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731.
732.
AIRSEA SystemⅡ已应用于我国的海洋重力测量,它主要由高精度的垂直加速度计和稳定平台组成。前者用于测量总加速度,后者使加速度计保持精确的垂直指向。海洋重力测量时,由于测量船水平加速度的存在,稳定平台难以精确维持水平从而使加速度计偏离正确指向,由此产生了水平加速度改正。为了提高测量精度,给出水平加速度改正计算的两种方法,即两步法和一步法。为有效地减弱因水平加速度改正不完善产生的系统性误差,提出了水平加速度改正的预滤波方法。最后利用中沙大环礁实测数据对此改正方法进行了验证和分析。 相似文献
733.
734.
735.
根据像底点与航摄像片外方位角元素的几何关系,本文提出利用像底点求解IMU视准轴误差,以提高POS系统直接对地目标定位精度的方法,并用一组实际飞行的数据进行了试验。结果表明,该方法可有效提高POS系统直接对地目标定位的精度,而无需布设特定的检校场和地面控制点。对带POS系统的城区大比例尺航空影像对地目标定位有一定实用价值。 相似文献
736.
《The Journal of geography》2012,111(1):10-19
Abstract A Learning Support System (LSS) that emphasizes experiential research in natural environments using the cutting-edge technologies of GIS and multimedia has been developed for teaching environmental literacy to undergraduate students at the University of Georgia. Computers are used as cognitive tools to create a context in which students become interns in an ecological research center. Students are instructed to conduct research in the form of two field laboratories (the stream and forest laboratories). They accomplish their tasks by collecting data in the field (the State Botanical Garden of Georgia near the campus). They enter the data in the Learning Support System (LSS), and are guided to formulate hypotheses relating to stream water quality and human impact on forest succession for testing. Students also interact with the Environmental Research Support Site (ERSS) within the LSS for explanations to their findings. A specially customized Arc View GIS program within the LSS provides a tool to students for spatial analysis in the case of the forest laboratory. Students and faculty evaluations as well as final examination results confirmed the receptiveness of students to the LSS approach and its effectiveness in the learning of environmental literacy. 相似文献
737.
The transverse cutting mechanism of the checkerboard paving robot, which directly affects the robot’s operation, is regarded as the most important part in a transverse pavement system. This research aims at designing a new system which is appropriate for desert working environments and finishing cutting the rope of straw. In this paper, computer emulating technology is applied, and with three-dimensional modeling by Pro/ENGINEER software, can observe whether or not interference exists. Also, the speed and displacement parameters of the main parts of the cutting system are given by the dynamic emulation by Automatic Dynamic Analysis of Mechanical System (ADAMS), which theoretically assures the feasibility and appropriateness of the cutting mechanism. Finally, results of the experiment shows that the cutting mechanism can cut the straw strand as designed. 相似文献
738.
Exploring the Effects of Microscale Structural Heterogeneity of Forest Canopies Using Large-Eddy Simulations 总被引:4,自引:4,他引:0
Gil Bohrer Gabriel G. Katul Robert L. Walko Roni Avissar 《Boundary-Layer Meteorology》2009,132(3):351-382
The Regional Atmospheric Modeling System (RAMS)-based Forest Large-Eddy Simulation (RAFLES), developed and evaluated here,
is used to explore the effects of three-dimensional canopy heterogeneity, at the individual tree scale, on the statistical
properties of turbulence most pertinent to mass and momentum transfer. In RAFLES, the canopy interacts with air by exerting
a drag force, by restricting the open volume and apertures available for flow (i.e. finite porosity), and by acting as a heterogeneous
source of heat and moisture. The first and second statistical moments of the velocity and flux profiles computed by RAFLES
are compared with turbulent velocity and scalar flux measurements collected during spring and winter days. The observations
were made at a meteorological tower situated within a southern hardwood canopy at the Duke Forest site, near Durham, North
Carolina, U.S.A. Each of the days analyzed is characterized by distinct regimes of atmospheric stability and canopy foliage
distribution conditions. RAFLES results agreed with the 30-min averaged flow statistics profiles measured at this single tower.
Following this intercomparison, two case studies are numerically considered representing end-members of foliage and midday
atmospheric stability conditions: one representing the winter season with strong winds above a sparse canopy and a slightly
unstable boundary layer; the other representing the spring season with a dense canopy, calm conditions, and a strongly convective
boundary layer. In each case, results from the control canopy, simulating the observed heterogeneous canopy structure at the
Duke Forest hardwood stand, are compared with a test case that also includes heterogeneity commensurate in scale to tree-fall
gaps. The effects of such tree-scale canopy heterogeneity on the flow are explored at three levels pertinent to biosphere-atmosphere
exchange. The first level (zero-dimensional) considers the effects of such heterogeneity on the common representation of the
canopy via length scales such as the zero-plane displacement, the aerodynamic roughness length, the surface-layer depth, and
the eddy-penetration depth. The second level (one-dimensional) considers the normalized horizontally-averaged profiles of
the first and second moments of the flow to assess how tree-scale heterogeneities disturb the entire planar-averaged profiles
from their canonical (and well-studied planar-homogeneous) values inside the canopy and in the surface layer. The third level
(three-dimensional) considers the effects of such tree-scale heterogeneities on the spatial variability of the ejection-sweep
cycle and its propagation to momentum and mass fluxes. From these comparisons, it is shown that such microscale heterogeneity
leads to increased spatial correlations between attributes of the ejection-sweep cycle and measures of canopy heterogeneity,
resulting in correlated spatial heterogeneity in fluxes. This heterogeneity persisted up to four times the mean height of
the canopy (h
c
) for some variables. Interestingly, this estimate is in agreement with the working definition of the thickness of the canopy
roughness sublayer (2h
c
–5h
c
). 相似文献
739.
北京地区暴雨个例的观测敏感区研究 总被引:1,自引:1,他引:0
利用基于集合预报的相关方法对2009年7月23日发生在北京及周边地区的暴雨过程的观测敏感区进行了分析。通过WRF(Weather Research Forecast)三维变分方法对初始场进行随机扰动,形成30个初始集合样本,做了预报时效为12 h的集合预报。利用该方法分析检验区(北京及周边地区)累积降水[14:00(北京时间,下同)至20:00]相对于初始时刻(08:00)各基本要素的敏感性,确定感性要素及其对应的区域。研究发现初步确定的敏感性要素为水汽和温度,对应的敏感区分别位于北京的西南侧和北京的东北侧,且通过实况分析可知初步确定的敏感性要素和对应的敏感区具有明确的物理意义。还进一步通过观测系统模拟试验(OSSE)的资料同化验证所确定的敏感区,结果表明在水汽对应的敏感区内同化水汽对降水的预报结果有明显的改进;在温度对应的敏感区内同化温度,降水的预报准确率有了明显的提高,说明了初步确定的敏感性要素和敏感区的正确性。在水汽对应的敏感区内同化水汽的同时在温度对应的敏感区内同化温度,使降水预报的技巧有大幅度的提高,说明了温度和水汽的共同作用对提高降水预报准确率贡献最大。因此,通过基于集合预报的相关方法能够快速的确定敏感区。研究结果将为确定北京暴雨的观测敏感区提供参考。 相似文献
740.
If Jupiter and the Sun both formed directly from the same well-mixed proto-solar nebula, then their atmospheric compositions should be similar. However, direct sampling of Jupiter’s troposphere indicates that it is enriched in elements such as C, N, S, Ar, Kr, and Xe by 2-6× relative to the Sun (Wong, M.H., Lunine, J.I., Atreya, S.K., Johnson, T., Mahaffy, P.R., Owen, T.C., Encrenaz, T. [2008]. 219-246). Most existing models to explain this enrichment require an extremely cold proto-solar nebula which allows these heavy elements to condense, and cannot easily explain the observed variations between these species. We find that Jupiter’s atmospheric composition may be explained if the Solar System’s disk heterogeneously accretes small amounts of enriched material such as supernova ejecta from the interstellar medium during Jupiter’s formation. Our results are similar to, but substantially larger than, isotopic anomalies in terrestrial material that indicate the Solar System formed from multiple distinct reservoirs of material simultaneously with one or more nearby supernovas (Trinquier, A., Birck, J.-L., Allegre, C.J. [2007]. Astrophys. J. 655, 1179-1185). Such temporal and spatial heterogeneities could have been common at the time of the Solar System’s formation, rather than the cloud having a purely well-mixed ‘solar nebula’ composition. 相似文献