全文获取类型
收费全文 | 5562篇 |
免费 | 19篇 |
国内免费 | 18篇 |
专业分类
航空 | 2565篇 |
航天技术 | 2075篇 |
综合类 | 191篇 |
航天 | 768篇 |
出版年
2021年 | 31篇 |
2019年 | 36篇 |
2018年 | 79篇 |
2017年 | 44篇 |
2016年 | 48篇 |
2014年 | 111篇 |
2013年 | 135篇 |
2012年 | 120篇 |
2011年 | 190篇 |
2010年 | 122篇 |
2009年 | 212篇 |
2008年 | 286篇 |
2007年 | 134篇 |
2006年 | 130篇 |
2005年 | 148篇 |
2004年 | 151篇 |
2003年 | 166篇 |
2002年 | 204篇 |
2001年 | 227篇 |
2000年 | 90篇 |
1999年 | 137篇 |
1998年 | 162篇 |
1997年 | 108篇 |
1996年 | 151篇 |
1995年 | 181篇 |
1994年 | 152篇 |
1993年 | 92篇 |
1992年 | 128篇 |
1991年 | 59篇 |
1990年 | 56篇 |
1989年 | 123篇 |
1988年 | 45篇 |
1987年 | 51篇 |
1986年 | 55篇 |
1985年 | 203篇 |
1984年 | 155篇 |
1983年 | 111篇 |
1982年 | 135篇 |
1981年 | 188篇 |
1980年 | 59篇 |
1979年 | 44篇 |
1978年 | 42篇 |
1977年 | 44篇 |
1975年 | 46篇 |
1974年 | 43篇 |
1973年 | 38篇 |
1972年 | 39篇 |
1971年 | 34篇 |
1970年 | 39篇 |
1969年 | 45篇 |
排序方式: 共有5599条查询结果,搜索用时 15 毫秒
221.
222.
Experiments of prolonged bedrest in antiorthostatic position are conducted in order to simulated cardio-circulatory modifications observed in weightlessness. Until now, no studies of r.C.B.F. have been effected in these conditions. Six young, healthy volunteers (average age 23.8) were placed in strict bedrest and in antiorthostatic position -4 degrees for 7 days. The r.C.B.F. measurements were studied by 133Xe inhalation method using a 32 detectors system. Studies were made first in basal conditions, then between the 6th and 12th hr, and finally between the 72nd and the 78th hr after the beginning of the experiment. Three of the subjects received 0.450 mg of Clonidine daily during the experiment. In the subjects having taken no Clonidine, we observed a constant increase in r.C.B.F. (12, 17 and 16% respectively) in the first 12 hr; at the 72nd hour, all values had returned to basal state. This findings agrees with the well known notion of a rapid correction of hemodynamic disturbances observed in the first days of weightlessness. In the subjects treated with Clonidine, the increase of r.C.B.F. did not occur. Several mechanisms of action are possible; the Clonidine affecting either the heart by inhibiting volemic atrial receptors or the brain by direct vasoconstriction. 相似文献
223.
P. B. Price 《Space Science Reviews》1973,15(1):69-88
The composition of cosmic rays and solar particles is reviewed with emphasis on the question of whether they are representative samples of Galactic and solar matter. The composition of solar particles changes with energy and from flare to flare. A strong excess of heavy elements at energies below a few MeV/nuc decreases with energy, and at energies above 15 MeV/nuc the composition of solar particles resembles that of galactic cosmic rays somewhat better than that of the solar atmosphere. The elements Ne through Pb have remarkably similar abundances in cosmic ray sources and in the matter of the solar system. The lighter elements are depleted in cosmic rays, whereas U and Th may be enriched or not, depending on whether the meteoritic or solar abundance of Th is used. Two prototype sources of cosmic rays are considered: gas with solar system composition but enriched in elements with Z > 8 during acceleration and emission (by analogy with solar particle emission), and highly evolved matter enriched in r-process elements such as U, Th and transuranic elements. The energy-dependence of cosmic ray composition suggests that both sources may contribute at different energies.Miller Institute Professor, 1972–73. 相似文献
224.
A summary of research and development work on high-power microwave filters is given here: first, design considerations in high-power microwave filters?power handling capacity, waveform distortion, multimode suppression, and stop and pass band characteristics; second, a discussion of filtering devices and techniques in two broad categories-dominant mode filters and harmonic filters (discussion covers methods of operation and significant characteristics of each filter developed to date); and third, a discussion of applications and limitations of filters in various systems and interference situations. Also included are the availability, cost, size, weight, etc. as functions of frequency for the various filter types. 相似文献
225.
Ground penetrating radar VIY-2 总被引:2,自引:0,他引:2
Prokhorenko V.P. Ivashchuk V.E. Korsun S.V. 《Aerospace and Electronic Systems Magazine, IEEE》2005,20(7):16-18
VIY-2 ground penetrating radar (GPR) with unique sounding possibilities and use simplicity is presented at this paper. VIY-2 GPR combines all units (synchronizer, transmitting and receiving modules, powering, and antenna system) into single case. The VIY-2 GPR communicates with computer via standard interface RS232 or USB1.0. Technical solutions utilized by the VIY-2 GPR reduce deployment time and simplify surveying process. The VIY-2 GPR design features and its components interaction are considered at this paper. Some field results are also presented here. The VIY-2 GPR design concept allows reducing the data acquisition time,, optimizing the time-varying gain control function, applying depth-stacking dependence, controlling the surveying window position and interference reducing by pulse repetition frequency randomizing. 相似文献
226.
Basic equations of hydrodynamics in an elastic pipeline that are found using the vectorial notation of Newton’s law for a viscous fluid are presented. An analog of Poiseuille’s formula for the fluid flowrate in the elastic pipeline and the results of the comparative analysis of a viscous flow in elastic and rigid pipelines are given. It is shown that the ratio of the maximal fluid velocity in the pipeline cross section and the section-average velocity is similar for elastic and rigid pipelines. 相似文献
227.
S. Carnochan P. Moore S. Ehlers 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,16(12)
Achieving orbital accuracies in the radial direction for ERS-1 commensurate with those for TOPEX/Poseidon is of utmost importance for the integration of the two altimeter data sets. This paper outlines a procedure whereby the radial orbit error for ERS-1 is recovered as a time series expansion in the form of a finite Fourier series with additional terms for atmospheric drag, solar radiation pressure, and initial state vector mismodelling. Using a least squares collocation method with constraints derived from the JGM2 gravity field co-variance matrix, the radial error is recovered using both dual crossovers and ERS-1 single satellite crossovers. Aggregate arcs are then used to derive the ERS-1 orbit error over the repeat period of 35 days. The results are presented in the improvement of fit in the dual crossover, ERS-1 crossover and altimetry data sets as well as the recovery of an altimeter bias for the two satellites. 相似文献
228.
Improving GPS-Based Landing System Performance using an Empirical Barometric Altimeter Confidence Bound 总被引:2,自引:0,他引:2
Shau-Shiun Jan Gebre-Egziabher D. Walter T. Enge P. 《IEEE transactions on aerospace and electronic systems》2008,44(1):127-146
This paper develops an empirical confidence bound for barometric altimeter altitude errors and shows how this bound may improve the performance of GPS-based approach and landing systems. This empirical bound is developed using historical meteorological data collected at a set of geographically diverse locations over a thirty year period. The confidence bound developed is shown to provide a Gaussian overbound on altimeter altitude errors in standard atmospheric conditions between a 10-5 and 10-6 confidence level. This confidence bound is integrated into the standard methodology for analyzing the performance of GPS-based landing systems and the results of a performance trade study using the confidence bound are presented. The results show that incorporating the empirical barometric altimeter confidence bound provides an increase in the coterminous United States (CONUS) service volume for lateral precision with vertical guidance (LPV) type approaches. While this increase is approximately 2% for an L1 single-frequency GPS user, it jumps to roughly 40% for an L5 single-frequency user. 相似文献
229.
The considerable evidence that Mars once had a wetter, more clement, environment motivates the search for past or present life on that planet. This evidence also suggests the possibility of restoring habitable conditions on Mars. While the total amounts of the key molecules--carbon dioxide, water, and nitrogen--needed for creating a biosphere on Mars are unknown, estimates suggest that there may be enough in the subsurface. Super greenhouse gases, in particular, perfluorocarbons, are currently the most effective and practical way to warm Mars and thicken its atmosphere so that liquid water is stable on the surface. This process could take approximately 100 years. If enough carbon dioxide is frozen in the South Polar Cap and absorbed in the regolith, the resulting thick and warm carbon dioxide atmosphere could support many types of microorganisms, plants, and invertebrates. If a planet-wide martian biosphere converted carbon dioxide into oxygen with an average efficiency equal to that for Earth's biosphere, it would take > 100,000 years to create Earth-like oxygen levels. Ethical issues associated with bringing life to Mars center on the possibility of indigenous martian life and the relative value of a planet with or without a global biosphere. 相似文献
230.
Musal H.M. Thomas P.D. Vinokur M. 《IEEE transactions on aerospace and electronic systems》1970,(2):252-254
The evaluation of the double convolution integrat involved in the expression of the radar response to scattering from a turbulent ionized wake is simplified by the approximation technique presented here, so that the Doppler spectrum parameters can be explicitly expressed in terms of the wake characteristics. 相似文献