全文获取类型
收费全文 | 1998篇 |
免费 | 303篇 |
国内免费 | 247篇 |
专业分类
航空 | 1068篇 |
航天技术 | 1038篇 |
综合类 | 125篇 |
航天 | 317篇 |
出版年
2024年 | 5篇 |
2023年 | 49篇 |
2022年 | 54篇 |
2021年 | 94篇 |
2020年 | 106篇 |
2019年 | 98篇 |
2018年 | 93篇 |
2017年 | 69篇 |
2016年 | 82篇 |
2015年 | 89篇 |
2014年 | 157篇 |
2013年 | 131篇 |
2012年 | 129篇 |
2011年 | 167篇 |
2010年 | 126篇 |
2009年 | 161篇 |
2008年 | 142篇 |
2007年 | 102篇 |
2006年 | 67篇 |
2005年 | 107篇 |
2004年 | 48篇 |
2003年 | 42篇 |
2002年 | 44篇 |
2001年 | 46篇 |
2000年 | 42篇 |
1999年 | 36篇 |
1998年 | 41篇 |
1997年 | 37篇 |
1996年 | 18篇 |
1995年 | 24篇 |
1994年 | 41篇 |
1993年 | 17篇 |
1992年 | 24篇 |
1991年 | 22篇 |
1990年 | 16篇 |
1989年 | 9篇 |
1988年 | 11篇 |
1987年 | 1篇 |
1986年 | 1篇 |
排序方式: 共有2548条查询结果,搜索用时 875 毫秒
431.
单相机迎角测量中振动影响研究 总被引:1,自引:1,他引:0
从描述像空间坐标与物理空间坐标关系的共线方程出发,推导出单相机迎角测量中模型迎角关于相机外方位元素的计算公式。在单相机迎角测量中模型表面标记点展向坐标Y不变的假设下,研究了2m量级风洞中,相机位于5个不同位置时,3个角外方位元素(φ,ω,κ)和模型侧滑角β在-2°~2°之间振动对迎角测量结果的影响。模拟结果表明:φ和κ的振动对迎角测量结果影响较大,且该影响对相机位置和模型迎角的变化不敏感;在相机位于法向投影位置(相机光轴与Y轴平行)时,ω与β的振动对迎角测量结果影响非常小,可以忽略,但影响量随相机位置偏离法向投影位置迅速增加。 相似文献
432.
航空发动机导流叶片角度调节参数变化故障分析 总被引:1,自引:1,他引:0
某型发动机低压压气机进口导流叶片角度α1,其电子调节部分执行机构电磁活门的占空比(Sα),在整个调节过程中是起关键作用的重要参数,但在台架试车过程中曾多台次出现在不同次检查时Sα数值变化的故障。通过对α1主调节系统工作原理的深入分析,确定试车中Sα变化故障是由于燃油调节机构掺混入空气所致,并得到试车验证。本研究对发动机试车中类似故障的排除具有重要的参考意义。 相似文献
433.
434.
N.C. Joshi W. Uddin A.K. Srivastava R. Chandra N. Gopalswamy P.K. Manoharan M.J. Aschwanden D.P. Choudhary R. Jain N.V. Nitta H. Xie S. Yashiro S. Akiyama P. Mäkelä P. Kayshap A.K. Awasthi V.C. Dwivedi K. Mahalakshmi 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
435.
Sneha Yadav R.S. Dabas Rupesh M. Das A.K. Upadhayaya A.K. Gwal 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
The paper deals with the study of temporal and spatial variation of equatorial ionization anomaly (EIA) phenomenon along with its dependence on solar activity and season during the 19th solar cycle by using seven Indian ionosonde stations. Present study is an attempt to carry out the comprehensive study of EIA by using the limited number of ground based instruments. This has been achieved by performing the Gaussian fitting over the latitudinal distribution of F2-region critical frequency (foF2) data. Results reveal that the phenomenon of EIA has a strong dependence on solar activity and seasons. The EIA crest exhibits the feature of latitudinal shifting and expansion with increasing solar activity. It is found out that the effect of solar cycle and seasons on EIA is local time dependent. The observations were also compared with the IRI-2001 model predictions and results reveal that the model values are in general agreement with the observed values with some discrepancies, particularly during the high solar activity period and morning sector. The results have been discussed in the light of relative contribution from transequatorial interhemispheric neutral wind and strength of equatorial fountain process during different local time, season and solar activity levels. Furthermore, an attempt is made to parameterize the location and foF2 of the EIA crest by using the regression analysis. These results can be used to predict the latitudinal position and foF2 of the EIA crest for any given 12-month running average sunspot number (R12). 相似文献
436.
Xinan Yue William S. SchreinerChristian Rocken Ying-Hwa Kuo 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
During 2008, the solar activity is extremely low. The satellite observations show that the ionospheric height and electron density is much lower than the predictions by the international reference ionosphere (IRI) model. In this paper, we compared the slant total electron content (TEC) observed by the COSMIC satellites during 2008 with the IRI model results. It is found that the IRI model with IRI2001 and IRI2001 Cor. topside options will always overestimate the electron density in both lower and higher altitudes. But the rest two topside options (NeQuick, and TTS) tend to overestimate the electron density in the F layer and underestimate it in the topside altitudes. The switch altitude between overestimation and underestimation and the latitude-local time distribution of the model deviation depend on the topside option. The current investigation might be useful for the model improvement as well as data assimilation work based on the IRI model and the LEO TEC data. 相似文献
437.
B.A. Maruca J.C. Kasper 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
The Wind spacecraft’s Faraday cups (FC) continue to produce high-quality, in situ observations of thermal protons (i.e., ionized hydrogen) and α-particles (i.e., fully ionized helium) in the solar wind. By fitting a Wind/FC ion spectrum with a model velocity distribution function (VDF) for each particle species, values for density, bulk velocity, and temperature can be inferred. Incorporating measurements of the background magnetic field from the Wind Magnetic Field Investigation (MFI) allows perpendicular and parallel temperature components to be separated. Prior implementations of this analysis averaged the higher-cadence Wind/MFI measurements to match that of the Wind/FC ion spectra. However, this article summarizes recent and extensive revisions to the analysis software that, among other things, eliminate such averaging and thereby account for variations in the direction of the magnetic field over the time taken to measure the ions. A statistical comparison reveals that the old version consistently underestimates the temperature anisotropy of ion VDF’s: averaging over fluctuations in the magnetic field essentially blurs the perpendicular and parallel temperature components, which makes the plasma seem artificially more isotropic. The new version not only provides a more accurate dataset of ion parameters (which is well suited to the study of microkinetic phenomena), it also demonstrates a novel technique for jointly processing particle and field data. Such methods are crucial to heliophysics as wave-particle interactions are increasingly seen as playing an important role in the dynamics of the solar wind and similar space plasmas. 相似文献
438.
R. Bewick C. LückingC. Colombo J.P. SanchezC.R. McInnes 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
This paper examines the concept of a Sun-pointing elliptical Earth ring comprised of dust grains to offset global warming. A new family of non-Keplerian periodic orbits, under the effects of solar radiation pressure and the Earth’s J2 oblateness perturbation, is used to increase the lifetime of the passive cloud of particles and, thus, increase the efficiency of this geoengineering strategy. An analytical model is used to predict the orbit evolution of the dust ring due to solar-radiation pressure and the J2 effect. The attenuation of the solar radiation can then be calculated from the ring model. In comparison to circular orbits, eccentric orbits yield a more stable environment for small grain sizes and therefore achieve higher efficiencies when the orbit decay of the material is considered. Moreover, the novel orbital dynamics experienced by high area-to-mass ratio objects, influenced by solar radiation pressure and the J2 effect, ensure the ring will maintain a permanent heliotropic shape, with dust spending the largest portion of time on the Sun facing side of the orbit. It is envisaged that small dust grains can be released from a circular generator orbit with an initial impulse to enter an eccentric orbit with Sun-facing apogee. Finally, a lowest estimate of 1 × 1012 kg of material is computed as the total mass required to offset the effects of global warming. 相似文献
439.
O.V. Dudnik M.L. Kaiser 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
The analysis of observations of very high frequency radio noise intensity at the middle latitude on a frequency f = 500 MHz from 14th till 26th of October, 2003 is presented. These data are compared with the solar radio bursts in the range of frequencies 1–14 MHz registered by RAD2 receiver of the WAVES device installed on board the WIND spacecraft. 相似文献
440.
M. Ogurtsov M. Lindholm R. Jalkanen S.V. Veretenenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
A new summer temperature proxy was built for northern Fennoscandia in AD 1000–2004 using parameters of tree growth from a large region, extending from the Swedish Scandes to the Kola Peninsula. It was found that century-scale (55–140 year) cyclicity is present in this series during the entire time interval. This periodicity is highly significant and has a bi-modal structure, i.e. it consists of two oscillation modes, 55–100 year and 100–140 year variations. A comparison of the century-long variation in the northern Fennoscandian temperature proxy with the corresponding variations in Wolf numbers and concentration of cosmogenic 10Be in glacial ice shows that a probable cause of this periodicity is the modulation of regional climate by the secular solar cycle of Gleissberg. This is in line with the results obtained previously for a more limited part of the region (Finnish Lapland: 68–70° N, 20–30° E). Thus the reality of a link between long-term changes in solar activity and climate in Fennoscandia has been confirmed. Possible mechanisms of solar influence on the lower troposphere are discussed. 相似文献