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251.
美国FIA光学成像卫星项目失败的启示 总被引:1,自引:1,他引:1
“未来成像体系”(FIA)是美国政府大胆创新发展的新一代间谍卫星项目。此项目的取消或许是美国50年间谍卫星发展历史中最为惨痛的失败。文章介绍了FIA项目的发展背景、先进能力以及整个研制过程,并介绍了美国采取的一些补救措施。最后总结了FIA项目失败所揭示的一些经验教训。 相似文献
252.
Zhong Lingwei Liu Yu Li Juan Gu Zuo Jiang Haocheng Wang Haixing Tang Haibin 《中国航空学报》2010,23(1):15-21
Charge exchange(CEX) ions could inflict severe damages on the ion thruster optical system. This article is aimed at investigating the characteristics of the CEX ions and their influences upon the optical system by means of particle-in- cell(PIC) ion simulation and Monte Carlo collision(MCC) methods. The results from numerical simulation indicate that despite the fact that CEX ions appear in the entire beamlet region near the ion optical system, the ones that present themselves downstream of the accelerator grid have good reason for attracting more attention. As their trajectories are significantly affected by the local electric field, a great number of CEX ions are accelerated toward grids resulting in sputtering erosion. When the influences of the CEX ions are considered in the numerical simulation, there could hardly be observed augments in the screen grid current, but the accelerator grid current increases from zero to 1. 4% of the beamlet current. It can be understood from the numerical simulation that the CEX ions formed in the region far downstream of the accelerator grid should be blamed for the erosion on the downstream surface of the accelerator grid. 相似文献
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Luo Fei 《南京航空航天大学学报(英文版)》1994,(1)
DISTRIBUTEDOPTIC-FIBERFORCESENSORBASEDONMODECOUPLINGEFFECTSINHIGH-BIREFRINGENTFIBERLuoFei(DepartmentofTestsMeasurementsEngine... 相似文献
257.
V. Sreekanth 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
Climatological aerosol optical depths (AOD) over Bangalore, India have been examined to bring out the temporal heterogeneity in columnar aerosol characteristics. AOD values at 550 nm derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard NASA’s Terra and Aqua satellites, for the period of 2002–2011 have been analyzed (independently) for the purpose. Frequency distributions of the AOD values are examined to infer the monthly mean values. Monthly and seasonal variations of AOD are investigated in the light of regional synoptic meteorology. Climatological monthly and seasonal mean Terra and Aqua AOD values exhibited similar temporal variation patterns. Monthly mean AOD values increased from January, peaks during May and thereafter (except for a secondary peak during July) fall off to reach a minimum during December. Monsoon season recorded the highest climatological seasonal mean AOD, while winter season recorded the lowest. AOD values show an overall increasing trend on a yearly basis, which was found mainly due to sustained increase in the seasonal averaged AOD during summer. The results obtained in the present study are compared with that of the earlier studies over the same location and also with AOD over various other Indian locations. Finally, the radiative and climatic impacts are discussed. 相似文献
258.
半球谐振子金属化是半球谐振陀螺研制过程中的重要环节,针对半球表面薄膜制备均匀性难以实现的问题,提出了一种将薄膜沉积实验和光学模拟相结合的方法。本文采用电子束蒸发技术在半球上沉积Au薄膜,利用台阶仪测量球面上不同位点的薄膜厚度,将平面上的膜厚等效为半球曲面上的膜厚,研究球面薄膜的均匀性,得出了在半球内外表面上薄膜的膜厚分布;同时对薄膜沉积均匀性进行光学模拟,将半球探测器上辐照度等效为实验中沉积所得到的薄膜厚度,计算得出的半球探测器上辐照度分布与实验测量结果一致性较好,可为半球谐振子纳米薄膜的均匀性制备提供理论基础。 相似文献
259.
Simultaneous determination of aerosol- and surface characteristics from top-of-atmosphere reflectance using MERIS on board of ENVISAT 总被引:1,自引:0,他引:1
Wolfgang von Hoyningen-Huene Alexander A. Kokhanovsky John P. Burrows Veronique Bruniquel-Pinel Peter Regner Frederic Baret 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,37(12):2172-II
The determination of aerosol optical thickness (AOT) from nadir scanning multi-spectral radiometers, like SeaWiFS, MERIS or MODIS, requires the separation of spectral atmospheric and surface properties. Since SeaWiFS and MERIS do not provide information at 2.1 μm, like MODIS, the estimation of the surface reflectance cannot be made by the cross correlation approach described by Kaufman et al., 1997. The BAER approach (Bremen AErosol Retrieval), von Hoyningen-Huene et al., 2003, uses a linear mixing model of spectra for ‘green vegetation’ and ‘bare soil’, tuned by the NDVI, determining an apparent surface to enable this separation of aerosol and surface properties from VIS and NIR channels. Thus AOT can be derived over a wide range of land surfaces for wavelengths <0.67 μm. Using MERIS L1 data over Europe, the AOT retrieved is comparable with ground-based observations, provided by AERONET. Regional variation of AOT can be observed, showing the atmospheric variability for clear sky conditions by: large scale variation of aerosol turbidity, regional pollution, urban regions, effects of contrails and cases of aerosol-cloud interaction. Simultaneously with the spectral AOT also spectral surface reflectance is obtained, where all atmospheric influences have been considered (molecules, aerosols and absorbing gases (O3)) for channels with wavelengths <0.67 μm. The AOT is extrapolated by Angström power law to NIR channels and the atmospheric correction for land surface properties is performed, enabling the further investigation of land use and spectral land properties. 相似文献
260.