全文获取类型
收费全文 | 6967篇 |
免费 | 22篇 |
国内免费 | 13篇 |
专业分类
航空 | 3173篇 |
航天技术 | 2471篇 |
综合类 | 195篇 |
航天 | 1163篇 |
出版年
2021年 | 56篇 |
2019年 | 40篇 |
2018年 | 139篇 |
2017年 | 92篇 |
2016年 | 96篇 |
2015年 | 48篇 |
2014年 | 174篇 |
2013年 | 189篇 |
2012年 | 198篇 |
2011年 | 271篇 |
2010年 | 180篇 |
2009年 | 303篇 |
2008年 | 392篇 |
2007年 | 189篇 |
2006年 | 167篇 |
2005年 | 204篇 |
2004年 | 181篇 |
2003年 | 231篇 |
2002年 | 234篇 |
2001年 | 285篇 |
2000年 | 106篇 |
1999年 | 167篇 |
1998年 | 190篇 |
1997年 | 142篇 |
1996年 | 172篇 |
1995年 | 212篇 |
1994年 | 187篇 |
1993年 | 102篇 |
1992年 | 152篇 |
1991年 | 75篇 |
1990年 | 60篇 |
1989年 | 133篇 |
1988年 | 59篇 |
1987年 | 63篇 |
1986年 | 63篇 |
1985年 | 180篇 |
1984年 | 162篇 |
1983年 | 129篇 |
1982年 | 155篇 |
1981年 | 202篇 |
1980年 | 64篇 |
1979年 | 48篇 |
1978年 | 50篇 |
1977年 | 41篇 |
1976年 | 41篇 |
1975年 | 38篇 |
1974年 | 47篇 |
1972年 | 41篇 |
1971年 | 40篇 |
1970年 | 39篇 |
排序方式: 共有7002条查询结果,搜索用时 15 毫秒
401.
A new current injected equivalent circuit approach (CIECA) to modeling switching dc-dc converter power stages is developed, which starts with the current injected approach and results in either a set of equations which completely describe input and out-put properties or an equivalent linear circuit model valid at small signal, low frequency levels. This approach to modeling switching dc-dc converter power stages has the merits but not the demerits of both the electronic equivalent circuit state space average approach and the current injected control type approach, namely, 1) the modeling is very clear and is simple whether the converter operates in continuous or discontinuous inductor conduction modes, 2) the modeling results in an equivalent circuit which is very close to the actual converter, and 3) the equivalent circuit can be used directly in the computer for theoretical predictions like SPICE, etc. 相似文献
402.
403.
Flutter and time errors are critical factors in all instrumentation recording. This paper presents analytically the relationship between flutter, time base error, and time base error difference (sometimes called jitter), and the effects of these factors on direct and FM recording. Methods of measuring these quantities are discussed and experimental examples are given. Finally, a measurement method for the interchannel time error is presented, and some trends of correlation among all channels are discussed. 相似文献
404.
405.
The Discrete Address Beacon System (DABS) is scheduled to replace the Air Traffic Control Beacon System (ATCRBS) as the prime sensing tool for air traffic control in the 1980's. The design of DABS requires knowledge of the tracking accuracy obtainable as a function of various system parameters. In particular, the potential value of air-to-ground telemetry of turn-rate data and its effect on intermittent positive control (IPC) must be assessed. The design of DABS tracking algorithms is discussed, and the results obtained with a Kalman tracker simulation are given. The major finding is that turn-rate telemetry of moderate accuracy can improve tracking by a factor of 6 to 10. 相似文献
406.
This paper analyzes a hybrid navigation concept that uses signals from a radio interferometer mounted on a spinning geostationary satellite, preliminary position estimates from self-contained equipment, and stored a priori information on the past performance of this equipment. The craft-borne processor, optimum in the maximum a posteriori (MAP) sense, is designed to estimate position coordinates using only the incoming radio signals, although improved estimates result if the other two items are available. An error analysis starts with the derivation of an estimation error covariance matrix, whose elements depend on additive receiver noise and the physical parameters of the system. A minimum 1? estimation error in position is obtained by trading off these parameters. The effects of other major error sources, such as tropospheric phase fluctuations, multipath, and craft altitude uncertainty, are added to the estimation error to give a total 1? position error on the order of 3.7 to 5.6 km. 相似文献
407.
J.S. Kaastra R. Lieu T. Tamura F.B.S. Paerels J.W. den Herder 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(12):2504-2508
We discuss the detection of soft excess X-ray emission in a sample of 19 clusters of galaxies observed by XMM-Newton. In 6/19 clusters evidence for a soft X-ray excess is found. Four of these clusters show soft X-ray and O VII line emission from gas with a temperature of 0.2 keV. The centroid of this oxygen line is consistent with the redshift of the cluster. The intensity and spatial extend of the soft excess agrees with previous PSPC measurements. These observations are interpreted as emission from warm-hot intergalactic medium filaments, with density enhancements near the cluster centers, consistent with theoretical predictions. In the other two soft excess clusters a non-thermal origin is consistent with the data. 相似文献
408.
P.B. Marty S. Bardeau O. Czoske H. Ebeling J.P. Kneib R. Sadat I. Smail 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(12):2509-2515
We present an analysis of seven clusters observed by XMM-Newton as part of our survey of 17 most X-ray luminous clusters of galaxies at z 0.2 selected for a comprehensive and unbiased study of the mass distribution in massive clusters. Using the public software FTOOLS and XMMSAS we have set up an automated pipeline to reduce the EPIC MOS and pn spectro-imaging data, optimized for extended sources analysis. We also developped a code to perform intensive spectral and imaging analysis particularly focussing on proper background estimate and removal. XMM-Newton deep spectro-imaging of these clusters allowed us to fit a standard β-model to their gas emission profiles as well as a standard MEKAL emission model to their extracted spectra, and test their inferred characteristics against already calibrated relations. 相似文献
409.
F A Cucinotta W Schimmerling J W Wilson L E Peterson P B Saganti J F Dicello 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(6):1383-1389
Methods used to project risks in low-Earth orbit are of questionable merit for exploration missions because of the limited radiobiology data and knowledge of galactic cosmic ray (GCR) heavy ions, which causes estimates of the risk of late effects to be highly uncertain. Risk projections involve a product of many biological and physical factors, each of which has a differential range of uncertainty due to lack of data and knowledge. Using the linear-additivity model for radiation risks, we use Monte-Carlo sampling from subjective uncertainty distributions in each factor to obtain an estimate of the overall uncertainty in risk projections. The resulting methodology is applied to several human space exploration mission scenarios including a deep space outpost and Mars missions of duration of 360, 660, and 1000 days. The major results are the quantification of the uncertainties in current risk estimates, the identification of factors that dominate risk projection uncertainties, and the development of a method to quantify candidate approaches to reduce uncertainties or mitigate risks. The large uncertainties in GCR risk projections lead to probability distributions of risk that mask any potential risk reduction using the "optimization" of shielding materials or configurations. In contrast, the design of shielding optimization approaches for solar particle events and trapped protons can be made at this time and promising technologies can be shown to have merit using our approach. The methods used also make it possible to express risk management objectives in terms of quantitative metrics, e.g., the number of days in space without exceeding a given risk level within well-defined confidence limits. 相似文献
410.
Experimental drop tube of the metallurgy department of Grenoble 总被引:1,自引:0,他引:1
J. P. Praizey B. Vinet J. Bost 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1988,8(12):155-161
The drop tube which will be available in the “Centre d'Etudes Nucléaires de Grenoble” is described. Its main features are the following: - Dimensions : Drop height : 47.1 m Drop time : 3.1 s Tube inside diameter : 0.2 m - Experimental atmosphere : 1 Ultra-vacuum : 10−6 to 10−7 Pa - Residual gravity level : 10−8 to 10−9 g according to the vacuum level and drop diameter.
This facility is unique insofar as it enables experiments to be performed under ultra-vacuum conditions which, by delaying the formation of surface oxides, should contribute to improving maximum undercooling values.
The techniques used for obtaining small metallic drops (0.5 to 3 mm) are described. The availability of this instrument for the scientific community is also foreseen by the french sponsoring organizations (CEA, CNES, CNRS) ; some practicle informations will be given to potential experimenters. 相似文献