全文获取类型
收费全文 | 5985篇 |
免费 | 14篇 |
国内免费 | 16篇 |
专业分类
航空 | 2628篇 |
航天技术 | 1979篇 |
综合类 | 32篇 |
航天 | 1376篇 |
出版年
2021年 | 61篇 |
2019年 | 44篇 |
2018年 | 229篇 |
2017年 | 165篇 |
2016年 | 145篇 |
2015年 | 59篇 |
2014年 | 160篇 |
2013年 | 195篇 |
2012年 | 191篇 |
2011年 | 287篇 |
2010年 | 240篇 |
2009年 | 331篇 |
2008年 | 374篇 |
2007年 | 251篇 |
2006年 | 128篇 |
2005年 | 181篇 |
2004年 | 156篇 |
2003年 | 186篇 |
2002年 | 132篇 |
2001年 | 191篇 |
2000年 | 84篇 |
1999年 | 106篇 |
1998年 | 129篇 |
1997年 | 87篇 |
1996年 | 97篇 |
1995年 | 151篇 |
1994年 | 139篇 |
1993年 | 80篇 |
1992年 | 111篇 |
1991年 | 44篇 |
1990年 | 48篇 |
1989年 | 101篇 |
1988年 | 35篇 |
1987年 | 42篇 |
1986年 | 40篇 |
1985年 | 148篇 |
1984年 | 114篇 |
1983年 | 98篇 |
1982年 | 112篇 |
1981年 | 152篇 |
1980年 | 49篇 |
1979年 | 49篇 |
1978年 | 36篇 |
1977年 | 33篇 |
1976年 | 24篇 |
1975年 | 23篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1972年 | 26篇 |
1971年 | 21篇 |
排序方式: 共有6015条查询结果,搜索用时 656 毫秒
181.
Krishna M. V. Apparao 《Space Science Reviews》1994,69(3-4):255-329
Observations of X-ray emission from Be star/X-ray binaries are reviewed. Some optical characteristics of these binaries are also presented. Theoretical aspects pertaining to the X-ray emissions are given. 相似文献
182.
Parameters of expanding magnetic loops and arches and of mass flows generated by them in the corona have been computed in a 1D two-fluid approximation. Two possible trigger mechanisms of the coronal transients have been considered: (i) sudden increase of the background magnetic field strength, and (ii) heating and compression plasma inside these magnetic structures. We discuss the formation of shock waves and their dependence on dynamics and geometry of the magnetic structures. 相似文献
183.
K. Schindler 《Space Science Reviews》1979,23(3):365-374
The present theories of tail structures are reviewed. It is shown that any satisfactory model for the quiet tail has to include at least two space dimensions. Several approaches to two-dimensional structures and some recent results of a study that includes the third dimension are reviewed. It is demonstrated that the observed gross structural properties of the quiet magnetotail can be reasonably well understood from these theories. A brief discussion of selected aspects of tail dynamics is added.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978. 相似文献
184.
K. G. Ivanov 《Space Science Reviews》1982,32(1-2):49-63
The configuration, and the structure of shock waves from isolated solar flares and from a series of flares are discussed and classified.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia. 相似文献
185.
Simplified analysis of PWM converters using model of PWM switch.II. Discontinuous conduction mode 总被引:1,自引:0,他引:1
For pt.I see ibid., vol.26, no.3, p.490-6 (1990). According to the method of state-space averaging, when a pulsewidth modulation (PWM) converter enters discontinuous conduction mode (DCM), the inductor current state is lost from the average model of the converter. It is shown that there is neither theoretical nor experimental justification for the disappearance of the inductor state as claimed by the method of state-space averaging. For example, when the model of the PWM switch in DCM is substituted in the buck, boost, or buck-boost converter while the inductor is left intact, the average model has two poles: the first pole f p1 agrees with the single pole of state-space averaging, while the second pole f p2 occurs in the range f p2⩾F s/π. It is shown that the right-half plane zeros present in the control-to-output transfer functions of the boost, buck-boost, and Cuk converters in continuous conduction mode are also present in discontinuous conduction mode 相似文献
186.
Trunk G.V. Cantrell B.H. Gordon W.B. 《IEEE transactions on aerospace and electronic systems》1979,(6):883-887
The Cramer-Rao bound for an unbiased estimate of the elevation angle of a target in the presence of multipath is calculated for the symmetric (target and image symmetric about the elevation symmetry plane of antenna) and nonsymmetric cases for an antenna consisting of 21 elements. These bounds are compared to the maximum likelihood estimates and it is found that the rms error of the maximum likelihood estimate (which has a bias) is below the Cramer-Rao bound for unbiased estimates. 相似文献
187.
188.
Trunk G.V. Gordon W.B. Cantrell B.H. 《IEEE transactions on aerospace and electronic systems》1990,26(1):146-153
A technique which uses maximum-likelihood estimates (MLEs) of target Doppler and target amplitude is developed for rejecting clutter residues. Multiple estimates are made and consistency checks are applied to the estimates. Simulation results indicate that for large clutter-to-noise ratios (C /N ⩾55 dB) the probability of false alarm from clutter residues is reduced from 1.0 to below 0.01 相似文献
189.
J. D. Anderson J. W. Armstrong J. K. Campbell F. B. Estabrook T. P. Krisher E. L. Lau 《Space Science Reviews》1992,60(1-4):591-610
The gravitation and celestial mechanics investigations during the cruise phase and Orbiter phase of the Galileo mission depend on Doppler and ranging measurements generated by the Deep Space Network (DSN) at its three spacecraft tracking sites in California, Australia, and Spain. Other investigations which also rely on DSN data, and which like ours fall under the general discipline of spacecraft radio science, are described in a companion paper by Howard et al. (1992). We group our investigations into four broad categories as follows: (1) the determination of the gravity fields of Jupiter and its four major satellites during the orbital tour, (2) a search for gravitational radiation as evidenced by perturbations to the coherent Doppler link between the spacecraft and Earth, (3) the mathematical modeling, and by implication tests, of general relativistic effects on the Doppler and ranging data during both cruise and orbiter phases, and (4) an improvement in the ephemeris of Jupiter by means of spacecraft ranging during the Orbiter phase. The gravity fields are accessible because of their effects on the spacecraft motion, determined primarily from the Doppler data. For the Galilean satellites we will determine second degree and order gravity harmonics that will yield new information on the central condensation and likely composition of material within these giant satellites (Hubbard and Anderson, 1978). The search for gravitational radiation is being conducted in cruise for periods of 40 days centered around solar opposition. During these times the radio link is least affected by scintillations introduced by solar plasma. Our sensitivity to the amplitude of sinusoidal signals approaches 10-15 in a band of gravitational frequencies between 10-4 and 10-3 Hz, by far the best sensitivity obtained in this band to date. In addition to the primary objectives of our investigations, we discuss two secondary objectives: the determination of a range fix on Venus during the flyby on 10 February, 1990, and the determination of the Earth's mass (GM) from the two Earth gravity assists, EGA1 in December 1990 and EGA2 in December 1992. 相似文献
190.
A Fast Beamforming Algorithm for Large Arrays 总被引:3,自引:0,他引:3
This beamforming algorithm is written specifically for array radars in which the number of array elements K is very large compared with the number of jammers L the radar is designed to suppress. It uses a set of M noise vectors to construct a basis for the jammer component of the antenna output vectors. The component of the quiescent weight vector orthogonal to each basis vector is calculated, renormalized to unit length, and identified as the adapted weight vector. This algorithm is effective in the suppression of many types of jammers. The number of noise samples M required in the construction of the adapted weight vector is approximately equal to L. In the special case of L narrowband noise jammers, for example, a choice of M = L usually reduces the receiver output jammer power to a few dBs above the white noise background. It is permissible to have M相似文献