全文获取类型
收费全文 | 7063篇 |
免费 | 8篇 |
国内免费 | 34篇 |
专业分类
航空 | 3624篇 |
航天技术 | 2558篇 |
综合类 | 32篇 |
航天 | 891篇 |
出版年
2021年 | 39篇 |
2019年 | 46篇 |
2018年 | 69篇 |
2014年 | 114篇 |
2013年 | 163篇 |
2012年 | 142篇 |
2011年 | 206篇 |
2010年 | 150篇 |
2009年 | 240篇 |
2008年 | 323篇 |
2007年 | 163篇 |
2006年 | 179篇 |
2005年 | 185篇 |
2004年 | 165篇 |
2003年 | 221篇 |
2002年 | 138篇 |
2001年 | 224篇 |
2000年 | 138篇 |
1999年 | 182篇 |
1998年 | 219篇 |
1997年 | 140篇 |
1996年 | 203篇 |
1995年 | 257篇 |
1994年 | 238篇 |
1993年 | 148篇 |
1992年 | 172篇 |
1991年 | 91篇 |
1990年 | 84篇 |
1989年 | 192篇 |
1988年 | 78篇 |
1987年 | 85篇 |
1986年 | 81篇 |
1985年 | 259篇 |
1984年 | 195篇 |
1983年 | 167篇 |
1982年 | 171篇 |
1981年 | 239篇 |
1980年 | 83篇 |
1979年 | 62篇 |
1978年 | 67篇 |
1977年 | 66篇 |
1976年 | 47篇 |
1975年 | 81篇 |
1974年 | 51篇 |
1973年 | 60篇 |
1972年 | 66篇 |
1971年 | 54篇 |
1970年 | 58篇 |
1969年 | 55篇 |
1967年 | 38篇 |
排序方式: 共有7105条查询结果,搜索用时 15 毫秒
231.
This paper presents a target identification method based on an estimation of the natural frequencies of oscillation in transient radar signatures. The emphasis is placed upon signal modeling and estimation ation strategy rather than relating resonance locations to physical structures. Salient features of this identification method are: 1) target aspect angle is not needed, 2) multiple targets of the same type can be illuminated simultaneously, and 3) bandpass interrogation ion pulses can be used. The latter feature is compatible with existing radar facilities. The method is applied to some simulated transfer functions, and factors affecting estimate accuracy are discussed. 相似文献
232.
233.
Moyer L.R. Morgan C.J. Rugger D.A. 《IEEE transactions on aerospace and electronic systems》1989,25(4):584-587
An exact expression for the bistatic resolution-cell area (A B) is developed for the special case in which either the transmit or receive antenna has a broad azimuth beamwidth or is omnidirectional. Quantitative examples are presented to illustrate the variation of A B with location. A comparison is made with the performance of a commonly used approximation formula 相似文献
234.
235.
The nonlinear modeling and analysis of dc-dc converters has been done by averaging and discrete-sampling techniques. The averaging technique is simple, but inaccurate, as the modulation frequencies approach the theoretical limit of one-half the switching frequency. The discrete technique is accurate even at high frequencies, but is very complex and cumbersome. An improved model is developed by combining the aforementioned techniques. This new model is easy to implement in circuit and state variable forms and is accurate to the theoretical limit. 相似文献
236.
E. J. Smith 《Space Science Reviews》1983,34(1):101-110
Interplanetary shock observations since the prior Solar Terrestrial Physics Symposium in 1978 are reviewed. Since the interval coincides with the recent solar maximum, emphasis is placed on shocks associated with transient solar phenomena, including coronal transients and eruptive prominences as well as flares. A good correlation between shocks and Storm Sudden Commencements has persisted into the recent maximum. Shocks have been identified that are associated with disappearing filaments and coronal transients rather than with flares. Significant progress has been made in the indirect observation of shocks near the Sun as a result of radio wave measurements in interplanetary space and measurement of the scintillation and spectral broadening of spacecraft radio transmissions. Preliminary results regarding the thickness of interplanetary shocks have appeared. Several quasi-parallel shocks propagating more nearly along, rather than across, the magnetic field have been identified. The plasma drivers accompanying interplanetary shocks have received increased attention and distinctive features have been found in electron, ion and magnetic field data. 相似文献
237.
J. P. Cox J. C. Wheeler C. J. Hansen D. S. King A. N. Cox S. W. Hodson 《Space Science Reviews》1980,27(3-4):529-535
The radial pulsations of very luminous, low-mass models (L/M 104, solar units), which are possible representatives of the R CrB stars, have been examined. These pulsations are extremely nonadiabatic. We find that there are in some cases at least one extra (strange) mode which makes interpretation difficult. The blue instability edges are also peculiar, in that there is an abrupt excursion of the blue edge to the blue for L/M sufficiently large. The range of periods of the model encompasses observed periods of the Cepheid-like pulsations of actual R CrB stars. 相似文献
238.
Magnetic Position and Orientation Tracking System 总被引:3,自引:0,他引:3
Raab F.H. Blood E.B. Steiner T.O. Jones H.R. 《IEEE transactions on aerospace and electronic systems》1979,(5):709-718
Three-axis generation and sensing of quasi-static magneticdipole fields provide information sufficient to determine both the position and orientation of the sensor relative to the source. Linear rotation transformations based upon the previous measurements are applied to both the source excitation and sensor output vectors, yielding quantities that are linearly propotional to small changes in the position and orientation. Changes are separated using linear combinations of sensor output vectors, transformed to the desired coordinate frame, and used to update the previous measurements. Practical considerations for a head-tracking application are discussed. 相似文献
239.
H. Borg L. -A. Holmgren B. Hultovist F. Cambou H. Rème A. Bahnsen G. Kremser 《Space Science Reviews》1978,22(5):511-535
Some preliminary new results are presented of the keV plasma experiment on GEOS-1. Electrons and ions have been observed to stream along the magnetic field lines in the dayside magnetosphere from the ionospheric side of the satellite towards the equatorial plane during magnetic storms, with streaming velocities corresponding to a kinetic energy of the order of a keV. The opposite streaming velocity has also been seen, but primarily in the ions only and with a smaller flux ratio for the two opposite directions along the field lines. The transition between the two opposite streaming directions, as seen by the satellite, has been found to occur even in a fraction of a second.In magnetic storm conditions azimuthal asymmetries in ion fluxes corresponding to electric field intensities of several tens of millivolts per meter have been observed. 相似文献
240.
J. L. Birck 《Space Science Reviews》1991,56(1-2):141-146
According to their chemical composition, rich in volatile compounds, comets are thought to be primitive materials. They may provide prime samples for the study of nucleosynthetic components of the solar system and of the processes occurring during the formation of the outer planets. Their origin is largely a matter of conjecture. Chromium isotopic measurements in carbonaceous chondrites illustrate how the non-volatile part of cometary material can be investigated both for isotopic heterogeneity and for the extinct nuclide 53Mn. Questions like the possible presence of 26Al as a heat source can also be addressed by these measurements. 相似文献