全文获取类型
收费全文 | 5208篇 |
免费 | 8篇 |
国内免费 | 10篇 |
专业分类
航空 | 2206篇 |
航天技术 | 1757篇 |
综合类 | 24篇 |
航天 | 1239篇 |
出版年
2021年 | 48篇 |
2019年 | 31篇 |
2018年 | 166篇 |
2017年 | 122篇 |
2016年 | 133篇 |
2015年 | 52篇 |
2014年 | 146篇 |
2013年 | 195篇 |
2012年 | 162篇 |
2011年 | 225篇 |
2010年 | 178篇 |
2009年 | 282篇 |
2008年 | 294篇 |
2007年 | 163篇 |
2006年 | 122篇 |
2005年 | 153篇 |
2004年 | 142篇 |
2003年 | 176篇 |
2002年 | 119篇 |
2001年 | 191篇 |
2000年 | 78篇 |
1999年 | 105篇 |
1998年 | 114篇 |
1997年 | 99篇 |
1996年 | 95篇 |
1995年 | 156篇 |
1994年 | 128篇 |
1993年 | 62篇 |
1992年 | 88篇 |
1991年 | 24篇 |
1990年 | 39篇 |
1989年 | 83篇 |
1988年 | 40篇 |
1987年 | 34篇 |
1986年 | 40篇 |
1985年 | 133篇 |
1984年 | 115篇 |
1983年 | 85篇 |
1982年 | 100篇 |
1981年 | 138篇 |
1980年 | 38篇 |
1979年 | 24篇 |
1978年 | 43篇 |
1977年 | 25篇 |
1976年 | 19篇 |
1975年 | 37篇 |
1974年 | 26篇 |
1973年 | 21篇 |
1972年 | 28篇 |
1971年 | 19篇 |
排序方式: 共有5226条查询结果,搜索用时 328 毫秒
801.
B.K. Flint 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(4):135-142
A number of new high efficiency optical coatings have recently been developed at wavelengths in the ultraviolet and vacuum ultraviolet for use in space science experiments. The development of these coatings has resulted in the design and manufacture of wavelength selective filters used in reflectance at near normal incidence that have high VUV reflectance with little near-UV and visible reflectance. Very narrow band transmission filters are now available at wavelengths as short as 1500Å with bandwidths as narrow as 35–40Å. In addition, high efficiency anti-reflection coatings as well as neutral density filters are now available at many wavelengths in this region.Material limitations, more severe than those in the visible, place certain restrictions on the size, wavelength, and degree of selectivity that is achievable with present technology. A summary of the current commercial technology along with the material limitations is presented. 相似文献
802.
Kim N.P. Stanbery B.J. Burgess R.M. Mickelsen R.A. McClelland R.W. King B.D. Gale R.P. 《Aerospace and Electronic Systems Magazine, IEEE》1989,4(11):23-32
Mechanically stacked tandem cells consisting of GaAs thin-film upper cells and CuInSe2 thin-film lower cells have been developed to meet the increasing power needs projected for future spacecraft. The authors report the fabrication of the first highly efficient lightweight GaAs/CuInSe2 tandem cell on a 2-mil thick substrate, update recent performance improvements in thin-film GaAs/CuInSe2 tandem cells, and discuss their application to space power systems. The efficiency of 4-cm2 cells has improved to 21.6% AM0, the highest ever reported for a thin-film photovoltaic cell. Lightweight 4-cm2 tandem cells have been successfully fabricated with efficiencies as high as 20.8%. These cells weighed about 180 mg without optimized substrate trimming. Radiation and operating temperature effects on GaAs/CuInSe2 tandem cells are also discussed, and an interconnect scheme to form a voltage-matched string is described 相似文献
803.
Incremental planning to control time-constrained blackboard-basedproblem solver [vehicle monitoring]
A planner that improves the control decisions made by a blackboard-based problem solver is described. This planner interleaves planning and execution. It sketches out major plan steps but incrementally details actions for only the next step since earlier actions influence how (and whether) to pursue a plan. The planner also reasons about time constraints, revising plans to meet problem-solving deadlines and deciding when it has done enough work on a problem. Implementation of these mechanisms is outlined, and experiments are discussed showing how they reduce overall computation and improve problem solving 相似文献
804.
H.-E. Frhlich V. Müller H. Oleak 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,3(10-12)
Dusty pregalactic Population III objects may provide a mechanism for an effective thermalization of the star radiation. They may generate the observed microwave background and so the high cosmic photon entropy. Assuming a tepid universe a smaller primordial entropy contribution results in reasonable mass scales and amplification factors of pregalactic density fluctuations. 相似文献
805.
B. Nebeling K. Roessler G. Schmitz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1986,6(12):207-210
The redox properties of irradiated liquid and frozen H2O, NH3 and H2O/NH3 mixtures at 298 K and 77 K, resp., towards some simple organic molecules have been checked by injecting carrierfree 11C atoms and analyzing their chemical state by means of radiochromatography. The reactions and the stability of organic products versus radiation dose (in this study by MeV protons) depend on temperature, phase state, mobility of radicals, their concentration and reactivity. Especially dangerous are the reactive OH and O2H radicals which oxidize organic material to inorganic CO2. Highest stability has been found at low temperatures (solid state, reduced mobility of radicals) and for systems containing H-donors (H2O/NH3 mixtures), which reduce the concentration of oxidizing radicals. The fact that many bodies in space consist of H2O-ice with NH3 and CH4 additives at temperatures between 10 and 150 K is promising in view of the survival of organic matter under high doses of radiation. 相似文献
806.
The problem of estimating the time difference of arrival of a signal with unknown spectrum to two receivers is treated. A signal model containing both spectral and delay parameters is derived. The model parameters are computed by a two-step procedure: (1) the modified Yule-Walker equations are used to estimate the autoregressive spectrum of the source signal, and (2) a frequency domain squared error criterion is minimized to provide the delay estimate. The performance of the proposed technique is illustrated by simulation results. 相似文献
807.
S B Curtis W Atwell R Beever A Hardy 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1986,6(11):269-274
In order to make an assessment of radiation risk during manned missions in space, it is necessary first to have as accurate an estimation as possible of the radiation environment within the spacecraft to which the astronauts will be exposed. Then, with this knowledge and the inclusion of body self-shielding, estimations can be made of absorbed doses for various body organs (skin, eye, blood-forming organs, etc.). A review is presented of our present knowledge of the radiation environments and absorbed doses expected for several space mission scenarios selected for our development of the new radiation protection guidelines. The scenarios selected are a 90-day mission at an altitude (450 km) and orbital inclinations (28.5 degrees, 57 degrees and 90 degrees) appropriate for NASA's Space Station, a 15-day sortie to geosynchronous orbit and a 90-day lunar mission. All scenarios chosen yielded dose equivalents between five and ten rem to the blood forming organs if no large solar particle event were encountered. Such particle events could add considerable exposure particularly to the skin and eye for all scenarios except the one at 28.5 degrees orbital inclination. 相似文献
808.
B.N. Swanenburg 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(4):71
The NASA Gamma-Ray Observatory, GRO, will carry two instruments for low energy gamma-ray astronomy. The ‘Oriented Scintillation Spectrometer Experiment - OSSE’ represents the latest step in the evolution of collimated detectors. A large detection area, simultaneous source and background observation and rigorous control over systematic errors yield significant improvements in sensitivity over earlier instruments. The ‘Imaging Compton Telescope - COMPTEL’ brings the proven concept of the Compton telescope to the state of the art level. Position sensitive scintillation detectors make it possible to generate sky images with a resolution of about 2° over a f.o.v. of about 1 sr. The complementary nature of these two experiments promises a first in-depth exploration of the sky in a wavelength range which covers the transition from the X-ray sky to the apparently unrelated high energy gamma-ray sky. Possible directions of further evolution of these experiments will be discussed. 相似文献
809.
Shats S. Bobrovsky B.Z. Shaked U. 《IEEE transactions on aerospace and electronic systems》1988,24(6):670-677
The problem of minimum variance discrete-time state estimation of a continuous-time double integrator via noisy continuous-time measurements is considered. The error covariance matrices of this estimation are calculated and analyzed. The relations between these covariance matrices and the error covariance matrix of the optimal continuous-time filter are obtained, and a way for determining the required sampling period is proposed. A commonly used approximated model is investigated; it is shown to be inappropriate unless a specific improvement is introduced in the model 相似文献
810.
Bhat A.K.S. Biswas A. Iyengar B.S.R. 《IEEE transactions on aerospace and electronic systems》1995,31(3):1186-1193
A series-parallel resonant converter employing (LC)(LC)-type tank circuit operating in lagging power factor (PF) mode is presented and analyzed using complex ac circuit analysis. Design curves are obtained and the converter is optimized under certain constraints. Detailed Space Integrated Control Experiment (SPICE) simulation results are presented to evaluate the performance of the designed converter under varying load conditions. Results obtained from an experimental converter are also presented. The results obtained from the theory, SPICE simulation, and the experimental converter are compared. The proposed converter has high efficiency from full load to very light load (<10%). Switching frequency variation required for a wide change in the load (near load open circuit to full load) is narrow compared with the series resonant converter (SRC) 相似文献