全文获取类型
收费全文 | 6327篇 |
免费 | 48篇 |
国内免费 | 33篇 |
专业分类
航空 | 2825篇 |
航天技术 | 2099篇 |
综合类 | 88篇 |
航天 | 1396篇 |
出版年
2022年 | 28篇 |
2021年 | 65篇 |
2018年 | 197篇 |
2017年 | 167篇 |
2016年 | 129篇 |
2015年 | 71篇 |
2014年 | 163篇 |
2013年 | 210篇 |
2012年 | 181篇 |
2011年 | 289篇 |
2010年 | 217篇 |
2009年 | 305篇 |
2008年 | 357篇 |
2007年 | 222篇 |
2006年 | 164篇 |
2005年 | 202篇 |
2004年 | 154篇 |
2003年 | 195篇 |
2002年 | 153篇 |
2001年 | 205篇 |
2000年 | 120篇 |
1999年 | 142篇 |
1998年 | 153篇 |
1997年 | 127篇 |
1996年 | 122篇 |
1995年 | 162篇 |
1994年 | 159篇 |
1993年 | 92篇 |
1992年 | 115篇 |
1991年 | 48篇 |
1990年 | 58篇 |
1989年 | 100篇 |
1988年 | 45篇 |
1987年 | 44篇 |
1986年 | 46篇 |
1985年 | 158篇 |
1984年 | 128篇 |
1983年 | 113篇 |
1982年 | 92篇 |
1981年 | 169篇 |
1980年 | 46篇 |
1979年 | 42篇 |
1978年 | 42篇 |
1977年 | 38篇 |
1976年 | 34篇 |
1975年 | 50篇 |
1974年 | 29篇 |
1972年 | 45篇 |
1971年 | 34篇 |
1967年 | 27篇 |
排序方式: 共有6408条查询结果,搜索用时 15 毫秒
991.
M.J. Hagyard R.L. Moore A.G. Emslie 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(7):71-80
We present observational results and their physical implications garnered from the deliberations of the FBS Magnetic Shear Study Group on magnetic field shear in relation to flares. The observed character of magnetic shear and its involvement in the buildup and release of flare energy are reviewed and illustrated with emphasis on recent results from the Marshall Space Flight Center vector magnetograph. It is pointed out that the magnetic field in active regions can become sheared by several processes, including shear flow in the photosphere, flux emergence, magnetic reconnection, and flux submergence. Modeling studies of the buildup of stored magnetic energy by shearing are reported which show ample energy storage for flares. Observational evidence is presented that flares are triggered when the field shear reaches a critical degree, in qualitative agreement with some theoretical analyses of sheared force-free fields. Finally, a scenario is outlined for the class of flares resulting from large-scale magnetic shear; the overall instability driving the energy release results from positive feedback between reconnection and eruption of the sheared field. 相似文献
992.
Ramachandra K.V. Srinivasan V.S. 《IEEE transactions on aerospace and electronic systems》1977,(4):419-423
An X, Y, Z Kalman tracking filter is described and its steady state characteristics are analytically determined when the radar sensor meaures range, bearing, and elevation (?, ?, ?) at uniform intervals of time, T seconds. The relationship between the quantities measured by the sensor (?, ?,?) and the Cartesian coordinate system (X, Y, Z) is explicitly considered. 相似文献
993.
The application of importance sampling to the efficient simulation of low-probability events is discussed. Several examples are given to illustrate how the technique works for processes involving single and multiple statistical inputs. A procedure is also given for handling the case of multiple inputs where each is from a different statistical process. 相似文献
994.
M.G. Kivelson K.W. Behannon T.E. Cravens I. de Pater T.V. Johnson D.L. Matson H. Masursky D.J. Southwood V.M. Vasyliunas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(3):5-14
A was presented in conjunction with the Twenty-fourth Plenary Meeting of the Committee on Space Research. This paper summarizes the talks presented and places the remaining papers of this volume in context. 相似文献
995.
Phase-locked-loop (PLL) bit synchronizers often employ integrate-and-dump type phase detectors that provide phase error information only at discrete points in time. Usually these phase detectors are followed by sample-and-hold circuits to produce a stairstep error voltage as the input to a standard analog circuit loop filter. When the loop is configured in this manner, it is referred to as a hybrid PLL. Sampled-data analysis methods (Z transforms) are used to determine the stability and transient response of this loop. 相似文献
996.
R.L. Dowden 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(2):173-182
Non-linear amplification of a monochromatic VLF transmission in the magnetosphere produces an output fluctuating in amplitude and phase, and so is not monochromatic. This output can usually be resolved into two waves: one similar to the input, but stronger, and a higher frequency sideband or “embryo emission”. The established theory is outlined as far as it goes. 相似文献
997.
According to the informational theory of emotions, emotions in humans depend on the power of some need (motivation) and the estimation by the subject of the probability (possibility) of the need satisfaction (the goal achievement). Low probability of need satisfaction leads to negative emotions, actively minimized by the subject. Increased probability of satisfaction, as compared to earlier forecast, generates positive emotions, which the subject tries to maximize, i.e. to enhance, to prolong, to repeat. The informational theory of emotions encompasses their reflective function, the laws of their appearance, the regulatory significance of emotions, and their role in organization of behavior. The level of emotional stress influences the operator's performance. A decrease in the emotional tonus leads to drowsiness, lack of vigilance, missing of significant signals and to slower reactions. An extremely high stress level disorganizes the activity, complicates it with a trend toward incorrect actions and reactions to insignificant signals (false alarms). The neurophysiological mechanisms of the influence of emotions on perceptual activity and operator performance as well as the significance of individuality are discussed. 相似文献
998.
The use of generators by the military is examined. The factors determining generator size and weight for a given power rating and the generator scaling laws are discussed. Size and weight reduction is shown to be primarily a function of improvements in material properties and cooling techniques 相似文献
999.
In the above-titled paper (see ibid., vol.AES-23, p.568-82, July 1987) M.I. Dadi and J.R. Marks II studied the relative efficiencies of the Neyman-Pearson optimal detector with respect to the linear and sign detectors, for the detection of a constant signal in additive Laplace noise. By applying the central limit theorem, they derived expressions for three types of asymptotic relative efficiencies (AREs). However, as noted in the above paper, the Gaussian approximation to the sign detector fails to yield the correct asymptotic efficiency. The commenter derives the correct ARE of the optimal detector with respect to the sign detector for the Laplace noise 相似文献
1000.
Parts of a study conducted to examine state-of-the-art power systems applicable to future military spacecraft are summarized. The study focused on burst-mode megawatt-class CW power, such as might be applied to SDIO directed energy systems, but lower-power, continuous-duty subsystems were included in less detail. A set of simple mass and volume algorithms has been developed to approximate several prime systems, and these were incorporated into a Lotus 1-2-3 spreadsheet. Among the power subsystems included in that study were primary batteries, alkaline primary fuel cells, and combustion turbogenerators. These systems, which are the most likely candidates for mobile battlefield power, are described 相似文献