全文获取类型
收费全文 | 8443篇 |
免费 | 9篇 |
国内免费 | 26篇 |
专业分类
航空 | 4363篇 |
航天技术 | 2818篇 |
综合类 | 202篇 |
航天 | 1095篇 |
出版年
2021年 | 53篇 |
2019年 | 49篇 |
2018年 | 102篇 |
2016年 | 50篇 |
2014年 | 134篇 |
2013年 | 190篇 |
2012年 | 180篇 |
2011年 | 266篇 |
2010年 | 175篇 |
2009年 | 293篇 |
2008年 | 348篇 |
2007年 | 198篇 |
2006年 | 190篇 |
2005年 | 201篇 |
2004年 | 178篇 |
2003年 | 251篇 |
2002年 | 250篇 |
2001年 | 315篇 |
2000年 | 165篇 |
1999年 | 205篇 |
1998年 | 255篇 |
1997年 | 181篇 |
1996年 | 239篇 |
1995年 | 298篇 |
1994年 | 270篇 |
1993年 | 165篇 |
1992年 | 199篇 |
1991年 | 117篇 |
1990年 | 107篇 |
1989年 | 217篇 |
1988年 | 102篇 |
1987年 | 108篇 |
1986年 | 101篇 |
1985年 | 281篇 |
1984年 | 224篇 |
1983年 | 192篇 |
1982年 | 203篇 |
1981年 | 275篇 |
1980年 | 98篇 |
1979年 | 78篇 |
1978年 | 89篇 |
1977年 | 70篇 |
1976年 | 67篇 |
1975年 | 89篇 |
1974年 | 70篇 |
1973年 | 62篇 |
1972年 | 86篇 |
1971年 | 75篇 |
1970年 | 63篇 |
1969年 | 64篇 |
排序方式: 共有8478条查询结果,搜索用时 344 毫秒
271.
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
272.
273.
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
274.
275.
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
276.
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
277.
A brief summary of the main results of magnetospheric ion composition measurements in general is first presented. PROGNOZ-7 measurements in the nightside plasma mantle are then described and analyzed. Some of the results are the following: In the nightside mantle not too far from midnight the properties of the mantle are sometimes consistent with the open magnetosphere model. However during most magnetic storm situations O+ ions appear in the mantle in large proportions and with high energies. The acceleration process affecting the ions has been found in several cases to give equal amounts of energy to all ions independent of mass. Along the flanks of the magnetosphere the flow of the plasma is often low or absent. The O+ content is high (up to 20%) and the energy spectrum of both ions and electrons may be very hot, even up to the level of the ring current plasma in the keV range.The O+ content in the plasma mantle is positively correlated with the magnetospheric activity level. The mantle, however, does not appear to be the dominating source for the storm time ring current. Direct acceleration of ionospheric ions onto the closed field lines of the plasma sheet and ring current is most likely the main source. The magnetopause on the nightside and along the flanks of the magnetosphere appears to be a fairly solid boundary for mantle ions of ionospheric origin. This is especially evident during periods with high geomagnetic activity, when the mantle is associated with fairly strong fluxes of O+ ions.An interesting observation in most of the mantle passages during geomagnetically disturbed periods is the occurrence of intense, magnetosheath like, regions deep inside the mantle. In some cases these regions with strong antisunward flow and with predominant magnetosheath ion composition was observed in the innermost part of the mantle, i.e. marking a boundary region between the lobe and the mantle. These magnetosheath penetration events are usually associated with strong fluxes of accelerated ionospheric ions in nearby parts of the mantle. Evanescent penetration regions with much reduced flow properties are frequently observed in the flank mantle. 相似文献
278.
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
279.
An improved moving target detector (MTD) (a digital signal processor) has been designed, constructed, and tested which successfully rejects all forms of radar clutter while providing reliable detection of all aircraft within the coverage of the radar. The MTD is being tested on both terminal and enroute surveillance radars for the FAA. This processor has been integrated with automatic tracking algorithms to give complete rejection of ground clutter, heavy precipitation, and angels (birds). 相似文献
280.
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