全文获取类型
收费全文 | 4428篇 |
免费 | 23篇 |
国内免费 | 7篇 |
专业分类
航空 | 2336篇 |
航天技术 | 1195篇 |
综合类 | 12篇 |
航天 | 915篇 |
出版年
2021年 | 30篇 |
2019年 | 27篇 |
2018年 | 179篇 |
2017年 | 154篇 |
2016年 | 77篇 |
2015年 | 45篇 |
2014年 | 68篇 |
2013年 | 85篇 |
2012年 | 118篇 |
2011年 | 235篇 |
2010年 | 205篇 |
2009年 | 252篇 |
2008年 | 265篇 |
2007年 | 215篇 |
2006年 | 78篇 |
2005年 | 141篇 |
2004年 | 96篇 |
2003年 | 113篇 |
2002年 | 70篇 |
2001年 | 125篇 |
2000年 | 73篇 |
1999年 | 83篇 |
1998年 | 106篇 |
1997年 | 70篇 |
1996年 | 95篇 |
1995年 | 131篇 |
1994年 | 102篇 |
1993年 | 67篇 |
1992年 | 91篇 |
1991年 | 34篇 |
1990年 | 39篇 |
1989年 | 86篇 |
1988年 | 43篇 |
1987年 | 37篇 |
1986年 | 35篇 |
1985年 | 96篇 |
1984年 | 92篇 |
1983年 | 82篇 |
1982年 | 82篇 |
1981年 | 90篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 29篇 |
1977年 | 27篇 |
1976年 | 19篇 |
1975年 | 35篇 |
1974年 | 25篇 |
1973年 | 18篇 |
1972年 | 33篇 |
1971年 | 18篇 |
排序方式: 共有4458条查询结果,搜索用时 390 毫秒
741.
Rajasekaran P.K. Srinath M.D. 《IEEE transactions on aerospace and electronic systems》1969,(6):984-988
The problem of minimum time frequency transitions in phase-locked loops with both phase and frequency controls applied is investigated using Pontryagin's maximum principle. Typically, a type II secondorder loop with a damping ratio of 0.707 subjected to a step change in the frequency of its input signal is considered and switchless control strategies that force the transients in the loop to settle down in minimum time are obtained. 相似文献
742.
The current status of the theoretical methods for producing the relevant atomic data is surveyed.Proceedings of the Conference Solar Physics from Space, held at at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980. 相似文献
743.
A direct iterative method of solving for Tandem equilibria by moving magnetic field lines in a manner to satisfy the linearized equilibrium equations converges much more rapidly than standard relaxation techniques, typically in under a fifty iterations. At the highest 's the number of iterations increase, but is still far less than other methods. In quadrupole tandem mirror equilibrium, octupole and higher distortions of the flux surfaces are important which forces us to abandon finite differences in the angle-like flux variable and resort to a spectral decomposition to solve the equilibrium equations. We display equilibria at the high expected for MFTF-B and show how Finite Larmor Radius (FLR) effects strongly suppress these azimuthal distortions. 相似文献
744.
B. B. Kreisman 《Cosmic Research》2005,43(2):84-106
A monodromy matrix calculated at a single arbitrary point of the periodic solution to a Hamiltonian system allows one to obtain both the direction of continuation for the family of solutions of the first (in Poincarés sense) kind and the multiplicity and direction of branching for periodic solutions of the second kind. In case of resonances 1 : 1 and 1 : 2 one needs to take into account the structure of elementary divisors of the monodromy matrix. Using the planar circular restricted three-body problem as an example, the infiniteness of the process of branching for a nonintegrable system and its finiteness for an integrable system are demonstrated. It is proved that periodic solutions of both first and second kinds which are obtained by continuation of symmetric periodic solutions of a restricted problem are also symmetric. The only exception is the case of resonance 1 : 1 and two second-order cells of the monodromy matrix in the Jordanian form. In this case, all periodic solutions of the second kind turned out to be nonsymmetrical. Examples of the families of nonsymmetrical periodic solutions are given.__________Translated from Kosmicheskie Issledovaniya, Vol. 43, No. 2, 2005, pp. 88–110.Original Russian Text Copyright © 2005 by Kreisman. 相似文献
745.
A. V. Bogomolov Yu. I. Denisov G. Ya. Kolesov M. I. Kudryavtsev Yu. I. Logachev O. V. Morozov S. I. Svertilov 《Cosmic Research》2005,43(5):307-313
We study the characteristics of fluxes of electrons with energy >80 keV in the near-Earth space regions corresponding to the drift shells L = 1.7, 1.4, and 1.1 observed during the entire period of the GRIF experiment onboard the Spectr module of the Mir orbital station from October 1995 to June 1997. The obtained geographic maps of the distribution of electron fluxes at the height of the station flight (400 km) and, also, the estimates of the spectra indicate that the South-Atlantic Anomaly provides for a mechanism of stable replenishment for shells with L < 1.5. The mechanism of stable replenishment of shells with L < 1.5 may be due to the scattering, in the residual atmosphere, of electrons from the inner radiation belt precipitating into the region of the South-Atlantic Anomaly. 相似文献
746.
The influence of zonal and meridional components of the neutral wind under anthropogenic impacts of water molecules on the ionospheres electron concentration distributed along a magnetic field tube is considered. Computer simulation method based on a mathematical model of the F2 region along a field tube is used.Translated from Kosmicheskie Issledovaniya, Vol. 43, No. 1, 2005, pp. 76–80.Original Russian Text Copyright © 2005 by Medvedev, Ishanov, Zenkin. 相似文献
747.
Jaekl P Zikovitz DC Jenkin MR Jenkin HL Zacher JE Harris LR 《Acta Astronautica》2005,56(9-12):1033-1040
We measured the amount of visual movement judged consistent with translational head movement under normal and microgravity conditions. Subjects wore a virtual reality helmet in which the ratio of the movement of the world to the movement of the head (visual gain) was variable. Using the method of adjustment under normal gravity 10 subjects adjusted the visual gain until the visual world appeared stable during head movements that were either parallel or orthogonal to gravity. Using the method of constant stimuli under normal gravity, seven subjects moved their heads and judged whether the virtual world appeared to move “with” or “against” their movement for several visual gains. One subject repeated the constant stimuli judgements in microgravity during parabolic flight. The accuracy of judgements appeared unaffected by the direction or absence of gravity. Only the variability appeared affected by the absence of gravity. These results are discussed in relation to discomfort during head movements in microgravity. 相似文献
748.
749.
This paper reports on the objectives of the German space processing programme with sounding rockets and the scientific results obtained thus far. 相似文献
750.
D.C. Hamilton 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(3):25-40
Observations of solar cosmic ray events far from the sun (?1 AU) became possible after the launch of Pioneer 10 in 1972. Four spacecraft have now travelled beyond the orbit of Jupiter - Pioneer 10/11 and Voyager 1/2 — and are producing a growing body of distant observations of solar cosmic ray events. Initial studies using Pioneer 10/11 data out to ~6 AU interpreted flare particle observations in terms of a diffusion model, including the effects of convection and adiabatic energy loss. This model enjoyed general success in explaining the time-intensity profiles in cases where the spacecraft connection longitude at the sun did not change significantly with time. The results implied that the radial diffusion coefficient (Kr) increased slowly with distance over that radial range. More recent results at larger distances imply that Kr may begin to decrease beyond ~5 AU. It is not yet clear whether the standard diffusion model will be adequate to explain solar events well beyond 5 AU. The fact that large events at very large distances can last up to two solar rotations implies that solar wind stream structure will also play a role in the event dynamics. In general, however, observations at large distances offer perhaps the best hope of separating interplanetary propagation effects from coronal storage and propagation effects which frequently dominate observed event profiles at 1 AU. 相似文献