全文获取类型
收费全文 | 18663篇 |
免费 | 58篇 |
国内免费 | 128篇 |
专业分类
航空 | 10082篇 |
航天技术 | 5522篇 |
综合类 | 248篇 |
航天 | 2997篇 |
出版年
2021年 | 160篇 |
2018年 | 244篇 |
2016年 | 173篇 |
2014年 | 436篇 |
2013年 | 514篇 |
2012年 | 440篇 |
2011年 | 623篇 |
2010年 | 438篇 |
2009年 | 790篇 |
2008年 | 830篇 |
2007年 | 408篇 |
2006年 | 437篇 |
2005年 | 421篇 |
2004年 | 452篇 |
2003年 | 558篇 |
2002年 | 487篇 |
2001年 | 605篇 |
2000年 | 367篇 |
1999年 | 467篇 |
1998年 | 449篇 |
1997年 | 329篇 |
1996年 | 395篇 |
1995年 | 457篇 |
1994年 | 451篇 |
1993年 | 359篇 |
1992年 | 337篇 |
1991年 | 250篇 |
1990年 | 235篇 |
1989年 | 407篇 |
1988年 | 209篇 |
1987年 | 247篇 |
1986年 | 233篇 |
1985年 | 639篇 |
1984年 | 519篇 |
1983年 | 416篇 |
1982年 | 487篇 |
1981年 | 614篇 |
1980年 | 244篇 |
1979年 | 190篇 |
1978年 | 189篇 |
1977年 | 146篇 |
1976年 | 156篇 |
1975年 | 191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 15 毫秒
61.
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
62.
63.
V D Kern F D Sack 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,27(5):941-949
Apical cells of moss protonemata represent a single-celled system that perceives and reacts to light (positive and negative phototropism) and to gravity (negative gravitropism). Phototropism completely overrides gravitropism when apical cells are laterally irradiated with relatively high red light intensities, but below a defined light intensity threshold gravitropism competes with the phototropic reaction. A 16 day-long exposure to microgravity conditions demonstrated that gravitropism is allowed when protonemata are laterally illuminated with light intensities below 140 nmol m-2s-1. Protonemata that were grown in darkness in microgravity expressed an endogenous tendency to grow in arcs so that the overall culture morphology resembled a clockwise spiral. However this phenomenon only was observed in cultures that had reached a critical age and/or size. Organelle positioning in dark-grown apical cells was significantly altered in microgravity. Gravisensing most likely involves the sedimentation of starch-filled amyloplasts in a well-defined area of the tip cell. Amyloplasts that at 1-g are sedimented were clustered at the apical part of the sedimentation zone in microgravity. Clustering observed in microgravity or during clino-rotation significantly differs from sedimentation-induced plastid aggregations after inversion of tip cells at 1-g. 相似文献
64.
C Medvedovsky B V Worgul Y Huang D J Brenner F Tao J Miller C Zeitlin E J Ainsworth 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):475-482
Because activities in space necessarily involve chronic exposure to a heterogeneous charged particle radiation field it is important to assess the influence of dose-rate and the possible modulating role of heavy particle fragmentation on biological systems. Using the well-studied cataract model, mice were exposed to plateau 600 MeV/amu 56Fe ions either as acute or fractionated exposures at total doses of 5 - 504 cGy. Additional groups of mice received 20, 360 and 504 cGy behind 50 mm of polyethylene, which simulates body shielding. The reference radiation consisted of 60Co gamma radiation. The animals were examined by slit lamp biomicroscopy over their three year life spans. In accordance with our previous observations with heavy particles, the cataractogenic potential of the 600 MeV/amu 56Fe ions was greater than for low-LET radiation and increased with decreasing dose relative to gamma-rays. Fractionation of a given dose of 56Fe ions did not reduce the cataractogenicity of the radiation compared to the acute regimen. Fragmentation of the beam in the polyethylene did not alter the cataractotoxicity of the ions, either when administered singly or in fractions. 相似文献
65.
66.
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
67.
68.
69.
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
70.
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