全文获取类型
收费全文 | 8850篇 |
免费 | 11篇 |
国内免费 | 36篇 |
专业分类
航空 | 4257篇 |
航天技术 | 3054篇 |
综合类 | 198篇 |
航天 | 1388篇 |
出版年
2021年 | 74篇 |
2019年 | 50篇 |
2018年 | 153篇 |
2017年 | 93篇 |
2016年 | 85篇 |
2014年 | 165篇 |
2013年 | 218篇 |
2012年 | 218篇 |
2011年 | 319篇 |
2010年 | 211篇 |
2009年 | 361篇 |
2008年 | 400篇 |
2007年 | 225篇 |
2006年 | 190篇 |
2005年 | 228篇 |
2004年 | 233篇 |
2003年 | 287篇 |
2002年 | 272篇 |
2001年 | 327篇 |
2000年 | 181篇 |
1999年 | 223篇 |
1998年 | 262篇 |
1997年 | 189篇 |
1996年 | 243篇 |
1995年 | 299篇 |
1994年 | 267篇 |
1993年 | 147篇 |
1992年 | 222篇 |
1991年 | 96篇 |
1990年 | 98篇 |
1989年 | 223篇 |
1988年 | 88篇 |
1987年 | 87篇 |
1986年 | 100篇 |
1985年 | 268篇 |
1984年 | 207篇 |
1983年 | 182篇 |
1982年 | 214篇 |
1981年 | 256篇 |
1980年 | 91篇 |
1979年 | 66篇 |
1978年 | 79篇 |
1977年 | 60篇 |
1976年 | 63篇 |
1975年 | 69篇 |
1974年 | 67篇 |
1972年 | 64篇 |
1971年 | 60篇 |
1970年 | 54篇 |
1969年 | 53篇 |
排序方式: 共有8897条查询结果,搜索用时 62 毫秒
821.
Rajat Acharya Bijoy Roy M.R. Sivaraman Ashish Dasgupta 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
It is important to use models developed specifically for the equatorial ionospheric estimation for real-time applications, particularly in Satellite Navigation. This work demonstrates a methodology for improved predictions of VTEC in real time using the model developed for the equatorial ionosphere by the authors. This work has been done using TEC data of the low solar activity period of 2005 obtained using dual frequency GPS receivers installed under the GAGAN project of ISRO. For the purpose, the model is first used in conjunction with Kriging technique. Improvement in accuracy is observed when compared with the estimations from the model alone using the measurements as true reference. Further improvement is obtained by Bayesian combination of these estimates with independent Neural Network based predictions. Statistical performance of improvement is provided. An improvement of ∼1 m in confidence level of estimation of VTEC is obtained. 相似文献
822.
823.
R. R. Brown 《Space Science Reviews》1966,5(3):311-387
The present experimental knowledge of electron precipitation in the auroral zone is summarized, using observations primarily
from riometer, balloon, rocket and satellite studies. This includes the association of electron precipitation events with
aurora, magnetic disturbances as well as temporal and spatial aspects of the particle bombardment. 相似文献
824.
825.
The possibility of using solar sails in Earth orbit is investigated. The characteristic parameters of a solar sail consisting of six spheres attached by mutually orthogonal long rods are estimated. 相似文献
826.
T Naitoh M Yamashita A Izumi-Kurotani I Takabatake R J Wassersug 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2000,25(10):2015-2018
Amphibians possess the ability to vomit in response to a variety of stimuli that provoke emesis in mammals. Pharmacological studies have establish that the ejection of gastric contents and the basic mechanism for vomiting have been phylogenetically conserved among these tetrapods. As part of on-going comparative studies on emesis in vertebrates, we previously documented that some postmetamorphic anurans and salamander larvae experience motion-induced emesis when exposed to the provocative stimulus of parabolic aircraft flight. However, more recent experiments suggest that there are strict conditions for inducing emesis in amphibians exposed to parabolic flight and that amphibians are not as sensitive to this stimulus as mammals. Further studies on emesis in lower vertebrates may help us understand the processes that cause emesis in abnormal gravitational regimes. 相似文献
827.
Purpose of the work is to analyze and to summarize the data of investigations into human hemodynamics performed over 20 years aboard orbital stations Salyut-7 and Mir with participation of 26 cosmonauts on space flights (SF) from 8 to 438 days in duration. The ultrasonic techniques and occlusive plethysmography demonstrated dynamics of changes in the cardiovascular system during SF of various durations. The parameters of general hemodynamics, the pumping function of the heart and arterial circulation in the brain remained stable in all the space flights; however, there were alterations in peripheral circulation associated with blood redistribution and hypovolemie in microgravity. The anti-gravity distribution of the vascular tone decayed gradually as unneeded. The most considerable changes were observed in leg vessels, equally in arteries (decrease in resistance) and veins (increase in maximum capacity). The lower body negative pressure test (LBNP) revealed deterioration of the gravity-dependent reactions that changed for the worse as SF duration extended. The cardiovascular deconditioning showed itself as loss of descent acceleration tolerance and orthostatic instability in the postflight period. 相似文献
828.
829.
Nicole D. Buckley Claude P. Champagne Adriana I. Masotti Lisa E. Wagar Thomas A. Tompkins Julia M. Green-Johnson 《Acta Astronautica》2011,68(7-8):731-738
Astronauts face numerous health challenges during long-duration space missions, including diminished immunity, bone loss and increased risk of radiation-induced carcinogenesis. Changes in the intestinal flora of astronauts may contribute to these problems. Soy-based fermented food products could provide a nutritional strategy to help alleviate these challenges by incorporating beneficial lactic acid bacteria, while reaping the benefits of soy isoflavones. We carried out strain selection for the development of soy ferments, selecting strains of lactic acid bacteria showing the most effective growth and fermentation ability in soy milk (Streptococcus thermophilus ST5, Bifidobacterium longum R0175 and Lactobacillus helveticus R0052). Immunomodulatory bioactivity of selected ferments was assessed using an in vitro challenge system with human intestinal epithelial and macrophage cell lines, and selected ferments show the ability to down-regulate production of the pro-inflammatory cytokine interleukin-8 following challenge with tumour necrosis factor-alpha. The impact of fermentation on vitamin B1 and B6 levels and on isoflavone biotransformation to agluconic forms was also assessed, with strain variation-dependent biotransformation ability detected. Overall this suggests that probiotic bacteria can be successfully utilized to develop soy-based fermented products targeted against health problems associated with long-term space travel. 相似文献
830.
Performance analysis of echolocation systems requires knowledge of the probability density function (pdf) or cumulative distribution function (cdf) of a matched filter output. A method is presented to estimate these and other probability functions from data by estimating the failure rate function, a function employed in reliability theory. The method can also be used to derive approximations to closed-form probability functions. The method is demonstrated using experimental sonar and radar clutter data and a closed-form radar clutter model 相似文献