全文获取类型
收费全文 | 395篇 |
免费 | 1篇 |
国内免费 | 2篇 |
专业分类
航空 | 154篇 |
航天技术 | 81篇 |
综合类 | 3篇 |
航天 | 160篇 |
出版年
2021年 | 3篇 |
2019年 | 5篇 |
2018年 | 4篇 |
2017年 | 5篇 |
2016年 | 2篇 |
2015年 | 3篇 |
2014年 | 8篇 |
2013年 | 16篇 |
2012年 | 7篇 |
2011年 | 38篇 |
2010年 | 15篇 |
2009年 | 31篇 |
2008年 | 19篇 |
2007年 | 19篇 |
2006年 | 19篇 |
2005年 | 11篇 |
2004年 | 7篇 |
2003年 | 17篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1991年 | 4篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1968年 | 6篇 |
1967年 | 7篇 |
1966年 | 3篇 |
1965年 | 1篇 |
1963年 | 1篇 |
排序方式: 共有398条查询结果,搜索用时 9 毫秒
201.
Since 1972, nearly continuous observations of coronal holes and their associated photospheric magnetic fields have been made using a variety of satellite and ground-based equipment. The results of comparisons of these observations are reviewed and it is demonstrated that the structure and evolution of coronal holes is basically governed by the large-scale distribution of photospheric magnetic flux. Non-polar holes form in the decaying remnants of bipolar magnetic regions in areas with a large-scale flux imbalance. There is strong indirect evidence that the magnetic field in coronal holes is always open to interplanetary space but not all open-field regions have associated coronal holes. The well-observed declining phase of the last solar cycle was characterized by stable magnetic field and coronal hole patterns which were associated with recurrent, high-speed wind streams and interplanetary magnetic field patterns at the Earth. The ascending phase of the current cycle has been characterized by transient magnetic field and coronal hole patterns which tend to occur at high solar latitudes. This shift in magnetic field and coronal hole patterns has resulted in a less obvious and more complicated association with high-speed wind streams at the Earth.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Visiting Scientist, Kitt Peak National Observatory. 相似文献
202.
单转子风扇的三维反问题气动设计 总被引:3,自引:1,他引:3
采用准三维流函数反问题设计方法与三维 N-S求解方法的相互迭代对单转子风扇进行气动设计。在叶片初步计算中得到叶栅进出口气流角沿径向分布 ,并将它作为本文的目标函数。采用准三维反问题求解方法 ,依次构造出各个 S1流面上的叶片几何形状和气流角分布。然后再采用 N-S方程的求解方法 ,对叶片进行全三维流场的数值计算。通过 N-S方程计算结果与目标函数的对比 ,重新修正叶片出口气流角分布 ,并作为下一次反问题设计的目标函数。经过反问题与 N-S方程求解的反复迭代 ,最终得到满足设计要求的叶型。 相似文献
203.
John Bosco Habarulema Lee-Anne McKinnell Pierre J. Cilliers Ben D.L. Opperman 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
The propagation of radio signals in the Earth’s atmosphere is dominantly affected by the ionosphere due to its dispersive nature. Global Positioning System (GPS) data provides relevant information that leads to the derivation of total electron content (TEC) which can be considered as the ionosphere’s measure of ionisation. This paper presents part of a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Vertical total electron content (VTEC) was calculated for four GPS receiver stations using the Adjusted Spherical Harmonic (ASHA) model. Factors that influence TEC were then identified and used to derive input parameters for the NN. The well established factors used are seasonal variation, diurnal variation, solar activity and magnetic activity. Comparison of diurnal predicted TEC values from both the NN model and the International Reference Ionosphere (IRI-2001) with GPS TEC revealed that the IRI provides more accurate predictions than the NN model during the spring equinoxes. However, on average the NN model predicts GPS TEC more accurately than the IRI model over the GPS locations considered within South Africa. 相似文献
204.
205.
206.
John Heise 《Space Science Reviews》1985,40(1-2):79-90
Hot white dwarfs are objects that copiously emit in the Extreme Ultraviolet and soft X-ray range. They are the brightest sources seen in the Low Energy Telescope of EXOSAT, with countrates up to 25 cnts/s. in contrast to their optical and UV spectrum the total flux and spectral distribution at soft X-ray energies are highly sensitive to the effective temperature, structure and elemental composition of the dwarf's atmosphere. The imaging soft X-ray experiments onboard EXOSAT cover with large sensitivity the spectral region where the peak of emission of hot white dwarfs is expected to occur.I here review some of the (preliminary) results obtained so far with broadband X-ray photometry on a dozen or so white dwarfs, and some of the high-resolution spectra obtained for three white dwarfs with the grating spectrometers. 相似文献
207.
Raymond M. Wheeler Gary W. Stutte Cheryl L. Mackowiak Neil C. Yorio John C. Sager William M. Knott 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(5):798-806
Plants can provide a means for removing carbon dioxide (CO2) while generating oxygen (O2) and clean water for life support systems in space. To study this, 20 m2 stands of potato (Solanum tuberosum L.) plants were grown in a large (113 m3 vol.), atmospherically closed chamber. Photosynthetic uptake of CO2 by the stands was detected about 10 DAP (days after planting), after which photosynthetic rates rose rapidly as stand ground cover and total light interception increased. Photosynthetic rates peaked ca. 50 DAP near 45 μmol CO2 m−2 s−1 under 865 μmol m−2 s−1 PPF (average photosynthetic photon flux), and near 35 μmol CO2 m−2 s−1 under 655 μmol m−2 s−1 PPF. Short term changes in PPF caused a linear response in stand photosynthetic rates up to 1100 μmol m−2 s−1 PPF, with a light compensation point of 185 μmol m−2 s−1 PPF. Comparisons of stand photosynthetic rates at different CO2 concentrations showed a classic C3 response, with saturation occurring near 1200 μmol mol−1 CO2 and compensation near 100 μmol mol−1 CO2. In one study, the photoperiod was changed from 12 h light/12 h dark to continuous light at 58 DAP. This caused a decrease in net photosynthetic rates within 48 h and eventual damage (scorching) of upper canopy leaves, suggesting the abrupt change stressed the plants and/or caused feedback effects on photosynthesis. Dark period (night) respiration rates increased during early growth as standing biomass increased and peaked near 9 μmol CO2 m−2 s−1 ca. 50 DAP, after which rates declined gradually with age. Stand transpiration showed a rapid rise with canopy ground cover and peaked ca. 50 DAP near 8.9 L m−2 d−1 under 860 μmol m−2 s−1 PPF and near 6.3 L m−2 d−1 under 650 μmol m−2 s−1 PPF. Based on the best photosynthetic rates from these studies, approximately 25 m2 of potato plants under continuous cultivation would be required to support the CO2 removal and O2 requirements for one person. 相似文献
208.
209.
John M. Logsdon 《Space Policy》1997,13(4):273-277
The US space program has emerged from a difficult period of budget cuts and uncertainty about its goals leaner, fitter and much more focused. This contrasts with the world's other major spacefaring countries, which find themselves either the victim of severe economic circumstances (Russia), politically floundering (Europe) or no longer certain of their priorities (Japan). The author briefly appraises each major programme before analysing the current US success. He warns that leadership must not be squandered as it was in some fields after Apollo. 相似文献
210.
This paper develops a set of mathematical tools for system design or analysis of a type of pseudorandom coded ranging system used in several space programs. Certain probabilities of failure of the system to perform the ranging function and the times required for the system to perform prescribed ranging functions are defined and related to system parameters. A set of sample calculations is presented for clarification of the computational techniques. 相似文献