全文获取类型
收费全文 | 4521篇 |
免费 | 5篇 |
国内免费 | 21篇 |
专业分类
航空 | 2319篇 |
航天技术 | 1689篇 |
综合类 | 21篇 |
航天 | 518篇 |
出版年
2018年 | 54篇 |
2017年 | 36篇 |
2014年 | 64篇 |
2013年 | 99篇 |
2012年 | 75篇 |
2011年 | 125篇 |
2010年 | 91篇 |
2009年 | 151篇 |
2008年 | 242篇 |
2007年 | 91篇 |
2006年 | 98篇 |
2005年 | 107篇 |
2004年 | 130篇 |
2003年 | 147篇 |
2002年 | 83篇 |
2001年 | 114篇 |
2000年 | 90篇 |
1999年 | 87篇 |
1998年 | 138篇 |
1997年 | 91篇 |
1996年 | 117篇 |
1995年 | 131篇 |
1994年 | 151篇 |
1993年 | 86篇 |
1992年 | 121篇 |
1991年 | 56篇 |
1990年 | 56篇 |
1989年 | 133篇 |
1988年 | 47篇 |
1987年 | 55篇 |
1986年 | 73篇 |
1985年 | 172篇 |
1984年 | 115篇 |
1983年 | 106篇 |
1982年 | 111篇 |
1981年 | 155篇 |
1980年 | 66篇 |
1979年 | 47篇 |
1978年 | 44篇 |
1977年 | 41篇 |
1975年 | 53篇 |
1974年 | 39篇 |
1973年 | 46篇 |
1972年 | 40篇 |
1971年 | 43篇 |
1970年 | 38篇 |
1969年 | 45篇 |
1968年 | 30篇 |
1967年 | 30篇 |
1966年 | 30篇 |
排序方式: 共有4547条查询结果,搜索用时 546 毫秒
811.
This paper reports on the objectives of the German space processing programme with sounding rockets and the scientific results obtained thus far. 相似文献
812.
Miles R.B. Kimball R.B. Frey W.H. 《IEEE transactions on aerospace and electronic systems》1971,(4):578-584
Outdated and rigorous curricula are not only producing engineers unsuited to the challenges of the 1970's, but they are scaring off many of the most creative and intelligent students. Engineering education has become an endurance test; the excitement has been lost. Most students strive only to get out. Engineering schools have not yet realized the importance of the involvement of engineers in current environmental problems. They continue to present only the dry technical courses and seem unconcerned about the student's sense of social perspective or his motivation. This paper proposes a freshman awareness seminar plus project oriented learning in the following years as a remedy for this sad situation. 相似文献
813.
Cantrell B.H. Gordon W.B. Trunk G.V. 《IEEE transactions on aerospace and electronic systems》1981,(2):213-221
The maximum likelihood estimates of the elevation angles of two closely spaced targets within the beamwidth is considered. For an array divided into three subapertures, a simple, closed form solution is found whose accuracy compares favorably to the maximum likelihood estimate which uses all the individual elements. Simulation results are presented for the case of a radar target located over a smooth reflecting surface. 相似文献
814.
W. M. Glencross 《Space Science Reviews》1982,33(1-2):151-160
Early models of the Chromosphere-Corona transition zone of the Sun considered it to be a static plane-parallel region. From these it became clear that the layer was extremely thin and had an important role in the conduction of energy from the Corona. More recent observations show mass motions of order 10 km s-1, which means that heating and cooling of the moving plasma has an important affect on the energy balance, while transient effects producing far higher velocities are also common. Studies of plasma motion through the zone are clearly relevant to the initial heating of material which enters the solar wind.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind. 相似文献
815.
Margolis M.D. Messenger G.C. Fitch S.H. 《IEEE transactions on aerospace and electronic systems》1972,(6):771-779
A hardness assurance program suitable for a retaliatory or deterrent missile system is discussed. The program consists of three phases: the establishment of suitable controls and screens to ensure piece part hardness, a supplier data monitoring program to measure hardness related parameters in a timely fashion to obtain advanced warning of potential problems, and a hardness assurance verification testing (HAVT) program to periodically measure the radiation response of parts, circuits, subsystems, and systems on a sample basis to ensure proper functioning in simulated radiation environments. All of the data relevant to hardness assurance is stored in computer memory and routinely interrogated with programs which compare current results with the engineering baseline data and other data from preceding lots. Other computer programs enable statistical estimation of system response to the radiation environment with confidence based on the expanding data base provided by the HAVT program. 相似文献
816.
The performance of the modulation cancellation altimeter is investigated. Several sources of error are considered, specifically: 1) terrain averaging error; 2) output noise fluctuations; 3) errors due to phase shifts; and 4) acquisition ambiguities. The predominant parameters affecting these errors are investigated with a view towards optimizing the performance of the altimeter. 相似文献
817.
W.M. Alexander J.D. Corbin 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(8):107-110
Recent studies of the lunar ejecta from lunar impacts of interplanetary dust particles indicate that during favorable lunar phases, over 80% of the submicron ejecta enters the earth's magnetosphere. This “pulse” of lunar ejecta produced by the sporadic meteor background will follow the random variations of the sporadic flux. An additional enhancement of this flux can be related to major meteor showers. Since the annual periods of these showers occur during varying lunar phase angles, magnetosphere ejecta flux associated with major showers will vary depending on the coincidence of shower periods and favorable lunar phase angles. The results of an analysis of the “pulse” of ejecta flux in the magnetosphere during the Quadrantids, Geminids, Leonids, and Perseids meteor showers are presented. These results are compared to the satellite measurements of 1959 Eta and HEOS II. 相似文献
818.
Echoes from complex targets due to chirp waveforms show different time responses for different chirp rates, because of the ambiguity of range and radial velocity information. A graphical method is described which enables the derivation of the time response for an arbitrary chirp rate when the scattering cross-section distribution in the range and radial velocity plane is known for complex targets. The graphical method can also be used to generate an approximately realistic scattering cross-section distribution when the time responses for two different chirp rates are given. An example is shown which applies to the computer simulation of the radar echo from a waking reentry space vehicle. 相似文献
819.
Temesmber C.L. Thompson W.E. 《IEEE transactions on aerospace and electronic systems》1975,(2):183-189
Useful new properties are obtained with a coherent pulse-burst waveform if the FM slopes are allowed to change from pulse to pulse. A design feature using such a "slope-coded" waveform is the ability to transfer, in a controllable manner, ambiguity volume from the range ambiguity peaks to regions located between the peaks (in the frequency direction). Slope coding thus provides a useful mechanism for matching the waveform to the environment; i. e., it permits the signal designer to make some compromise between range ambiguity-peak heights and low-velocity clutter rejection. 相似文献
820.
H. Rott 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(11):241-246
The potential of SAR systems for monitoring the seasonal snow cover and glaciers has been investigated based on an airborne experiment in the Austrian Alps and on Seasat SAR and Shuttle Imaging Radar-A data. X- and C-band SAR are useful sensors for mapping wet snow packs, while in L-band snow-covered and snow-free surfaces often cannot be separated. SAR data in all three frequency bands provide valuable glaciological information. 相似文献