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251.
Real-time studies on microalgae under microgravity 总被引:2,自引:0,他引:2
Using remote sensing technique, we investigated real-time Nostoc sphaeroides Kütz (Cyanobacterium) in Closed System under microgravity by SHENZHOU-2 spacecraft in January 2001. The experiments had 1 g centrifuges in space for control and ground control group experiments were also carried out in the same equipments and under the same controlled condition. The data about the population growth of Nostoc sp. of experiments and temperature changes of system were got from spacecraft every minute. From the data, we can find that population growth of Nostoc sp. in microgravity group was higher than that of other groups in space or on ground, even though both the control 1 g group in space and 1 g group on ground indicated same increasing characteristics in experiments. The growth rate of 1.4 g group (centrifuged group on ground) was also promoted during experiment. The temperature changes of systems are also affected by gravity and light. Some aspects about those differences were discussed. From the discussion of these results during experiment, it can be found that gravity is the major factor to lead to these changes. 相似文献
252.
系统论述了原子氧和航天器表面常用聚合物材料反应的微观机理,并从宏观角度对这些材料的性能进行了评估。同时也总结了国外在原子氧防护技术研究方面所取得的最新成果。 相似文献
253.
254.
塔尔寺始建于明嘉靖三十九年(1560年).到万历五年(1577年)初具规模。整个寺院依山势而建.由众多殿宇.经堂,佛塔.僧舍等组成.布局严谨.建筑巍峨金碧辉煌.气势恢宏.是我国名的藏传佛教格鲁派六大寺院之一。除了历史和建筑塔尔寺的酥油花.壁画、堆绣从题材内容到艺术风格、工艺水平皆超过了其发源地而居雪域各寺之冠.闻名于海内外被誉为塔尔寺“三绝”。 相似文献
256.
Gang Hai Huan Xie Wenjia Du Menglian Xia Xiaohua Tong Rongxing Li 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(7):2120-2139
Slope correction is important to improve the accuracy of satellite radar elevation measurements by mitigating the slope-induced error (SE), especially over uneven ground surfaces. Although several slope correction methods have been proposed, guidance in the form of stepwise algorithm on how to implement these methods in processing radar altimetric data at the coding level, and the differences among these methods need to be presented and discussed systematically. In this paper, three existing types of slope correction methods—the direct method (DM), intermediate method (IM), and relocation method (RM, further divided into RM1 and RM2)—are described in detail. In addition, their main differences and features for various scientific applications are analyzed. We conduct a systematic experiment with CryoSat-2 Low Resolution Mode (LRM) data in a physically stable area around Dome Argus in East Antarctica, where in-situ measurements were available for comparison. The slope correction is implemented separately using the three methods, with the latest high-accuracy Reference Elevation Model of Antarctica (REMA) as the a-priori topography model. The bias and precision of the slope-corrected CryoSat-2 data results from the RM2 is ?0.18 ± 0.86 m based on the comparison with the field Global Navigation Satellite System (GNSS) data. The results from the RM2 indicate higher precision compared to those from the RM1. According to the correlation analysis of the slope-corrected CryoSat-2 data results (RM1 and RM2), the bias enlarges and the precision becomes worse when the surface slope increases from 0 to 0.85°. After a comprehensively comparative analysis, we find that the results from the RM1 and RM2 are superior in precision (0.93 m and 0.86 m) with respect to the GNSS data. The relatively low precision (1.22 m) from the IM is due to the potential error from the a-priori digital elevation model (DEM). The DM has the lowest precision (2.66 m). Another experiment over rough topography in West Antarctica is carried out for comparison, especially between the RM1 (precision of 15.27 m) and RM2 (precision of 16.25 m). In general, the RM is recommended for the SE elimination among the three methods. Moreover, the RM2 is firstly considered over smooth topography due to the superior performance in bias and precision, while the RM1 is more suggested over the rough topography because of the slightly smaller bias and better precision. The IM relies much on the accuracy of the a-prior DEM and is not usually recommended, because of the strict requirement in the sampling time between the radar altimetry data and the a-priori DEM to avoid any surface change over time. 相似文献
257.
一种基于分组截短PN码的SOFDM信道估计方法 总被引:1,自引:1,他引:0
杨童 《中国空间科学技术》2008,28(5):46
基于PN序列的信道估计方法的特点是计算过程简明,已被广泛应用在地面无线局域网系统中。在卫星正交频分多路复用传输体制(SOFDM)系统中引入传统PN序列信道估计方法的主要问题在于卫星信道的多径延时远大于SOFDM数据符号的持续时间,信道特性比较复杂,使得传统方法的估计精度严重下降。提出了一种改进的方法,根据信道的近似周期特性对数据帧进行合理的等长度分组,并把长PN序列改为短PN码来跟踪信道局部特性的变化,再对分组后的数据子帧分别进行信道估计。对该方法的设计思想进行了理论分析,并通过仿真验证了该方法的估计精度比传统方法至少提高了10倍。 相似文献
258.
贴装优化问题直接关系到表面贴装工艺的效率。本文以JUKI-2010/2020贴片机为试验平台,根据该机型结构及贴装过程,把贴装优化问题的描述为最小权重匹配问题(MWMP)以及非对称旅行商(TSP)问题,构建了一种新的优化模型:把时间优化转化为距离优化,并应用启发式算法进行软件编程。通过生产企业的6块PCB板的对比实验,证明了该数学模型和算法是合理的。 相似文献
259.
260.