全文获取类型
收费全文 | 3199篇 |
免费 | 10篇 |
国内免费 | 5篇 |
专业分类
航空 | 1192篇 |
航天技术 | 1041篇 |
综合类 | 19篇 |
航天 | 962篇 |
出版年
2022年 | 15篇 |
2021年 | 37篇 |
2019年 | 16篇 |
2018年 | 128篇 |
2017年 | 102篇 |
2016年 | 116篇 |
2015年 | 41篇 |
2014年 | 111篇 |
2013年 | 135篇 |
2012年 | 118篇 |
2011年 | 158篇 |
2010年 | 127篇 |
2009年 | 192篇 |
2008年 | 190篇 |
2007年 | 122篇 |
2006年 | 74篇 |
2005年 | 101篇 |
2004年 | 94篇 |
2003年 | 106篇 |
2002年 | 83篇 |
2001年 | 113篇 |
2000年 | 36篇 |
1999年 | 53篇 |
1998年 | 61篇 |
1997年 | 42篇 |
1996年 | 43篇 |
1995年 | 80篇 |
1994年 | 61篇 |
1993年 | 34篇 |
1992年 | 45篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 41篇 |
1988年 | 11篇 |
1987年 | 17篇 |
1986年 | 16篇 |
1985年 | 70篇 |
1984年 | 56篇 |
1983年 | 48篇 |
1982年 | 42篇 |
1981年 | 82篇 |
1980年 | 25篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 17篇 |
1976年 | 13篇 |
1975年 | 13篇 |
1974年 | 11篇 |
1972年 | 13篇 |
1971年 | 11篇 |
排序方式: 共有3214条查询结果,搜索用时 15 毫秒
941.
Window-Based and Rate-Based Transmission Control Mechanisms over Space-Internet Links 总被引:1,自引:0,他引:1
Ruhai Wang Gutha B.I. Rapet P.V. 《IEEE transactions on aerospace and electronic systems》2008,44(1):157-170
Space communications urgently need an effective transmission control mechanism. This paper presents an experimental, comparative analysis of window-based transmission control, rate-based transmission control, and a hybrid of the two over error-prone, congestion-free, high-latency, point-to-point space communication links simulated using the space-to-ground link simulation (SGLS) test-bed. The results revealed that the traffic shaping mechanism of rate-based transmission protocol is more effective than the bursting flow of window-based protocol over simulated space communication links with a high error rate and a long link delay. The window-based transmission mechanisms show performance degradation due to traffic bursts and frequent packet retransmissions caused by their acknowledgment (ACK)-clocked transmission control algorithms. Pure rate-control is always preferable to other mechanisms in the simulated congestion-free, error-prone, point-to-point, geostationary-Earth orbit (GEO)-space communication channels, and its advantages become more pronounced when the channel rates are asymmetric. The performance differences come from their different behavior in controlling data transmission. 相似文献
942.
K. Kudela M. Minarovjech V. Ruin M. Rybanský V. Kollr 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,36(12):2368-2371
The cross correlation of daily values of coronal hole areas at the eastern limb of the Sun constructed from the ground based measurements of the green coronal line and daily mean cosmic ray intensities over long time periods shows asymmetry: at the maximum of their 27 day cycle, cosmic ray intensities are better correlated with coronal hole areas 66 days before than with the current value. This indicates the potential for using coronal emission data as one of the parameters for eventual prediction of the level of cosmic ray flux at neutron monitor energies. 相似文献
943.
V.A. Krasnopolsky 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1987,7(12):25-27
S3 absorption cross section equals 6×10−17 cm2 at 400 nm, 6 × 10−19 cm2 at 500 nm (less by a factor of 4 than that given by Sanko), 4×10−20 cm2 at 600 nm. That of S4 equals 1.5 × 10−17 cm2 at 450 nm, 8 × 10−17 cm2 at 500 nm, and 4.7 × 10−17 cm2 at 600 nm. Preliminary evaluation of the S3 mixing ratio in the lower atmosphere of Venus is (8±3)×10−11 at 5 to 25km according to the Venera 14 measurements and several times lower at the locations of the Veneras-11 and -13. 相似文献
944.
R. Hanel B. Conrath D. Gautier P. Gierasch S. Kumar V. Kunde P. Lowman W. Maguire J. Pearl J. Pirraglia C. Ponnamperuma R. Samuelson 《Space Science Reviews》1977,21(2):129-157
The infrared investigation on Voyager uses two interferometers covering the spectral ranges 60–600 cm–1 (17–170 m) and 1000–7000 cm–1 (1.4–10 m), and a radiometer covering the range 8000–25 000 cm–1 (0.4–1.2 m). Two spectral resolutions (approximately 6.5 and 2.0 cm–1) are available for each of the interferometers. In the middle of the thermal channel (far infrared interferometer) the noise level is equivalent to the signal from a target at 50 K; in the middle of the reflected sunlight channel (near infrared interferometer) the noise level is equivalent to the signal from an object of albedo 0.2 at the distance of Uranus.For planets and satellites with substantial atmospheres, the data will be used to investigate cloud and gas composition (including isotopic ratios), haze scale height, atmospheric vertical thermal structure, local and planetary circulation and dynamics, and planetary energy balance. For satellites with tenuous atmospheres, data will be gathered on surface and atmospheric composition, surface temperature and thermal properties, local and global phase functions, and surface structure. For Saturn's rings, the composition and radial structure, particle size and thermal characteristics will be investigated. Comparative studies of the planets and their satellite systems will be carried out.Paris Observatory.Cornell University.Jet Propulsion Laboratory.University of Maryland. 相似文献
945.
Recently the galactic plane has been observed in the low and medium energy gamma-ray range in the directions towards the center and anticenter. Spectral measurements are now available at those energies, where the contribution from π°-decay gamma rays can be neglected. The high MeV-fluxes observed in both parts of the Galaxy are an indication of either a strong electron induced component or a high contribution from unresolved sources. Several interstellar cosmic-ray electron spectra have been used to calculate the contribution from electron bremsstrahlung and inverse Compton collisions with optical, infrared and 2.7 K black-body photons. From these calculations restrictions on the interstellar electron spectrum are derived. 相似文献
946.
The design of a suboptimal terminal guidance system for reentry vehicles with a constraint on the body attitude angle at impact is studied. Permissible range of the miss distance and the body attitude angle at impact is specified. The problem is formulated as a linear quadratic control problem. The Riccati equation is derived to provide time-varying feedback gains. The desired scheme is suboptimal. The region of initial states for which the system meets the specifications becomes smaller as the initial height of the reentry vehicle at initial time is decreased. 相似文献
947.
R. F. Wimmer-Schweingruber N. U. Crooker A. Balogh V. Bothmer R. J. Forsyth P. Gazis J. T. Gosling T. Horbury A. Kilchenmann I. G. Richardson J. D. Richardson P. Riley L. Rodriguez R. von Steiger P. Wurz T. H. Zurbuchen 《Space Science Reviews》2006,123(1-3):177-216
While interplanetary coronal mass ejections (ICMEs) are understood to be the heliospheric counterparts of CMEs, with signatures
undeniably linked to the CME process, the variability of these signatures and questions about mapping to observed CME features
raise issues that remain on the cutting edge of ICME research. These issues are discussed in the context of traditional understanding,
and recent results using innovative analysis techniques are reviewed. 相似文献
948.
During the last several years significant progress has been made in understanding MHD turbulence in the Earth’s plasma sheet.
Due to the statistically transitory properties of fluctuations, finite size and boundary effects, however, issues of fundamental
importance remain unresolved. Here we concentrate on such intrinsic features of plasma sheet turbulence as its origin and
dynamical nature. In particular, we investigate bursty bulk flow driven multi-scale transfer of energy towards the dissipation
scale, and provide evidence for the presence of non-linear interactions. We show that, in contrast with previous results,
Alfvénic fluctuations together with 2D eddy interactions may appear as important constituents of turbulence in the plasma
sheet. 相似文献
949.
J. H. Waite Jr. W. S. Lewis W. T. Kasprzak V. G. Anicich B. P. Block T. E. Cravens G. G. Fletcher W.-H. Ip J. G. Luhmann R. L. Mcnutt H. B. Niemann J. K. Parejko J. E. Richards R. L. Thorpe E. M. Walter R. V. Yelle 《Space Science Reviews》2004,114(1-4):113-231
The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation will determine the mass composition and number densities of neutral species and low-energy ions in key regions of the Saturn system. The primary focus of the INMS investigation is on the composition and structure of Titan’s upper atmosphere and its interaction with Saturn’s magnetospheric plasma. Of particular interest is the high-altitude region, between 900 and 1000 km, where the methane and nitrogen photochemistry is initiated that leads to the creation of complex hydrocarbons and nitriles that may eventually precipitate onto the moon’s surface to form hydrocarbon–nitrile lakes or oceans. The investigation is also focused on the neutral and plasma environments of Saturn’s ring system and icy moons and on the identification of positive ions and neutral species in Saturn’s inner magnetosphere. Measurement of material sputtered from the satellites and the rings by magnetospheric charged particle and micrometeorite bombardment is expected to provide information about the formation of the giant neutral cloud of water molecules and water products that surrounds Saturn out to a distance of ∼12 planetary radii and about the genesis and evolution of the rings.The INMS instrument consists of a closed ion source and an open ion source, various focusing lenses, an electrostatic quadrupole switching lens, a radio frequency quadrupole mass analyzer, two secondary electron multiplier detectors, and the associated supporting electronics and power supply systems. The INMS will be operated in three different modes: a closed source neutral mode, for the measurement of non-reactive neutrals such as N2 and CH4; an open source neutral mode, for reactive neutrals such as atomic nitrogen; and an open source ion mode, for positive ions with energies less than 100 eV. Instrument sensitivity is greatest in the first mode, because the ram pressure of the inflowing gas can be used to enhance the density of the sampled non-reactive neutrals in the closed source antechamber. In this mode, neutral species with concentrations on the order of ≥104 cm−3 will be detected (compared with ≥105 cm−3 in the open source neutral mode). For ions the detection threshold is on the order of 10−2 cm−3 at Titan relative velocity (6 km sec−1). The INMS instrument has a mass range of 1–99 Daltons and a mass resolutionM/ΔM of 100 at 10% of the mass peak height, which will allow detection of heavier hydrocarbon species and of possible cyclic hydrocarbons such as C6H6.The INMS instrument was built by a team of engineers and scientists working at NASA’s Goddard Space Flight Center (Planetary Atmospheres Laboratory) and the University of Michigan (Space Physics Research Laboratory). INMS development and fabrication were directed by Dr. Hasso B. Niemann (Goddard Space Flight Center). The instrument is operated by a Science Team, which is also responsible for data analysis and distribution. The INMS Science Team is led by Dr. J. Hunter Waite, Jr. (University of Michigan).This revised version was published online in July 2005 with a corrected cover date. 相似文献
950.
Lembege B. Giacalone J. Scholer M. Hada T. Hoshino M. Krasnoselskikh V. Kucharek H. Savoini P. Terasawa T. 《Space Science Reviews》2004,110(3-4):161-226
The physics of collisionless shocks is a very broad topic, which has been well studied for many decades. However, there are a number of important issues which remain unresolved. Moreover, there have been new findings, which cast doubt on well-established ideas. The purpose of this review is to address a subset of unresolved problems in collisionless shock physics from a theoretical and/or numerical modeling point of view. The topics which are addressed are: the nonstationarity of the shock front, the heating and dynamics of electrons through the shock layer, particle diffusion in turbulent electric and magnetic fields, particle acceleration, and the interaction of pickup ions with collisionless shocks. 相似文献