全文获取类型
收费全文 | 5149篇 |
免费 | 16篇 |
国内免费 | 15篇 |
专业分类
航空 | 2235篇 |
航天技术 | 1694篇 |
综合类 | 26篇 |
航天 | 1225篇 |
出版年
2021年 | 51篇 |
2019年 | 25篇 |
2018年 | 195篇 |
2017年 | 145篇 |
2016年 | 125篇 |
2015年 | 51篇 |
2014年 | 145篇 |
2013年 | 175篇 |
2012年 | 156篇 |
2011年 | 237篇 |
2010年 | 175篇 |
2009年 | 265篇 |
2008年 | 267篇 |
2007年 | 162篇 |
2006年 | 105篇 |
2005年 | 150篇 |
2004年 | 147篇 |
2003年 | 164篇 |
2002年 | 112篇 |
2001年 | 159篇 |
2000年 | 72篇 |
1999年 | 91篇 |
1998年 | 117篇 |
1997年 | 80篇 |
1996年 | 103篇 |
1995年 | 148篇 |
1994年 | 108篇 |
1993年 | 75篇 |
1992年 | 100篇 |
1991年 | 39篇 |
1990年 | 38篇 |
1989年 | 84篇 |
1988年 | 33篇 |
1987年 | 36篇 |
1986年 | 35篇 |
1985年 | 141篇 |
1984年 | 105篇 |
1983年 | 93篇 |
1982年 | 96篇 |
1981年 | 142篇 |
1980年 | 43篇 |
1979年 | 42篇 |
1978年 | 37篇 |
1977年 | 39篇 |
1976年 | 32篇 |
1975年 | 29篇 |
1974年 | 29篇 |
1972年 | 27篇 |
1970年 | 22篇 |
1969年 | 24篇 |
排序方式: 共有5180条查询结果,搜索用时 515 毫秒
901.
This paper presents a theoretically and practically grounded architecture of design methods combination. A possibility and completeness of developing integral design solutions and intensifying the design process are provided by redistribution of emphases of numerical and natural experiments. 相似文献
902.
903.
Structures for radar detection in compound Gaussian clutter 总被引:1,自引:0,他引:1
Sangston K.J. Gini F. Greco M.V. Farina A. 《IEEE transactions on aerospace and electronic systems》1999,35(2):445-458
The problem of coherent radar target detection in a background of non-Gaussian clutter modeled by a compound Gaussian distribution is studied here. We show how the likelihood ratio may be recast into an estimator-correlator form that shows that an essential feature of the optimal detector is to compute an optimum estimate of the reciprocal of the unknown random local power level. We then proceed to show that the optimal detector may be recast into yet another form, namely a matched filter compared with a data-dependent threshold. With these reformulations of the optimal detector, the problem of obtaining suboptimal detectors may be systematically studied by either approximating the likelihood ratio directly, utilizing a suboptimal estimate in the estimator-correlator structure or utilizing a suboptimal function to model the data-dependent threshold in the matched filter interpretation. Each of these approaches is studied to obtain suboptimal detectors. The results indicate that for processing small numbers of pulses, a suboptimal detector that utilizes information about the nature of the non-Gaussian clutter can be implemented to obtain quasi-optimal performance. As the number of pulses to be processed increases, a suboptimal detector that does not require information about the specific nature of the non-Gaussian clutter may be implemented to obtain quasi-optimal performance 相似文献
904.
905.
906.
Sai F. Lee 《航空学报》1993,14(12):596-602
<正> The terrain profile at several airports has caused some difficulty with autolands on the DC-10 and MD-80. Some operators have reported that the pilot had to disconnect the flight guidance system during flare and land manually. At present, the autoland guidance system is not uniquely designed to handle the variations of terrain. Since there is no accurate substitute for radio altitude(h_(rad)) the only variable left to improve is radio altitude rate (h_(rad)). 相似文献
907.
908.
N.C Bissinger N.A Blagoveshchensky A.A Gubanov V.N Gusev V.P Starukhin N.V Voevodenko S.M Zadonsky 《Aerospace Science and Technology》1998,2(8):505-514
Results of a numerical (CFD) study of the influence of the forebody shape on local flow parameters at a bottom-mounted inlet entrance are presented. The free-stream Mach number is assumed to be 3.5–7.0. Some recommendations on forebody shape optimization are provided. Main characteristics of the air inlet are evaluated. 相似文献
909.
N N Gurovsky O G Gazenko B A Adamovich E A Ilyin A M Genin V I Korolkov A A Shipov A R Kotovskaya V A Kondratyeva L V Serova 《Acta Astronautica》1980,7(1):113-121
In the 18.5-day flight of the Soviet biosatellite Cosmos-936 (3-22, August 1977) com-parative investigations of the physiological effects of prolonged weightlessness (20 rats) and artificial gravity of 1 g (10 rats) were carried out. Throughout the flight artificial gravity was generated by means of animal rotation in two centrifuges with a radius of 320mm. Postflight examination of animals and treatment of the flight data were performed by Soviet scientists in collaboration with the specialists from Bulgaria, Czechoslovakia, the German Democratic Republic, Hungary, Poland, Rumania, France and the U.S.A. During the flight the total motor activity of the weightless rats was higher and their body temperature was lower than those of the centrifuged animals. Postflight examination of the weightless rats showed a greater percentage of errors during maze an increase in water intake and a decrease in diuresis; a fall of the resistance of peripheral red cells; an increase in the conditionally pathogenic microflora in the mouth; a decrease of oxygen consumption, carbon dioxide production and energy expenditures; a drop in the static physical endurance; a decline in the capacity to keep balance on the rail; an increase in the latent period of the lifting reflex, etc. The centrifugal animals displayed lesser or no change of the above type. These findings together with the biochemical and morphological data give evidence that during and after flight adaptive processes in the centrifuged rats developed better. 相似文献
910.
Input data of the system are two-dimensional images and one-dimensional distributions of total and polarized solar emission at 5.2 cm wavelength obtained with SSRT. Together with photoheliograms, magnetograms, Hα-filtergrams and characteristics of active regions received from other sources, they form the initial database. The first stage includes superimposing the images, identifying microwave sources with active regions, assigning NOAA numbers to the sources, and determining for each active region the heliolatitude, extent, and inclination angle of the group's axis to the equator. These data are used to calculate the boundaries of longitude zones for each active region. A next stage involves determining the brightness temperatures of microwave sources less than the polarization distribution, the degree of polarization, and microwave emission flux, as well as calculating the parameters of microwave sources. Each parameter is assigned its own value of the weight factor, and the sum of values is used to draw the conclusion about the flare occurrence probability in each active region and on the Sun in general. 相似文献