全文获取类型
收费全文 | 6177篇 |
免费 | 28篇 |
国内免费 | 13篇 |
专业分类
航空 | 2954篇 |
航天技术 | 2125篇 |
综合类 | 192篇 |
航天 | 947篇 |
出版年
2021年 | 50篇 |
2019年 | 45篇 |
2018年 | 108篇 |
2017年 | 70篇 |
2016年 | 66篇 |
2015年 | 38篇 |
2014年 | 123篇 |
2013年 | 151篇 |
2012年 | 148篇 |
2011年 | 215篇 |
2010年 | 142篇 |
2009年 | 245篇 |
2008年 | 296篇 |
2007年 | 160篇 |
2006年 | 145篇 |
2005年 | 166篇 |
2004年 | 154篇 |
2003年 | 192篇 |
2002年 | 222篇 |
2001年 | 258篇 |
2000年 | 110篇 |
1999年 | 150篇 |
1998年 | 179篇 |
1997年 | 131篇 |
1996年 | 178篇 |
1995年 | 224篇 |
1994年 | 170篇 |
1993年 | 102篇 |
1992年 | 145篇 |
1991年 | 55篇 |
1990年 | 61篇 |
1989年 | 142篇 |
1988年 | 60篇 |
1987年 | 57篇 |
1986年 | 59篇 |
1985年 | 185篇 |
1984年 | 162篇 |
1983年 | 128篇 |
1982年 | 152篇 |
1981年 | 177篇 |
1980年 | 51篇 |
1979年 | 44篇 |
1978年 | 40篇 |
1977年 | 46篇 |
1975年 | 50篇 |
1974年 | 49篇 |
1973年 | 32篇 |
1972年 | 51篇 |
1971年 | 36篇 |
1969年 | 34篇 |
排序方式: 共有6218条查询结果,搜索用时 343 毫秒
541.
Musal H.M. Thomas P.D. Vinokur M. 《IEEE transactions on aerospace and electronic systems》1970,(2):252-254
The evaluation of the double convolution integrat involved in the expression of the radar response to scattering from a turbulent ionized wake is simplified by the approximation technique presented here, so that the Doppler spectrum parameters can be explicitly expressed in terms of the wake characteristics. 相似文献
542.
Red-cell mass determinations were performed before and after the first two Skylab missions. The data showed a 14% mean decrease in red-cell mass after the 28-day mission and a 12% mean decrease after the 59-day mission. The red-cell mass returned to premission levels more slowly after the shorter (28-day) than after the longer mission. Plasma volume decreases were found after each mission. with the crew from the longer mission showing the greater change (13% vs. 8.4%). Postmission decreases in red-cell mass and plasma volume have been a general finding in crewmen who return from short or long spaceflight. 相似文献
543.
The influence of sudden increases of electron content on the accurate determination of the position of a satellite is investigated based on a spherically stratified ionospheric model. Using the total electron content information from Faraday rotation measurements, a procedure is presented whereby the corrections of satellite position due to the unpredicted electron increase can be accounted for without the need to know the spatial distribution of the additional electrons. 相似文献
544.
Cross Polarization in Radomes: A Program for Its Computation 总被引:1,自引:0,他引:1
A study is made of transmission by radomes, with particular reference to cross polarization, and of the consequent radiation patterns of the radome with its aerial for both linear and circular polarizations. The work is embodied in a computer program which considers a scanner of specifiable size, position, and power distribution at different orientations inside a multilayer radome of given dimensions, class of shape, and construction in terms of the number and properties of its layers. Initially, rays are traced from points on the scanner, and details of their paths and propagation are presented in tables of preliminary results; later, diffraction theory is used. Polar diagrams of cross polarization, of main beam, etc., are presented in final tables for the system. Nearly all the parameters are specifiable, and so the program has reasonably general applicability, and it can also assess the effects of bandwidth, tapering of layer thickness, complex permittivity, and other parameters of the system. 相似文献
545.
Smith G.H. Cunningham D.R. Ziemer R.E. 《IEEE transactions on aerospace and electronic systems》1973,(4):548-555
Plots of performance degradation are used to compare the effects of fading and intersymbol interference in a two- component specular multipath digital communications channel. Similar plots are then used to compare two practical receivers designed to combat the interference. Degradation plots are shown to allow easy identification of each receiver's range of usefulness, as well as identification of variance bounds demanded of channel parameter estimates which the receivers require. 相似文献
546.
Kau S. Kumar K.S.P. Granley G.B. 《IEEE transactions on aerospace and electronic systems》1969,(6):906-911
The theory of nonlinear filtering has been applied to the problem of the attitude determination of a satellite. Two approximation methods have been studied with a view to practical implementation. Computer simulation results are included. 相似文献
547.
The laser beam affords a valuable carrier medium for an optical calmunication system, but such a system for transmission through the atmosphere is subjected to wide fluctuations in received signal strength because of turbulence. An electrooptical automatic gain control system em has been studied and built which can substantially reduce the effect of these fluctuations. The effect of system parameters on the control characteristics are considered. 相似文献
548.
Cullers DK 《Acta Astronautica》1986,13(1):31-37
Highly monochromatic signals, such as TV carriers, can be detected sensitively by using a narrow filter (b < or = 1 Hz) and performing power accumulation on its output. If instead a low-duty-cycle pulsed signal of the same total energy is present, the sensitivity of a square law device, followed by a thresholding operation (to eliminate most samples containing only noise), followed by the algorithm to be described, is greater by about 7 dB in typical cases. This is particularly interesting to SETI because such a pulsed signal is exactly what is sent by a rotating beacon with a directional antenna. Such a pulsed signal is, therefore, a good candidate for an extraterrestrial beacon. Software for detecting this signal type is now ready for field testing with the NASA Multichannel Spectrum Analyzer (MCSA). 相似文献
549.
Scharf L.L. Harthill W.P. Moose P.H. 《IEEE transactions on aerospace and electronic systems》1969,(4):672-673
The relative merits of intercept and pure pursuit homing are investigated by comparing the expected flight times to a nonmaneuvering target. The analysis is performed for the initial target heading angle uniformly distributed with arbitrary mean and variance. Two specific examples illustrate that intercept homing holds little statistical advantage over pure pursuit homing except when the speed advantage of the missile is very small. 相似文献
550.
J.M. Russell E.E. Remsberg L.L. Gordley J.C. Gille P.L. Bailey 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):169-172
The LIMS experiment was launched on the Nimbus 7 satellite for the purpose of sounding the vertical structure of temperature and key upper atmosphere trace gases on a global scale. The technique of thermal infrared limb sounding was used to obtain measurements of O3, H2O, NO2, and HNO3. LIMS collected data almost continuously from late October to late May over the latitude range from 64°S to 84°N. Two of the gases, NO2 and HNO3, are important elements in the NOx chain of chemical reactions leading to ozone destruction. We will describe results for these gases in terms of zonal mean profiles and latitudinal distributions. The period selected for study is January–May 1979, when a major stratospheric warming occurred. 相似文献