全文获取类型
收费全文 | 3818篇 |
免费 | 3篇 |
国内免费 | 9篇 |
专业分类
航空 | 1713篇 |
航天技术 | 1407篇 |
综合类 | 10篇 |
航天 | 700篇 |
出版年
2021年 | 28篇 |
2018年 | 56篇 |
2017年 | 51篇 |
2016年 | 47篇 |
2014年 | 65篇 |
2013年 | 105篇 |
2012年 | 88篇 |
2011年 | 153篇 |
2010年 | 107篇 |
2009年 | 150篇 |
2008年 | 197篇 |
2007年 | 111篇 |
2006年 | 79篇 |
2005年 | 112篇 |
2004年 | 127篇 |
2003年 | 124篇 |
2002年 | 82篇 |
2001年 | 117篇 |
2000年 | 60篇 |
1999年 | 86篇 |
1998年 | 107篇 |
1997年 | 77篇 |
1996年 | 70篇 |
1995年 | 109篇 |
1994年 | 125篇 |
1993年 | 64篇 |
1992年 | 77篇 |
1991年 | 32篇 |
1990年 | 42篇 |
1989年 | 68篇 |
1988年 | 31篇 |
1987年 | 27篇 |
1986年 | 38篇 |
1985年 | 125篇 |
1984年 | 112篇 |
1983年 | 92篇 |
1982年 | 77篇 |
1981年 | 155篇 |
1980年 | 31篇 |
1979年 | 34篇 |
1978年 | 37篇 |
1977年 | 34篇 |
1976年 | 29篇 |
1975年 | 34篇 |
1974年 | 29篇 |
1973年 | 31篇 |
1972年 | 41篇 |
1971年 | 22篇 |
1970年 | 22篇 |
1969年 | 21篇 |
排序方式: 共有3830条查询结果,搜索用时 15 毫秒
81.
The equations of motion of a momentum biased spacecraft are derived in a general form. The spacecraft is assumed to be orbiting in a near-elliptical orbit. An aerodynamic torque model which accounts for the atmospheric superrotation is assumed. The equilibrium attitude angles are obtained in terms of modified Bessel functions. Analytic expressions for the long-term motion of the momentum biased axis are derived for special cases. The analysis is applicable to the Magsat mission. 相似文献
82.
Observations of NGC 5194/95 with the Einstein HRI show a very strong nuclear X-ray source, surrounded by a diffuse flux, three point sources and the companion. The diffuse flux, which correlates well with the radio continuum, is likely to originate from the disk population with age 2·109 yrs. The large luminosity from the nuclear source, together with optical and radio observations, shows that it belongs to the low luminosity active nuclei, thus extending this class to luminosities less than 1040 erg/s. 相似文献
83.
S. Mereghetti G. F. Bignami P. A. Caraveo A. Goldwurm G. G. C. Palumbo 《Space Science Reviews》1985,40(3-4):495-501
A new EXOSAT (LE/CMA) observation of the region in Crux (R.A. 11h 45m, Dec. -62°) where Markert et al. (1981) reported the existence of two x-ray SNR's is presented. After cleaning the CMA field from the point source component, due to the UV emission of the numerous stars in the field, the smoothed x-ray contours are compared to the 408 MHz radio map of Caswell et al. (1983). The existence of two, well-separate x-ray emission regions is confirmed by EXOSAT, and the current x-ray/radio picture is not sufficent to distinguish clearly between the assumption of one or two (possibly interacting) SNR's in the region. 相似文献
84.
In the theory of signal detectability, the signal-to-noise ratio (SNR), defined as the quotient of the average received signal energy and the spectral density of the white Gaussian noise, is a fundamental parameter. For a signal which is exactly known, or known except for a random phase, this ratio uniquely defines the detection performance which can be achieved with a matched filter receiver. However, when the signal amplitude is a random parameter, the detection performance is changed and must be determined from the probability density function (pdf) of the amplitude. Relative to the case of a constant signal amplitude, such signal amplitude fluctuation usually degrades performance when a high probability of detection (Pd) is required, but improves performance at low values of Pd; the corresponding change in the required SNR is the so-called signal fluctuation loss Lf. Thus, since Lf in some cases represents an improvement in performance for low values of Pd, a question of at least theoretical interest is: how large might this improvement be, when the class of all signal amplitude pdf's is considered. The solution, presented here, results in a lower bound on the signal fluctuation loss Lf as a function of Pd, or equivalently an upper bound on Pd as a function of SNR. The corresponding most favorable pdf was determined using the Lagrange multiplier technique and results of a numerical maximization are included to provide insight into the general properties of the solution. 相似文献
85.
Position and Velocity Estimation Via Bearing Observations 总被引:2,自引:0,他引:2
The problem of performing target motion analysis using noisy bearing measurements derived from multiple observation platforms or from a single moving observer is addressed. For the latter case, the properties of an estimator based on a Cartesian model of the process are detailed. Methods of providing estimates both before an observer maneuver, when the process is unobservable, and following an observer maneuver are developed. The results of an experimental study are presented. 相似文献
86.
Middlebtook R.D. Kimble S.G. 《IEEE transactions on aerospace and electronic systems》1976,(2):162-172
The High Energy Astronomy Observatory (HEAO) main bus regulator contains a redundant error amplifier (REA) and a 12-section sequenced shunt transconductance amplifier (STA) in which not more than three and not less than one shunt section are in an active control mode regardless of a failure in any one of the twelve sections. The part of the work concerned with the shaping of the frequency response of the REA and STA minor feedback loops and of the teolator major feedback loop is described. Emphasis is on the application of a general design technique embodying a "design-analyze-measure iteration loop," in which a first-cut design and analysis is corrected ard improved after experimental observation of discrepancies and deficiencies. In particular, an incipient high-frequency oscillation was thereby reliably eliminated. It is demonstrated that analysis techniques involving a minimum of algebra can provide full quantitative information on an efficient and reliable system design. 相似文献
87.
Expressions for moving target indicator (MTI) improvement factor limitation due to pulse repetition frequency (PRF) staggering and loss of target detectability for various values of Doppler frequency in the passband are presented. It is also shown that the product of variance of stagger periods and clutter variance is an important parameter determining the performance of a staggered PRF MTI radar. 相似文献
88.
Paulikas G.A. Blake J.B. Imamoto S.S. 《IEEE transactions on aerospace and electronic systems》1975,(6):1138-1144
The Aerospace Corporation energetic electron-proton spectrometer operating on Applications Technology Satellite-6 (ATS-6) detects energetic electrons in four channels between 140 keV and greater than 32 MeV, and measures energetic protons in five energy channels between 2.3 and 80 MeV and energetic alpha particles in three channels between 9.4 and 94 MeV. After more than a year of operation in orbit, the experiment continues to return excellent data on the behavior of energetic magnetospheric electrons as well as information regarding the fluxes of solar protons and alpha particles. 相似文献
89.
A number of previously unclassified multiplets of Fexiv, xiii, xii, and xi produced by transitions of the type 3s 23p n -3s3p n+1 are identified in the XUV spectrum of the Sun. The iron lines account for most of the previously unidentified strong lines between 330 and 370 Å. Solar observations of especial value for the investigation of the 300–400 Å region were the slitless spectroheliograms of September 22, 1968 (Purcell and Tousey, 1969) and November 4, 1969 (Tousey, 1971) — on which the image of a flare was recorded. Other solar identifications in the same spectral region include the resonance lines of Nixvii and Nixviii, and one 3p-3d multiplet of Fexiii. The solar blend at 417 Å involving the Fexv inter-combination line and Sxiv is resolved. 相似文献
90.