全文获取类型
收费全文 | 2864篇 |
免费 | 657篇 |
国内免费 | 503篇 |
专业分类
航空 | 2522篇 |
航天技术 | 424篇 |
综合类 | 348篇 |
航天 | 730篇 |
出版年
2024年 | 11篇 |
2023年 | 57篇 |
2022年 | 91篇 |
2021年 | 147篇 |
2020年 | 117篇 |
2019年 | 123篇 |
2018年 | 106篇 |
2017年 | 164篇 |
2016年 | 154篇 |
2015年 | 122篇 |
2014年 | 210篇 |
2013年 | 179篇 |
2012年 | 227篇 |
2011年 | 218篇 |
2010年 | 188篇 |
2009年 | 205篇 |
2008年 | 176篇 |
2007年 | 224篇 |
2006年 | 195篇 |
2005年 | 180篇 |
2004年 | 155篇 |
2003年 | 139篇 |
2002年 | 112篇 |
2001年 | 95篇 |
2000年 | 104篇 |
1999年 | 55篇 |
1998年 | 55篇 |
1997年 | 42篇 |
1996年 | 27篇 |
1995年 | 31篇 |
1994年 | 22篇 |
1993年 | 25篇 |
1992年 | 22篇 |
1991年 | 11篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 1篇 |
1986年 | 2篇 |
排序方式: 共有4024条查询结果,搜索用时 15 毫秒
871.
Iu.V. Cherniak I.E. Zakharenkova A. Krankowski I.I. Shagimuratov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
The plasmaspheric electron content (PEC) was estimated by comparison of GPS TEC observations and FORMOSAT-3/COSMIC radio occultation measurements at the extended solar minimum of cycle 23/24. Results are retrieved for different seasons (equinoxes and solstices) of the year 2009. COSMIC-derived electron density profiles were integrated up to the height of 700 km in order to retrieve estimates of ionospheric electron content (IEC). Global maps of monthly median values of COSMIC IEC were constructed by use of spherical harmonics expansion. The comparison between two independent measurements was performed by analysis of the global distribution of electron content estimates, as well as by selection specific points corresponded to mid-latitudes of Northern America, Europe, Asia and the Southern Hemisphere. The analysis found that both kinds of observations show rather similar diurnal behavior during all seasons, certainly with GPS TEC estimates larger than corresponded COSMIC IEC values. It was shown that during daytime both GPS TEC and COSMIC IEC values were generally lower at winter than in summer solstice practically over all specific points. The estimates of PEC (h > 700 km) were obtained as a difference between GPS TEC and COSMIC IEC values. Results of comparative study revealed that for mid-latitudinal points PEC estimates varied weakly with the time of a day and reached the value of several TECU for the condition of solar minimum. Percentage contribution of PEC to GPS TEC indicated the clear dependence from the time with maximal values (more than 50–60%) during night-time and lesser values (25–45%) during day-time. 相似文献
872.
Shridhar D. Jawak Alvarinho J. Luis 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
An enhanced digital elevation model (DEM) of the Larsemann Hills region, east Antarctica, is constructed synergistically by using highly accurate ground-based GPS measurements, satellite-derived laser altimetry (GLAS/ICESat) and Radarsat Antarctic Mapping Project (RAMPv2) DEM-based point elevation dataset. Our DEM has a vertical accuracy of about 1.5 times better than RAMPv2 DEM and seven times better than GLAS/ICESAT-based DEM. The accuracy is improved by validating the RAMPv2 DEM elevation by supplementing with GLAS/ICESat and DGPS survey data, when compared to that of DEM constructed by using GLAS/ICESat or RAMPv2 alone. With the use of accurate GPS data as ground control points reference elevations, the DEM extracted is much more accurate with least mean RMSE of 34.5 m than that constructed by using a combination of GLAS/ICESat and RAMPv2 as true reference. The newly constructed DEM 7 achieves highest accuracy with the least average elevation difference of 0.27 m calculated using 46 ground reference points. Available DEMs of Antarctic region generated by using radar altimetry and the Antarctic Digital Database indicate elevation variations in the range of 50–100 m, which necessitates the generation of local DEM and its validation by using ground truth. This is our first attempt of fusing multi-temporal, multi-sensor and multi-source elevation data to generate a DEM of any part of Antarctica, in order to address the ice elevation change to infer the ice mass balance. Our approach focuses on the strengths of each elevation data source to produce an accurate DEM. 相似文献
873.
874.
Under the demand of strategic air traffic flow management and the concept of trajectory based operations (TBO),the network-wide 4D flight trajectories planning (N4DFTP) problem has been investigated with the purpose of safely and efficiently allocating 4D trajectories (4DTs) (3D position and time) for all the flights in the whole airway network.Considering that the introduction of large-scale 4DTs inevitably increases the problem complexity,an efficient model for strategic level conflict management is developed in this paper.Specifically,a bi-objective N4DFTP problem that aims to minimize both potential conflicts and the trajectory cost is formulated.In consideration of the large-scale,high-complexity,and multi-objective characteristics of the N4DFTP problem,a multi-objective multi-memetic algorithm (MOMMA) that incorporates an evolutionary global search framework together with three problem-specific local search operators is implemented.It is capable of rapidly and effectively allocating 4DTs via rerouting,target time controlling,and flight level changing.Additionally,to balance the ability of exploitation and exploration of the algorithm,a special hybridization scheme is adopted for the integration of local and global search.Empirical studies using real air traffic data in China with different network complexities show that the pro posed MOMMA is effective to solve the N4DFTP problem.The solutions achieved are competitive for elaborate decision support under a TBO environment. 相似文献
875.
876.
介绍利用单片单板机接口电路完成对6V、30W白炽灯光源与其对应的标准白炽灯光源的色温比对标定工作。 相似文献
877.
由于航空总温热偶气动构件的复杂性,校验问题一直悬而未决。制成后的电偶热电特性迁移,及配套特性变化强调了配套校验是必要的。本文在着重分析了以上问题后,介绍了自行研制的已获国家专利的STT-1A系列总温热电偶校验器。 相似文献
878.
提出了一种使用Renishaw检查规检定三坐标测量机或数控机床空间误差的快速方法,此方法一次测量即可完成包含X、Y、Z三个坐标轴的各项误差影响的空间误差的检定,具有速度快、效率高、准确可靠等特点。 相似文献
879.
M.N. Kouahla G. Moreels M. Faivre J. Clairemidi J.W. Meriwether G.A. Lehmacher E. Vidal O. Veliz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
A new and original stereo imaging method is introduced to measure the altitude of the OH nightglow layer and provide a 3D perspective map of the altitude of the layer centroid. Near-IR photographs of the OH layer are taken at two sites separated by a 645 km distance. Each photograph is processed in order to provide a satellite view of the layer. When superposed, the two views present a common diamond-shaped area. Pairs of matched points that correspond to a physical emissive point in the common area are identified in calculating a normalized cross-correlation coefficient (NCC). This method is suitable for obtaining 3D representations in the case of low-contrast objects. An observational campaign was conducted in July 2006 in Peru. The images were taken simultaneously at Cerro Cosmos (12°09′08.2″ S, 75°33′49.3″ W, altitude 4630 m) close to Huancayo and Cerro Verde Tellolo (16°33′17.6″ S, 71°39′59.4″ W, altitude 2272 m) close to Arequipa. 3D maps of the layer surface were retrieved and compared with pseudo-relief intensity maps of the same region. The mean altitude of the emission barycenter is located at 86.3 km on July 26. Comparable relief wavy features appear in the 3D and intensity maps. It is shown that the vertical amplitude of the wave system varies as exp (Δz/2H) within the altitude range Δz = 83.5–88.0 km, H being the scale height. The oscillatory kinetic energy at the altitude of the OH layer is comprised between 3 × 10−4 and 5.4 × 10−4 J/m3, which is 2–3 times smaller than the values derived from partial radio wave at 52°N latitude. 相似文献
880.
对将增强碳/碳纤维块体切削成球锥外形的工艺,块体热膨胀系数异性轴指向可以极大地影响球锥热应力大小,存在优化设计的潜力。本文对增强碳/碳不同主轴方向弹性模量比值下的球锥最小热应力优化特性进行了研究,发现以1为界,不同模量对比关系下的优化特征是完全不同的。在本文加热条件下,当异性轴模量小于同性面模量时,优化曲线对温度场变化很敏感,以块体异性轴和球锥横截面成±42.9夹角时热应力最小;当异性轴模量大于同性面模量时,优化曲线随加热时间变化不大,以块体同性面与球锥对称轴平行或成小锐角时热应力最小。一般而言,工艺设计中应避免用模量大的主轴方向指向球锥对称轴方向。 相似文献