全文获取类型
收费全文 | 17104篇 |
免费 | 28篇 |
国内免费 | 123篇 |
专业分类
航空 | 9723篇 |
航天技术 | 4900篇 |
综合类 | 235篇 |
航天 | 2397篇 |
出版年
2021年 | 154篇 |
2018年 | 165篇 |
2016年 | 149篇 |
2014年 | 432篇 |
2013年 | 513篇 |
2012年 | 398篇 |
2011年 | 547篇 |
2010年 | 385篇 |
2009年 | 742篇 |
2008年 | 772篇 |
2007年 | 341篇 |
2006年 | 415篇 |
2005年 | 346篇 |
2004年 | 396篇 |
2003年 | 462篇 |
2002年 | 451篇 |
2001年 | 507篇 |
2000年 | 337篇 |
1999年 | 434篇 |
1998年 | 376篇 |
1997年 | 294篇 |
1996年 | 331篇 |
1995年 | 406篇 |
1994年 | 351篇 |
1993年 | 349篇 |
1992年 | 264篇 |
1991年 | 246篇 |
1990年 | 231篇 |
1989年 | 360篇 |
1988年 | 198篇 |
1987年 | 229篇 |
1986年 | 222篇 |
1985年 | 636篇 |
1984年 | 507篇 |
1983年 | 391篇 |
1982年 | 485篇 |
1981年 | 605篇 |
1980年 | 243篇 |
1979年 | 182篇 |
1978年 | 189篇 |
1977年 | 144篇 |
1976年 | 155篇 |
1975年 | 181篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
52.
A simple fiber-optic radar calibration target is described. Its operation is based on a wideband fiber, a laser transmitter that is directly modulated by the down-converted radar signal and an optical diode receiver recovering said signal. Further up-conversion having a common local oscillator with the first mixer ensures fidelity of the calibration return. Measured useful bandwidth exceeds 200 MHz and practically any radar RF frequency can be handled when suitable mixers are employed. Amplifiers can be added to the down-converted path as desired to compensate for the fiber loss. Modulation and LO sweep provide easy ways of introducing artificial fluctuations and Doppler frequencies. Particularly pulsed radars are readily tested with the proposed scheme as no restrictions are posed by the radar's TR-switch delays. 相似文献
53.
The microwave landing system (MLS) transmits angle, data, andrange information for use by airborne receivers. In this paper, theintegrity of the data functions is analyzed in terms of the probabilityof undetected errors remaining in the data. The data format andintegrity requirements were derived from the MLS standards andguidance material defined by the International Civil Aviation Organization (ICAO). Results show that the performancerequirements can be met by: 1) averaging the received data bits ofseveral samples of the same word using a majority voting;2) reducing the bit error rate at the output of the receiver'sdecoder; and 3) a combination of the above techniques. 相似文献
54.
55.
56.
Guerci J.R. Goetz R.A. DiModica J. 《IEEE transactions on aerospace and electronic systems》1994,30(4):1090-1093
A relatively simple method is presented which eliminates previously reported (Oct. 1985) erratic estimation performance associated with Cartesian formulations of the extended Kalman filter (EKF) for the 2D angle-only emitter location problem. The technique is based on an initialization procedure which combines a priori probability density function (pdf) information with single measurement a posteriori pdf information in a manner which is more efficient than the EKF. Simulation results are presented which demonstrate the utility of the technique as compared with a previously offered modified gain EKF 相似文献
57.
58.
L. B. F. M. Waters 《Space Science Reviews》1992,61(1-2):25-43
In this review the IR emission from circumstellar material is discussed, both of ionized gas and dust grains, and the astrophysical information that can be extracted from such observations. Some emphasis is placed on the possibilities of stellar IR astronomy using a large space-borne telescope, especially with respect to the much better spatial and spectral resolution of such a telescope compared to the current generation of ground-based and space IR telescopes. 相似文献
59.
H. Thiemann R. W. Schunk 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(12):143-146
Heavy emission caused by impacting plasma ions results in a fast discharging effect of the initially large surface potentials on the dielectric solar cells. This eventually counteracts the energization process of the plasma ions to the cover glasses and leaves no significant electric fields. Thus, with an existing thermal plasma, electrons are again able to reach dielectric surfaces. Strong localized electric fields of the order of several 10 kV/cm form near the interconnector-cover glass interface. 相似文献
60.
The Cluster mission of the European Space Agency (ESA) will allow, for the first time three-dimensional measurements in key regions of the Earth's magnetosphere to be carried out. The European Numerical Simulation Network (ENSN) aims at providing a theoretical support to the mission. We describe the achievements of the ENSN during its first period of activity 1991–1994, during which the network was funded by the European Union. In particular, the ENSN has set up (i) thematic Working Groups on the prime scientific goals of the mission, (ii) a code development Working Group to develop numerical simulation codes specifically adapted to studying magnetospheric boundaries and the corresponding scale mixing, and (iii) software models of Cluster instruments to test in a numerical simulation what the set of four instruments will measure. 相似文献