全文获取类型
收费全文 | 1177篇 |
免费 | 232篇 |
国内免费 | 184篇 |
专业分类
航空 | 764篇 |
航天技术 | 401篇 |
综合类 | 117篇 |
航天 | 311篇 |
出版年
2024年 | 9篇 |
2023年 | 32篇 |
2022年 | 40篇 |
2021年 | 59篇 |
2020年 | 46篇 |
2019年 | 48篇 |
2018年 | 62篇 |
2017年 | 56篇 |
2016年 | 53篇 |
2015年 | 58篇 |
2014年 | 89篇 |
2013年 | 69篇 |
2012年 | 95篇 |
2011年 | 98篇 |
2010年 | 76篇 |
2009年 | 93篇 |
2008年 | 50篇 |
2007年 | 61篇 |
2006年 | 59篇 |
2005年 | 61篇 |
2004年 | 56篇 |
2003年 | 35篇 |
2002年 | 51篇 |
2001年 | 34篇 |
2000年 | 27篇 |
1999年 | 22篇 |
1998年 | 24篇 |
1997年 | 20篇 |
1996年 | 26篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 10篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 11篇 |
1987年 | 2篇 |
1986年 | 1篇 |
排序方式: 共有1593条查询结果,搜索用时 15 毫秒
81.
82.
变焦光学系统在空间探测领域中有着重要的应用需求,提出了一种基于五阶自由曲面垂轴偏移的变焦方法,实现了具有两倍变焦能力的光学系统模型。分析了五阶自由曲面透镜的变焦原理和像差模型,采用两组五阶自由曲面透镜设计并搭建了伽利略望远系统,通过控制五阶自由曲面透镜的垂轴偏移量,来改变透镜的光焦度。仿真结果分析表明:该方法的变焦曲线具有单调性,且垂轴偏移量在8mm 以内。与传统变焦方法相比,该变焦方法的特点是结构紧凑、变焦连续、控制方法简单,避免了大幅轴向变焦运动,对于可见光、红外空间光学相机轻小型化具有重要的应用价值。 相似文献
83.
DU Xin-kang WANG Tian-min WANG Cong CHEN Bu-liang ZHOU Long 《中国航空学报》2007,20(2):140-144
Some fundamental studies on the preparation, structure and optical properties of NbN films were carried out. NbN thin films were deposited by DC reactive magnetron sputtering at different N2 partial pressures and different substrate temperatures ranging from –50 ℃ to 600 ℃. X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) were employed to characterize their phase com- ponents, microstructures, grain sizes and surface morphology. Optical properties inclusive of refractive indexes, extinction coefficients and transmittance of the NbN films under different sputtering conditions were measured. With the increase in the N2 partial pressure, δ-NbN phase structure gets forming and the grain size and lattice constant of the cubic NbN increasing. The deposited NbN film has relatively high values of refractive index and extinction coefficient in the wavelength ranging from 240 nm to 830 nm. Substrate tem- perature exerts notable influences on the microstructure and optical transmittance of the NbN films. The grain sizes of the δ-NbN film remarkably increase with the rise of the substrate temperature, while the transmittance of the films with the same thickness decreases. Ultra-fine granular film with particle size of several nanometers forms when the substrate is cooled to –50 ℃, and a remarkable aug- mentation of transmittance could be noticed under so low a temperature. 相似文献
84.
85.
在未来月球探测中,需要对一些地形复杂的区域进行探测,而在这些区域软着陆,潜在的危险性增加,这就要求着陆时具有较高的定位精度并能够自动避险。当前,由于着陆时定点的误差较大,在小范围安全地域准确着陆很难。研究了一种精确定位、安全的软着陆方式,它在软着陆过程中增加了悬停阶段。在这一阶段中,通过着陆区危险地域的识别、着陆地点位置误差的计算,加上对着陆器横向漂移的控制技术,使着陆地点的精度以及着陆生存率大为提高,能够满足未来月球探测的软着陆要求。 相似文献
86.
V. Truhlik D. Bilitza L. Triskova 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
A database of electron temperature (Te) measurements comprising of most of the available satellite measurements in the topside ionosphere is used for studying the solar activity variations of the electron temperature Te at different latitudes, altitudes, local times and seasons. The Te data are grouped into three levels of solar activity (low, medium, high) at four altitude ranges, for day and night, and for equinox and solstices. We find that in general Te changes with solar activity are small and comparable in magnitude with seasonal changes but much smaller than the changes with altitude, latitude, and from day to night. In all cases, except at low altitude during daytime, Te increases with increasing solar activity. But this increase is not linear as assumed in most empirical Te models but requires at least a parabolic approximation. At 550 km during daytime negative as well as positive correlation is found with solar activity. Our global data base allows to quantify the latitude range and seasonal conditions for which these correlations occur. A negative correlation with solar activity is found in the invdip latitude range from 20 to 55 degrees during equinox and from 20 degrees onward during winter. In the low latitude (20 to −20 degrees invdip) F-region there is almost no change with solar activity during solstice and a positive correlation during equinox. A positive correlation is also observed during summer from 30 degrees onward. 相似文献
87.
88.
Yen-Hsyang Chu Chin-Lung SuHsiao-Tsung Ko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
With a network of ground-based ionosondes distributed around the world, the ionospheric peak electron density and its height measured by FORMOSAT-3/COSMIC satellites in terms of GPS radio occultation technique are extensively examined in this article. It is found that, in spite of the latitude, the mean values of the peak electron density measured by COSMIC satellites are systematically smaller than those observed by ground-based ionosondes. The discrepancy between them is dependent on the latitude, namely, it is small in low and mid-latitudes and large in high-latitude region. Moreover, statistical analysis shows that the slopes of the regression line that is best fitted to the scatter diagram of occultation-retrieved peak electron density (ordinate axis) versus ionosonde-observed peak density (abscissa axis) are universally less than one. This feature is believed to be the result of path average effect of non-uniform distribution of the electron density along the GSP ray during the occultation. A comparison between COSMIC-measured peak height and ionosonde-derived peak height hmF2 indicates that the former is systematically higher than the latter. The difference in the two can be as large as 20% or more in equatorial and low-latitude regions. This result implies that the peak height hmF2 derived from the virtual height through true height analysis based on Titheridge method seems to underestimate the true peak height. The correlation between COSMIC and ionosonde peak electron densities is analyzed and the result reveals that correlation coefficient seems to be dependent on the fluctuation of the occultation-retrieved electron density profile. The correlation will be higher (lower) for the electron density profiles with smaller (larger) fluctuations. This feature suggests that the inhomogeneous distribution of the electron density along the GPS ray path during the occultation plays an important role affecting the correlation between COSMIC and ionosonde measurements. 相似文献
89.
90.
GPS姿态测量的载波相位整周模糊度快速解算 总被引:8,自引:0,他引:8
根据 ARCE方法可以将模糊度搜索空间从所观测到的 m颗卫星的 m -1维降为 3维独立整周模糊度搜索空间,结合整数高斯变换及基线长度约束减小模糊度搜索空间,利用 Cholesky分解提高模糊度搜索效率,实验结果证明该方法能够快速解算整周模糊度适于实时姿态确定等应用。 相似文献