全文获取类型
收费全文 | 626篇 |
免费 | 39篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 35篇 |
地球物理 | 236篇 |
地质学 | 212篇 |
海洋学 | 42篇 |
天文学 | 92篇 |
综合类 | 6篇 |
自然地理 | 33篇 |
出版年
2023年 | 1篇 |
2022年 | 10篇 |
2021年 | 13篇 |
2020年 | 25篇 |
2019年 | 19篇 |
2018年 | 33篇 |
2017年 | 27篇 |
2016年 | 40篇 |
2015年 | 28篇 |
2014年 | 30篇 |
2013年 | 30篇 |
2012年 | 40篇 |
2011年 | 53篇 |
2010年 | 31篇 |
2009年 | 35篇 |
2008年 | 34篇 |
2007年 | 24篇 |
2006年 | 39篇 |
2005年 | 23篇 |
2004年 | 27篇 |
2003年 | 16篇 |
2002年 | 16篇 |
2001年 | 9篇 |
2000年 | 7篇 |
1999年 | 7篇 |
1998年 | 6篇 |
1997年 | 3篇 |
1996年 | 5篇 |
1995年 | 8篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1985年 | 4篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1973年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有668条查询结果,搜索用时 984 毫秒
581.
Precision spacecraft navigation using a low-cost GPS receiver 总被引:1,自引:1,他引:0
Oliver Montenbruck Paul Swatschina Markus Markgraf Stefano Santandrea Joris Naudet Etienne Tilmans 《GPS Solutions》2012,16(4):519-529
Within the PROBA-2 microsatellite mission, a miniaturized single-frequency GPS receiver based on commercial-off-the-shelf (COTS) technology is employed for onboard navigation and timing. A rapid electronic fuse protects against destructive single-event latch-ups (SEL) and enables a quasi-continuous receiver operation despite the inherent sensitivity to space radiation. While limited to single-frequency C/A-code tracking with a narrow-band frontend, the receiver is able to provide precision navigation services through processing of raw GPS measurements on ground as well as a built-in real-time navigation system. In both cases, ionospheric path delays are eliminated through a combination of L1 pseudorange and carrier phase measurements, which also offers a factor-of-two noise reduction relative to code-only processing. By comparison with satellite laser ranging (SLR) measurements, a 0.3-m (3D rms) accuracy is demonstrated for the PROBA-2 reduced dynamic orbit determinations using post-processed GPS orbit and clock products. Furthermore, the experimental onboard navigation system is shown to provide real-time position information with a 3D rms accuracy of about 1?m, which notably outperforms the specification of the Standard Positioning Service (SPS). In view of their lower hardware complexity, mass budget and power requirements as well as the reduced interference susceptibility, legacy C/A-code receivers can thus provide an attractive alternative to dual-frequency receivers even for demanding navigation applications in low Earth orbit. 相似文献
582.
在标准的伽玛暴余辉模型中,电子通过费米一级加速后形成单幂律能谱分布dn/dγe∝γe-p(p≈2.3),但在某些伽玛暴事件中观测到了平缓的电子能谱分布(即p<2).在单幂律谱和分段幂律谱两种情况下,分别给出了具有平缓电子能谱的伽玛暴余辉的解析光变曲线,并以GRB 060908为例进行了讨论.同时提出了伽玛暴低能谱危机的... 相似文献
583.
The impact of spatially non-uniform emissions on the turbulence dispersion of passive tracers in the convective boundary layer
is studied by means of large-eddy simulation. We explicitly calculated the different terms of the budget equations for the
concentrations, fluxes and variances, and used sub-domain averaging where each sub-domain is the typical size of a large-scale
model grid cell. We found that the concentration profiles in the sub-domain where the emission takes place are lightly affected
by the size of the emission release. This effect becomes more relevant in the downwind sub-domain. Although sub-domain averaged
fluxes are not affected by the emission source size, concentration variances are dramatically increased when the emission
shrinks. This increase originates from the mixing of highly concentrated air parcels with those of low concentrations. We
also found that the concentration variance at the surface is driven neither by the position of the emission source nor the
strength of the shear forcing but solely by the emission variance. 相似文献
584.
Goro Komatsu Gaetano Di Achille Ciprian Popa Stefano Di Lorenzo Jose Alexis Palmero Rodriguez 《Icarus》2009,201(2):474-491
The plains of Aurorae and Ophir in the equatorial region of Mars display geomorphic evidence indicative of extensive but generally short-lived paleohydrological processes. Elaver Vallis in Aurorae Planum south of Ganges Chasma is an outflow channel system >180 km long, and here inferred to have formed by cataclysmic spillover flooding from a paleolake(s) contained in the Morella crater basin. Ganges Cavus is an enormous 5-km-deep depression of probable collapse origin located in the Morella basin. The fluid responsible for the infilling of the Morella basin likely emerged at least partially through Ganges Cavus or its incipient depression, and it may have been supplied also from small-scale springs in the basin. Similar paleohydrological processes are inferred also in Ophir Planum. It is reasonable to assume that water, sometimes sediment-laden and/or mixed with gases, was the responsible fluid for these phenomena although some of the observed features could be explained by non-aqueous processes such as volcanism. Water emergence may have occurred as consequences of ground ice melting or breaching of cryosphere to release water from the underlying hydrosphere. Dike intrusion is considered to be an important cause of formation for the cavi and smaller depressions in Aurorae and Ophir Plana, explaining also melting of ground ice or breaching of cryosphere. Alternatively, the depressions and crater basins may have been filled by regional groundwater table rising during the period(s) when cryosphere was absent or considerably thin. The large quantities of water necessary for explaining the paleohydrological processes in Aurorae and Ophir Plana could have been derived through crustal migration from the crust of higher plains in western Ophir Planum where water existed in confined aquifers or was produced by melting of ground ice due to magmatic heating or climatic shift, or from a paleolake in Candor Chasma further west. 相似文献
585.
Stefano Zibetti Daniele Pierini Gabriel W. Pratt 《Monthly notices of the Royal Astronomical Society》2009,392(2):525-536
We study six groups and clusters of galaxies suggested in the literature to be 'fossil' systems (i.e. to have luminous diffuse X-ray emission and a magnitude gap of at least 2 mag R between the first and the second ranked member within half of the virial radius), each having good quality X-ray data and Sloan Digital Sky Survey (SDSS) spectroscopic or photometric coverage out to the virial radius. The poor cluster AWM 4 is clearly established as a fossil system, and we confirm the fossil nature of four other systems (RX J1331.5+1108, RX J1340.6+4018, RX J1256.0+2556 and RX J1416.4+2315), while the cluster RX J1552.2+2013 is disqualified as fossil system. For all systems, we present the luminosity functions within 0.5 and 1 virial radius that are consistent, within the uncertainties, with the universal luminosity function of clusters. For the five bona fide fossil systems, having a mass range 2 × 1013 –3 × 1014 M⊙ , we compute accurate cumulative substructure distribution functions (CSDFs) and compare them with the CSDFs of observed and simulated groups/clusters available in the literature. We demonstrate that the CSDFs of fossil systems are consistent with those of normal observed clusters and do not lack any substructure with respect to simulated galaxy systems in the cosmological Λ cold dark matter (ΛCDM) framework. In particular, this holds for the archetype fossil group RX J1340.6+4018 as well, contrary to earlier claims. 相似文献
586.
Evan S. Gawlik Jerrold E. Marsden Philip C. Du Toit Stefano Campagnola 《Celestial Mechanics and Dynamical Astronomy》2009,103(3):227-249
This study investigates Lagrangian coherent structures (LCS) in the planar elliptic restricted three-body problem (ER3BP),
a generalization of the circular restricted three-body problem (CR3BP) that asks for the motion of a test particle in the
presence of two elliptically orbiting point masses. Previous studies demonstrate that an understanding of transport phenomena
in the CR3BP, an autonomous dynamical system (when viewed in a rotating frame), can be obtained through analysis of the stable
and unstable manifolds of certain periodic solutions to the CR3BP equations of motion. These invariant manifolds form cylindrical
tubes within surfaces of constant energy that act as separatrices between orbits with qualitatively different behaviors. The
computation of LCS, a technique typically applied to fluid flows to identify transport barriers in the domains of time-dependent
velocity fields, provides a convenient means of determining the time-dependent analogues of these invariant manifolds for
the ER3BP, whose equations of motion contain an explicit dependency on the independent variable. As a direct application,
this study uncovers the contribution of the planet Mercury to the Interplanetary Transport Network, a network of tubes through
the solar system that can be exploited for the construction of low-fuel spacecraft mission trajectories.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
587.
588.
589.
The thermal behaviour of microsommite (MC), davyne from Vesuvius (DV) and from Zabargad (DZ) was determined by X-ray single crystal data obtained employing a microfurnace connected to a four-circle diffractometer. Upon heating, the a parameter increased linearly, with similar thermal expansion rates for the three samples: the mean linear expansion coefficients,
a
, were 10.2(3)·10-6, 13.4(7)·10-6, 15.1(8)·10-1 K-1 for MC, DV and DZ respectively.At about 473 K both MC and DZ showed a discontinuity in the expansion of the c parameter. The mean linear expansion coefficient,
c
, changed abruptly from 16(4)·10-6 K-1 for both minerals below the discontinuity to 2(1)·10-6 and 3(1)·10-6 K-1 for MC and DZ, respectively, above the discontinuity. In DV, however, the
c
coefficient was constant between 293 und 827 K and equal to 1(2)·10-6 K-1. 相似文献
590.
Prosper Andrianjakavah Stefano Salvi Didier Béziat Damien Guillaume Michel Rakotondrazafy Bernard Moine 《Mineralium Deposita》2007,42(4):385-398
The Maevatanana deposits consist of gold-bearing quartz–sulphide veins crosscutting banded iron formation (BIF) within a metamorphosed
2.5 Ga greenstone belt. The host rocks are dominated by a sequence of migmatites, gneisses, amphibolites, magnetite-rich quartzites
and soapstones, intruded by large granitoid batholiths (e.g. the 0.8 Ga Beanana granodiorite). In the mineralised rocks, pyrite
is the dominant sulphide, in addition to accessory chalcopyrite and galena. Outside the immediate ore zone, the BIF is dominated
by quartz + magnetite ± hematite, accompanied by cummingtonite, albite and biotite. Gold occurs as globular grains (usually
<500 μm) within quartz crystals close to the sulphides and as invisible inclusions within pyrite and chalcopyrite (up to 2,500 ppm
Au content). Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (∼3.6 wt.%
eq. NaCl) aqueous fluid coexisting with a carbonic fluid. Evidence for fluid-phase immiscibility during ore formation includes
variable L/V ratios in the inclusions and the fact that inclusions containing different phase proportions occur in the same
area, growth zone, or plane. Laser Raman spectroscopy confirms that the vapour phase in these inclusions is dominated by CO2 but shows that it may contain small amounts of CH4 (<1 mol%), H2S (<0.05 mol%) and traces of N2. Fluid inclusion trapping conditions ranged from 220 to 380°C and averaged 250°C. Pressure was on the order of 1–2 kbar.
The abundant CO2 and low salinity of the inclusions suggest a metamorphic origin for the fluid. Likewise, the presence of H2S in the fluid and pyritisation of the wall-rock indicate that gold was likely transported by sulphide complexing. Fluid immiscibility
was probably triggered by the pressure released by fracturing of the quartzites during fault movements due to competence differences
with the softer greenstones. Fracturing greatly enhanced fluid circulation through the BIF, allowing reaction of the sulphide-bearing
fluids with the iron oxides. This caused pyrite deposition and concomitant Au precipitation, enhanced by fluid phase separation
as H2S partitioned preferentially into the carbonic phase. 相似文献