
交通大数据能够反映人类社会经济活动产生的位移与时空轨迹,不仅能够满足学者们在微观尺度更深入、更精细的研究粒度要求,而且能够为宏观尺度的研究提供广范围、多视角的连续性观测,其研究与发展为人文与经济地理学带来了新思路和新技术。本文以交通大数据的研究前沿为基础,梳理了区位论、时空行为、复杂网络、流空间等理论研究的发展方向,勾勒了“大数据时代”背景下人文与经济地理学的研究框架体系,探讨了新旧技术方法融合的可能性,并讨论了对各个分支学科相关研究的影响。接着,本文总结了交通大数据在人文与经济地理学的主要应用方向与趋势,主要包括出行即服务的交通规划理论与方法,“人工智能+大数据”的城市管理科学,大尺度交通流迁移的模拟,以及“流空间”与“场所空间”的多维度研究等。最后,本文指出了交通大数据应用在获取难度和数据有偏性等方面值得注意的问题并进行了展望。 相似文献
986.
We have performed new simulations of two different scenarios for the excitation and depletion of the primordial asteroid belt, assuming Jupiter and Saturn on initially circular orbits as predicted by the Nice Model of the evolution of the outer Solar System [Gomes, R., Levison, H.F., Tsiganis, K., Morbidelli, A., 2005. Nature 435, 466-469; Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461; Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R., 2005. Nature 435, 462-465]. First, we study the effects of sweeping secular resonances driven by the depletion of the solar nebula. We find that these sweeping secular resonances are incapable of giving sufficient dynamical excitation to the asteroids for nebula depletion timescales consistent with estimates for solar-type stars, and in addition cannot cause significant mass depletion in the asteroid belt or produce the observed radial mixing of different asteroid taxonomic types. Second, we study the effects of planetary embryos embedded in the primordial asteroid belt. These embedded planetary embryos, combined with the action of jovian and saturnian resonances, can lead to dynamical excitation and radial mixing comparable to the current asteroid belt. The mass depletion driven by embedded planetary embryos alone, even in the case of an eccentric Jupiter and Saturn, is roughly 10-20× less than necessary to explain the current mass of the main belt, and thus a secondary depletion event, such as that which occurs naturally in the Nice Model, is required. We discuss the implications of our new simulations for the dynamical and collisional evolution of the main belt. 相似文献
987.
988.
989.
Barney Warf 《The Professional geographer》1990,42(4):421-432
The spatial distribution of foreign-owned high technology firms in the United States is poorly understood, as is the degree to which emplyment in foreign-owned firms and US-owned firms coincides or differs in their distribution. Data from 1988 indicate that while total employment in foreign-owned high technology firms was not statistically different from that of US-owned firms, marked differences were evident in particular sectors, including software, advanced materials, and pharmaceuticals. By nation of ownership, only Swiss firms departed significantly from the US distribution. 相似文献
990.
一致性哈希算法在数据库集群上的拓展应用 总被引:2,自引:0,他引:2
在数据库集群的研究中,可扩展性是一个重要的性能指标。为实现在数据高速增长或部分集群服务器故障情况下,数据依然能够快速、可靠、安全的分布到新的集群服务器节点上的目的,就必须合理设置数据划分的策略。将Key-Value存储技术中使用的一致性哈希算法思想借鉴运用到并行分析型数据库集群中,提出针对大规模结构化类特殊数据的一致性哈希划分方法,并在MapReduce框架下设计具体的数据划分方案。最后,以TPC-DS作为测试基准,与同类系统进行性能对比测试,实验结果表明方案不仅有良好的划分性能,且扩展性也较好。 相似文献