全文获取类型
收费全文 | 5954篇 |
免费 | 9篇 |
国内免费 | 16篇 |
专业分类
航空 | 2706篇 |
航天技术 | 2274篇 |
综合类 | 24篇 |
航天 | 975篇 |
出版年
2021年 | 45篇 |
2019年 | 41篇 |
2018年 | 101篇 |
2017年 | 65篇 |
2016年 | 67篇 |
2014年 | 114篇 |
2013年 | 158篇 |
2012年 | 140篇 |
2011年 | 220篇 |
2010年 | 152篇 |
2009年 | 244篇 |
2008年 | 326篇 |
2007年 | 166篇 |
2006年 | 137篇 |
2005年 | 168篇 |
2004年 | 167篇 |
2003年 | 195篇 |
2002年 | 120篇 |
2001年 | 181篇 |
2000年 | 94篇 |
1999年 | 138篇 |
1998年 | 165篇 |
1997年 | 120篇 |
1996年 | 121篇 |
1995年 | 171篇 |
1994年 | 199篇 |
1993年 | 103篇 |
1992年 | 130篇 |
1991年 | 64篇 |
1990年 | 69篇 |
1989年 | 127篇 |
1988年 | 49篇 |
1987年 | 52篇 |
1986年 | 60篇 |
1985年 | 205篇 |
1984年 | 164篇 |
1983年 | 142篇 |
1982年 | 139篇 |
1981年 | 215篇 |
1980年 | 58篇 |
1979年 | 68篇 |
1978年 | 58篇 |
1977年 | 49篇 |
1976年 | 38篇 |
1975年 | 45篇 |
1974年 | 40篇 |
1973年 | 44篇 |
1972年 | 51篇 |
1971年 | 32篇 |
1970年 | 32篇 |
排序方式: 共有5979条查询结果,搜索用时 359 毫秒
911.
J.C. Henoux G. Chambe M. Semel B. Woodgate R. Shine J. Beckers 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(11):155-158
Linear polarization was observed in the S I 1437Å line in bright flaring points during the soft X-ray emission. The degree of polarization is about 25% and is detected at a signal to noise ratio of 2.9. The polarized electric vector is directed towards disk center to within 3°.This polarization could be due to collisional excitationm of S I by energetic electrons beamed in the vertical direction. We suggest that the heat flux in the region interconnecting the transition zone to the high chromosphere during the gradual phase of a flare could lead to an anisotropic excitation. Then the observed polarization would be due to vertical motions of the transition zone sweeping the preexisting chromosphere. 相似文献
912.
Comments on "Optimum Bandwidth of a Low-Pass Filter for Detection of a Pulse in Nonstationary Noise"
In a recent correspondence1 a calculation of the optimum bandwidth of a low-pass RC filter for the detection of a pulse signal in nonstationary noise was presented. The purpose of this correspondence is: 1) to point out additional references to the work which has been conducted in the stationary noise case, and 2) to present an interesting alternate derivation of the expected output noise power for the nonstationary noise case. 相似文献
913.
G.E. Thomas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(12):127-130
It is often observed that the stratospheric and mesospheric temperature structure undergoes transient disturbances from its averaged steady-state behavior. The causes may be traceable to gravity waves, planetary waves, solar proton and relativistic electron precipitation, etc. We examine the theoretical time behavior of the atmospheric temperature following the cessation of such heating phenomena as it relaxes toward its quiescent steady-state value. We also study the time-dependent response during a model stratospheric-warming/mesospheric-cooling event. In particular, we investigate the roles of eddy heat conduction, non-LTE cooling in the 15μm CO2 band, and an ambient vertical wind, and their relative importance as they depend upon altitude and time, in modifying temperature changes in this region. 相似文献
914.
The results of a survey of data on failures of aircraft electronic and electrical components that was conducted to identify problematic components are reported. The motivation for the work was to determine priorities for future work on the development of accident investigation techniques for aircraft electrical components. The primary source of data was the Airforce Mishap Database, which is maintained by the Directorate of Aerospace Safety at Norton Air Force Base. Published data from the Air Force Avionics Integrity Program (AVIP) and Hughes Aircraft were also reviewed. Statistical data from these three sources are presented. Two major conclusions are that problems with interconnections are major contributors to aircraft electrical equipment failures, and that environmental factors, especially corrosion, are significant contributors to connector problems 相似文献
915.
The issues associated with processing the outputs of an integrating acoustooptical spectrum analyzer (AOSA) to detect and identify the frequency and power of continuous wave (CW) signals are considered. The performance of the optimum detection strategy is presented, along with the Cramer-Rao bound on the performance of estimators of the frequency, and the performance of a simple peak-based frequency estimator. The performance of the maximum likelihood (ML) estimator of the power level is also presented. The results provide the behavior of the system as a function of the product of the aperture time and photodetector spacing amongst other parameters 相似文献
916.
917.
M. K. Tleulinov 《Russian Aeronautics (Iz VUZ)》2009,52(2):158-164
A method of direct integration with respect to time is proposed to solve dynamic problems of structural mechanics and comparison with the well-known methods is presented. 相似文献
918.
Radar Imaging of Mercury 总被引:1,自引:0,他引:1
John K. Harmon 《Space Science Reviews》2007,132(2-4):307-349
Earth-based radar has been one of the few, and one of the most important, sources of new information about Mercury during
the three decades since the Mariner 10 encounters. The emphasis during the past 15 years has been on full-disk, dual-polarization
imaging of the planet, an effort that has been facilitated by the development of novel radar techniques and by improvements
in radar systems. Probably the most important result of the imaging work has been the discovery and mapping of radar-bright
features at the poles. The radar scattering properties of these features, and their confinement to permanently shaded crater
floors, is consistent with volume backscatter from a low-loss volatile such as clean water ice. Questions remain, however,
regarding the source and long-term stability of the putative ice, which underscores the need for independent confirmation
by other observational methods. Radar images of the non-polar regions have also revealed a plethora of bright features, most
of which are associated with fresh craters and their ejecta. Several very large impact features, with rays and other bright
ejecta spreading over distances of 1,000 km or more, have been traced to source craters with diameters of 80–125 km. Among
these large rayed features are some whose relative faintness suggests that they are being observed in an intermediate stage
of degradation. Less extended ray/ejecta features have been found for some of the freshest medium-size craters such as Kuiper
and Degas. Much more common are smaller (<40 km diameter) fresh craters showing bright rim-rings but little or no ray structure.
These smaller radar-bright craters are particularly common over the H-7 quadrangle. Diffuse areas of enhanced depolarized
brightness have been found in the smooth plains, including the circum-Caloris planitiae and Tolstoj Basin. This is an interesting
finding, as it is the reverse of the albedo contrast seen between the radar-dark maria and the radar-bright cratered highlands
on the Moon. 相似文献
919.
Dimogianopoulos D.G. Hios J.D. Fassois S.D. 《IEEE transactions on aerospace and electronic systems》2007,43(4):1385-1396
A statistical method for the on-board detection and compensation of adverse oscillations in pilot-aircraft systems is presented. A recursively updated linear model representing the pilot-aircraft system is used to continuously check for stability loss. The instability-related oscillations are detected early, using a statistical hypothesis test. Furthermore, a specially designed stability augmentation system assists the pilot during the instability periods. The method's effectiveness is demonstrated via data obtained from a flight simulator and a detailed simulation model. 相似文献
920.
Smart M.C. Ratnakumar B.V. Whitcanack L. Chin K. Rodriguez M. Surampudi S. 《Aerospace and Electronic Systems Magazine, IEEE》2002,17(12):16-20
The low temperature charge and discharge characteristics of experimental MCMB-Li/sub x/Ni/sub y/Co/sub 1-y/O/sub 2/ cells containing different electrolytes were investigated. The use of low ethylene carbonate (EC)-content electrolyte formulations has resulted in good discharge performance to temperatures as low as -40/spl deg/C. The effect of charge voltage and charge current upon the individual electrode potentials at low temperature was investigated using the three electrode cells (containing lithium reference electrodes). In some cases, lithium plating was observed to occur upon low temperature charge, and found to be facilitated by high charge voltages, high charge currents, and poor anode kinetics. Electrochemical characterization of the cells has helped to establish the conditions under which lithium plating can occur by providing information regarding the polarization effects present at each electrode. 相似文献