全文获取类型
收费全文 | 6846篇 |
免费 | 14篇 |
国内免费 | 12篇 |
专业分类
航空 | 3327篇 |
航天技术 | 2204篇 |
综合类 | 21篇 |
航天 | 1320篇 |
出版年
2021年 | 54篇 |
2019年 | 36篇 |
2018年 | 217篇 |
2017年 | 166篇 |
2016年 | 114篇 |
2015年 | 59篇 |
2014年 | 159篇 |
2013年 | 191篇 |
2012年 | 194篇 |
2011年 | 331篇 |
2010年 | 250篇 |
2009年 | 388篇 |
2008年 | 412篇 |
2007年 | 252篇 |
2006年 | 144篇 |
2005年 | 218篇 |
2004年 | 175篇 |
2003年 | 215篇 |
2002年 | 120篇 |
2001年 | 221篇 |
2000年 | 106篇 |
1999年 | 140篇 |
1998年 | 165篇 |
1997年 | 133篇 |
1996年 | 134篇 |
1995年 | 180篇 |
1994年 | 176篇 |
1993年 | 101篇 |
1992年 | 127篇 |
1991年 | 63篇 |
1990年 | 54篇 |
1989年 | 129篇 |
1988年 | 66篇 |
1987年 | 60篇 |
1986年 | 61篇 |
1985年 | 152篇 |
1984年 | 139篇 |
1983年 | 106篇 |
1982年 | 143篇 |
1981年 | 170篇 |
1980年 | 54篇 |
1979年 | 44篇 |
1978年 | 58篇 |
1977年 | 33篇 |
1976年 | 33篇 |
1975年 | 43篇 |
1974年 | 38篇 |
1973年 | 37篇 |
1972年 | 37篇 |
1971年 | 30篇 |
排序方式: 共有6872条查询结果,搜索用时 343 毫秒
381.
382.
The problem treated here is the analysis of a class of nonlinear sampled-data systems with a Gaussian input signal. The nonlinear element is a symmetrical limiter (memoryless device) that feeds the linear element. Preceding the limiter is a sampler and zero-order hold unit for data reconstruction. The analysis is predicated on finding an equivalent gain for the nonlinear element such that the mean-square error is a minimum. The nonlinearity is thus replaced by a linear component and the system is then analyzed by conventional techniques. It is also shown that for appropriate sampling rates the sampled-data system is equivalent to a continuous system due to the action of the sampler-hold combination. All calculations pertaining to the limiter input signal require the use of a transfer function that is interior to the feedback loop. However, in developing this transfer function, the overall system is considered so that the system output may be evaluated after the value of equivalent gain has been found. 相似文献
383.
384.
This paper analyzes a hybrid navigation concept that uses signals from a radio interferometer mounted on a spinning geostationary satellite, preliminary position estimates from self-contained equipment, and stored a priori information on the past performance of this equipment. The craft-borne processor, optimum in the maximum a posteriori (MAP) sense, is designed to estimate position coordinates using only the incoming radio signals, although improved estimates result if the other two items are available. An error analysis starts with the derivation of an estimation error covariance matrix, whose elements depend on additive receiver noise and the physical parameters of the system. A minimum 1? estimation error in position is obtained by trading off these parameters. The effects of other major error sources, such as tropospheric phase fluctuations, multipath, and craft altitude uncertainty, are added to the estimation error to give a total 1? position error on the order of 3.7 to 5.6 km. 相似文献
385.
Sarles F.W. Stanley A.G. Roberge J.K. Godfrey B.W. 《IEEE transactions on aerospace and electronic systems》1973,(6):921-924
For direct measurement of the integrated radiation dose experienced in Earth synchronous orbit, p-i-n diodes were flown as radiation dosimeters on LES-6. The diode, which has a lifetime of 10-4 seconds in the intrinsic region, was originally developed as a neutron dosimeter, but can detect 1-MeV electron fluences as low as 1013 e·cm-2. Observations over three years in orbit are presented. 相似文献
386.
Trunk G.V. Cantrell B.H. Queen F.D. 《IEEE transactions on aerospace and electronic systems》1974,(5):574-582
The modified generalized sign test processor is a nonparametric, adaptive detector for 2-D search radars. The detector ranks a sample under test with its neighboring samples and integrates (on a pulse-to-pulse basis) the ranks with a two-pole filter. A target is declared when the integrated output exceeds two thresholds. The first threshold is fixed and yields a 10-6 probability of false alarm when the neighboring samples are independent and identically distributed. The second threshold is adaptive and maintains a low false-alarm rate when the integrated neighboring samples are correlated and when there are nonhomogeneities, such as extraneous targets, in the neighboring cells. Using Monte Carlo techniques, probability of false-alarm results, probability of detection curves, and angular accuracy curves have been generated for this detector. The detector was built and PPI photographs are used to indicate the detector's performance when the radar is operated over land clutter. 相似文献
387.
G. Schubert C. C. Counselman III J. Hansen S. S. Limaye G. Pettengill A. Seiff I. I. Shapiro V. E. Suomi F. Taylor L. Travis R. Woo R. E. Young 《Space Science Reviews》1977,20(4):357-387
With the possible exception of the lowest one or two scale heights, the dominant mode of circulation of Venus' atmosphere is a rapid, zonal, retrograde motion. Global albedo variations in the ultraviolet may reflect planetary scale waves propagating relative to the zonal winds. Other special phenomena such as cellular convection in the subsolar region and internal gravity waves generated in the interaction of the zonal circulation with the subsolar disturbance may also be revealed in ultraviolet imagery of the atmosphere. We discuss the contributions of experiments on the Orbiter and Entry Probes of Pioneer Venus toward unravelling the mystery of the planet's global circulation and the role played by waves, instabilities and convection therein. 相似文献
388.
F. M. Ipavich G. Gloeckler C. Y. Fan L. A. Fisk D. Hovestadt B. Klecker J. J. O'Gallagher M. Scholer 《Space Science Reviews》1979,23(1):93-101
We report initial measurements from the ULECA sensor of the Max-Planck-Institut/University of Maryland experiment on ISEE-1. ULECA is an electrostatic deflection — total energy sensor consisting of a collimator, deflection analyzer and an array of solid state detectors. The position of a given detector, which determines the energy per charge of an incident particle, together with the measured energy determine the particle's charge state. We find that a rich variety of phenomena are operative in the transthermal energy regime (10 keV/Q to 100 keV/Q) covered by ULECA. Specifically, we present observations of locally accelerated protons, alpha particles, and heavier ions in the magnetosheath and upstream of the Earth's bow shock. Preliminary analysis indicates that the behavior of these locally accelerated particles is most similar at the same energy per charge. 相似文献
389.
A. G. Yahnin I. V. Despirak A. A. Lubchich B. V. Kozelov N. P. Dmitrieva M. A. Shukhtina H. K Biernat 《Space Science Reviews》2006,122(1-4):97-106
Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation
of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the
data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The
poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated
with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are
generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that
the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison
of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase
shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm
development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic
plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for
monitoring of substorm intensity in terms of the magnetic flux and energy dissipation. 相似文献
390.
Y. Futaana S. Barabash A. Grigoriev D. Winningham R. Frahm M. Yamauchi R. Lundin 《Space Science Reviews》2006,126(1-4):315-332
As a part of the global plasma environment study of Mars and its response to the solar wind, we have analyzed a peculiar case
of the subsolar energetic neutral atom (ENA) jet observed on June 7, 2004 by the Neutral Particle Detector (NPD) on board
the Mars Express satellite. The “subsolar ENA jet” is generated by the interaction between the solar wind and the Martian
exosphere, and is one of the most intense sources of ENA flux observed in the vicinity of Mars. On June 7, 2004 (orbit 485
of Mars Express), the NPD observed a very intense subsolar ENA jet, which then abruptly decreased within ∼10 sec followed
by quasi-periodic (∼1 min) flux variations. Simultaneously, the plasma sensors detected a solar wind structure, which was
most likely an interplanetary shock surface. The abrupt decrease of the ENA flux and the quasi-periodic flux variations can
be understood in the framework of the global response of the Martian plasma obstacle to the interplanetary shock. The generation
region of the subsolar ENA jet was pushed towards the planet by the interplanetary shock; and therefore, Mars Express went
out of the ENA jet region. Associated global vibrations of the Martian plasma obstacle may have been the cause of the quasi-periodic
flux variations of the ENA flux at the spacecraft location. 相似文献