全文获取类型
收费全文 | 4314篇 |
免费 | 7篇 |
国内免费 | 8篇 |
专业分类
航空 | 2204篇 |
航天技术 | 1642篇 |
综合类 | 18篇 |
航天 | 465篇 |
出版年
2018年 | 58篇 |
2017年 | 47篇 |
2016年 | 33篇 |
2014年 | 66篇 |
2013年 | 91篇 |
2012年 | 74篇 |
2011年 | 131篇 |
2010年 | 91篇 |
2009年 | 148篇 |
2008年 | 228篇 |
2007年 | 85篇 |
2006年 | 72篇 |
2005年 | 99篇 |
2004年 | 125篇 |
2003年 | 153篇 |
2002年 | 79篇 |
2001年 | 148篇 |
2000年 | 89篇 |
1999年 | 109篇 |
1998年 | 141篇 |
1997年 | 101篇 |
1996年 | 114篇 |
1995年 | 140篇 |
1994年 | 164篇 |
1993年 | 82篇 |
1992年 | 116篇 |
1991年 | 48篇 |
1990年 | 58篇 |
1989年 | 115篇 |
1988年 | 40篇 |
1987年 | 56篇 |
1986年 | 63篇 |
1985年 | 142篇 |
1984年 | 88篇 |
1983年 | 98篇 |
1982年 | 105篇 |
1981年 | 134篇 |
1980年 | 56篇 |
1979年 | 46篇 |
1978年 | 43篇 |
1977年 | 38篇 |
1976年 | 26篇 |
1975年 | 40篇 |
1974年 | 36篇 |
1973年 | 32篇 |
1972年 | 36篇 |
1971年 | 35篇 |
1969年 | 35篇 |
1967年 | 25篇 |
1966年 | 26篇 |
排序方式: 共有4329条查询结果,搜索用时 109 毫秒
991.
A functional scheme, primary informative signals, a method of electrode system structural parameters synthesis and evaluation of a systematic error of the ion-marking sensor for aerodynamic angle and airspeed measurement are the focus of this paper. 相似文献
992.
Guiping Feng S. Jin T. Zhang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
Sea level changes are threatening the human living environments, particularly along the European Coasts with highly dense population. In this paper, coastal sea level changes in western and southern Europe are investigated for the period 1993–2011 using Global Positioning System (GPS), Tide Gauge (TG), Satellite Altimetry (SA), Gravity Recovery and Climate Experiment (GRACE) and geophysical models. The mean secular trend is 2.26 ± 0.52 mm/y from satellite altimetry, 2.43 ± 0.61 mm/y from TG+GPS and 1.99 ± 0.67 mm/y from GRACE mass plus steric components, which have a remarkably good agreement. For the seasonal variations, annual amplitudes of satellite altimetry and TG+GPS results are almost similar, while GRACE Mass+Steric results are a little smaller. The annual phases agree remarkably well for three independent techniques. The annual cycle is mainly driven by the steric contributions, while the annual phases of non-steric (mass component) sea level changes are almost a half year later than the steric sea level changes. 相似文献
993.
Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments 总被引:1,自引:0,他引:1
Paul B. Niles David C. Catling Gilles Berger Eric Chassefière Bethany L. Ehlmann Joseph R. Michalski Richard Morris Steven W. Ruff Brad Sutter 《Space Science Reviews》2013,174(1-4):301-328
Ongoing research on martian meteorites and a new set of observations of carbonate minerals provided by an unprecedented series of robotic missions to Mars in the past 15 years help define new constraints on the history of martian climate with important crosscutting themes including: the CO2 budget of Mars, the role of Mg-, Fe-rich fluids on Mars, and the interplay between carbonate formation and acidity. Carbonate minerals have now been identified in a wide range of localities on Mars as well as in several martian meteorites. The martian meteorites contain carbonates in low abundances (<1 vol.%) and with a wide range of chemistries. Carbonates have also been identified by remote sensing instruments on orbiting spacecraft in several surface locations as well as in low concentrations (2–5 wt.%) in the martian dust. The Spirit rover also identified an outcrop with 16 to 34 wt.% carbonate material in the Columbia Hills of Gusev Crater that strongly resembled the composition of carbonate found in martian meteorite ALH 84001. Finally, the Phoenix lander identified concentrations of 3–6 wt.% carbonate in the soils of the northern plains. The carbonates discovered to date do not clearly indicate the past presence of a dense Noachian atmosphere, but instead suggest localized hydrothermal aqueous environments with limited water availability that existed primarily in the early to mid-Noachian followed by low levels of carbonate formation from thin films of transient water from the late Noachian to the present. The prevalence of carbonate along with evidence for active carbonate precipitation suggests that a global acidic chemistry is unlikely and a more complex relationship between acidity and carbonate formation is present. 相似文献
994.
V.V. Paznukhov V.G. Galushko B.W. Reinisch 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
A technique for studying ionospheric wavelike phenomena, primarily AGW/TID events, is developed based on the solution of the problem of radio wave propagation in ionospheric plasma disturbed by wavelike processes. A perfectly reflecting surface model is used for representing TIDs propagating at ionospheric heights. This technique is a generalization of the Frequency-and-Angular Sounding (FAS) method developed earlier for oblique TID diagnostics using transmitters of opportunity. Trial measurements were made in November 2003 with two DPS-4 systems at Millstone Hill Observatory, providing experimental validation of the developed method by comparing the results of disturbance diagnostics to those simultaneously obtained with the original (oblique) FAS method. The TID parameters recovered during the November 2003 campaign suggest that the observed disturbances predominately propagated equatorward which likely indicates their sources to be in the auroral region. The equatorward propagating AGW/TIDs are typical for disturbed geomagnetic conditions which were observed during the campaign. Implementation of the generalized FAS technique in the DPS sounder allowed development of a dedicated data acquisition system for ionospheric disturbance diagnostics. Routine measurements with the developed technique using the existing world-wide network of Digisondes (GIRO) will make it possible to conduct large-scale studies of the AGW/TID phenomena. 相似文献
995.
Performance of 10- and 20-target MSE classifiers 总被引:2,自引:0,他引:2
Novak L.M. Owirka G.J. Brower W.S. 《IEEE transactions on aerospace and electronic systems》2000,36(4):1279-1289
MIT Lincoln Laboratory is responsible for developing the ATR (automatic target recognition) system for the DARPA-sponsored SAIP program; the baseline ATR system recognizes 10 GOB (ground order of battle) targets; the enhanced version of SAIP requires the ATR system to recognize 20 GOB targets. This paper presents ATR performance results for 10- and 20-target mean square error (MSE) classifiers using high-resolution SAR (synthetic aperture radar) imagery. 相似文献
996.
Uttam Kumar S. Kumar Raja Chiranjit Mukhopadhyay T.V. Ramachandra 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
Variable Endmember Constrained Least Square (VECLS) technique is proposed to account endmember variability in the linear mixture model by incorporating the variance for each class, the signals of which varies from pixel to pixel due to change in urban land cover (LC) structures. VECLS is first tested with a computer simulated three class endmember considering four bands having small, medium and large variability with three different spatial resolutions. The technique is next validated with real datasets of IKONOS, Landsat ETM+ and MODIS. The results show that correlation between actual and estimated proportion is higher by an average of 0.25 for the artificial datasets compared to a situation where variability is not considered. With IKONOS, Landsat ETM+ and MODIS data, the average correlation increased by 0.15 for 2 and 3 classes and by 0.19 for 4 classes, when compared to single endmember per class. 相似文献
997.
D Leugner T Streibel H Rocher G Reitz W Heinrich 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,22(4):511-515
Stacks of CR-39 plastic nuclear track detectors were mounted inside the MIR-station during the EUROMIR-94-mission. We present LET-spectra determined separately for long range cosmic ray heavy ions and for short range target fragments produced in nuclear interactions of cosmic rays and measured charge distributions for relativistic and stopping particles. 相似文献
998.
In order to realize the operational and service cost savings through the use of rechargeable batteries, the dismounted soldier is burdened with the weight, volume and/or charging logistics of the batteries. By providing the soldier with a high energy density source and a lightweight compact battery charger, the burden imposed by rechargeable batteries in the forward field can be minimized. Zinc-air batteries have the potential for meeting the energy demands of forward battlefield charging. They are attitude insensitive, have a high specific energy and are inherently inexpensive, lightweight and safe 相似文献
999.