低轨卫星通信系统星座设计与性能仿真     

刘绍奎  ;刘虎东 《全球定位系统》2014,(3):19-23

低轨卫星通信系统在通信领域发挥着越来越重要的作用。针对不同类型星座覆盖能力和星际链路建立条件存在的差异,设计了极轨道和倾斜圆轨道低轨卫星星座,利用STK验证了两种星座的覆盖能力和星际链路AER特性,利用NS2对两种星座卫星网络的时延、吞吐量和丢包率等性能指标进行了仿真,并对所得数据进行了对比分析,结果表明：在由32颗卫星（轨道高度1450 km ）构成的低轨星座中,极轨星座的全球覆盖能力、星际链路建立条件和卫星网络性能优于倾斜圆轨道星座,更适合于构建全球低轨卫星通信系统。  相似文献   

Investigation of Uncertainties of Establishment Schemes in Dynamic Global Vegetation Models简     

SONG Xiang  ZENG Xiaodong 《大气科学进展》2014,31(1):85-94

In Dynamic Global Vegetation Models （DGVMs）, the establishment of woody vegetation refers to flowering, fertiliza- tion, seed production, germination, and the growth of tree seedlings. It determines not only the population densities but also other important ecosystem structural variables. In current DGVMs, establishments of woody plant functional types （PFTs） are assumed to be either the same in the same grid cell, or largely stochastic. We investigated the uncertainties in the competition of establishment among coexisting woody PFTs from three aspects： the dependence of PFT establishments on vegetation states; background establishment; and relative establishment potentials of different PFTs. Sensitivity experi- ments showed that the dependence of establishment rate on the fractional coverage of a PFT favored the dominant PFT by increasing its share in establishment. While a small background establishment rate had little impact on equilibrium states of the ecosystem, it did change the timescale required for the establishment of alien species in pre-existing forest due to their disadvantage in seed competition during the early stage of invasion. Meanwhile, establishment purely fiom background （the scheme commonly used in current DGVMs） led to inconsistent behavior in response to the change in PFT specification （e.g., number of PFTs and their specification）. Furthermore, the results also indicated that trade-off between irtdividual growth and reproduction/colonization has significant influences on the competition of establishment. Hence, further development of es- tablishment parameterization in DGVMs is essential in reducing the uncertainties in simulations of both ecosystem structures and successions.  相似文献   

基于历史数据的黄海浒苔爆发规模预测     

刘璐  罗年学  赵前胜 《测绘通报》2022,(7):7-11

依据黄海浒苔初始覆盖面积及最大覆盖面积的历史数据,选取温度、光强、降水3种对于浒苔生长扩散最重要的影响因子,本文提出并建立了基于BP神经网络确定转换系数R的浒苔覆盖面积预测模型,可在浒苔出现初期即实现对本年度浒苔最大覆盖面积的模拟预测,并通过历史数据进行验证。结果表明,所预测的浒苔最大爆发情况与真实情况相符,研究成果可为浒苔的应急准备工作争取更多的时间提供一定参考。  相似文献   

浅析地理国情普查内容分类体系   总被引：1，自引：0，他引：1  

冷海芹 《东北测绘》2014,(6):118-121

为确保第一次全国地理国情普查的顺利进行,需要对普查内容分类体系有一致、深入的理解与认识。本文先后分析了地理国情普查内容分类的原则和采用的三级分类与编码体系,讨论了不同门类普查数据的表示方法,提出了基于决策树思想的一级类可靠判译方法,最后通过比较分析,指出了该分类体系与其他已有体系的异同,可为深入理解地理国情普查内容提供有益参考。  相似文献   

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刘根友  郝晓光  陈晓峰  孙保琪  彭碧波  周旭华  柳林涛  胡小刚 《大地测量与地球动力学》2007,27(5):115-118

????????????????????????????????Χ????????????????????????????????????????????????????????????4??IGSO????????????????????????Χ???γ55??????????γ75??  相似文献   

Evidence for climatic variability and its impact on human development during the Neolithic from Loughmeenaghan,County Sligo,Ireland     

Susann Stolze  Walter Dörfler  Thomas Monecke  Oliver Nelle 《第四纪科学杂志》2012,27(4):393-403

High‐resolution pollen and geochemical analyses conducted on a sediment profile from a small lake in County Sligo, Ireland, revealed that human development during the Neolithic was influenced by pronounced climatic oscillations. The primeval woodland around the lake experienced a considerable transformation coinciding with the elm decline at 3810 BC. The subsequent increase in summer temperatures and decrease in precipitation favoured wheat cultivation in the lake's catchment area, which was practised for approximately 140 years. A shift towards pastoral farming took place with the establishment of exceptionally dry conditions between 3650 and 3560 BC, when lake level and influx of allochthonous material were notably low. The onset of cool and wet conditions at the transition from the Early to Middle Neolithic possibly caused the initial decline of human activity in the area. Periods of particularly high precipitation during the Middle and early Late Neolithic contributed to the abandonment of the area by the first farmers. Comparison of the proxy record from the study site with other palynological and archaeological records from Ireland suggests that climatic variability on the decadal to centennial scale represented a primary control on the nature and duration of farming practices during the Neolithic. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Array Configurations That Tile the Plane     

John D. Bunton 《Experimental Astronomy》2001,11(1):193-206

Future radiotelescopes need ever increasing sensitivity. With the limits of receiver sensitivity being reached this can only be achieved by increasing collecting area. The increase in antenna numbers and the multi-resolution capabilities of these telescopes, such as the SKA, means a significant departure from existing designs is needed. In this article, some fundamental designs for antenna arrays that uniformly sample the UV plane are presented. The SKA is used to provide target specifications. The designs presented allow asymmetric configurations providing more freedom for site selection. They also allow a direct tradeoff between field of view and correlator complexity. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Characteristics and variation law of wind-blown sand delivered to the Ningxia-Inner Mongolia reach of the Yellow River under a changing environment     

Zhichun Yue  Ximin Yuan  Lugan Cao  Fuchang Tian  Chao Han  Hongwu Zhang 《国际泥沙研究》2022,37(2):188-201

Wind-blown sand is one of the key factors affecting the evolution of sediment transport,erosion,and deposition in rivers crossing desert areas.However,the differences and complex variations in the spatial and temporal distribution of the underlying surface conditions are seldom considered in research on the river inflow of wind-blown sand over a long time period.The Yellow River contains a large amount of sediment.The Ningxia-Inner Mongolia reach of the Yellow River was selected as the research ...  相似文献   

高植被覆盖区遥感矿化信息提取方法研究     

杨正萌  白卉  孟嵩 《吉林地质》2015,(2)

针对吉林省由于植被覆盖、水体、阴影等干扰因素,遥感矿化蚀变信息提取方法繁琐,提取结果不理想的问题,采取主成分分析和比值法相结合,采用遥感蚀变信息多层次分离技术,省去多次掩膜处理,尽可能的保留原始信息,减少了人为因素,提取方法更为简便且准确,为高植被覆盖区遥感矿化蚀变信息提取提供了新思路。  相似文献   

新疆吐鲁番胜金店墓地小麦遗存加工处理方式初探     

李晶静  张勇  张永兵  王龙  蒋洪恩 《第四纪研究》2015,35(1)

脱粒是谷物加工处理的重要环节之一.本文选择新疆吐鲁番胜金店墓地M9墓葬出土大量保存完好的小麦遗存进行原始脱粒方式分析,并尝试结合现代小麦原始脱粒方式模拟实验,将现代模拟实验结果与古代材料进行对比分析,从而初步探讨新疆吐鲁番地区先民的小麦脱粒方式.本文首先对胜金店墓地M9墓葬出土的小麦遗存进行初步分类,将不同的小麦脱粒产品与副产品(麦秆、花序轴、小穗轴等)进行分析和统计;其次设计并完成现代小麦脱粒模拟实验,包括手搓、徒手摔打、木棍击打等3种相对原始的小麦脱粒方式,统计不同脱粒方式所产生的小麦加工产品与副产品的组合比例;将现代模拟实验所得结果与M9墓葬出土小麦遗存进行进一步对比分析.研究结果表明,在距今约2000多年前,新疆吐鲁番胜金店先民可能使用木棍击打方式对小麦进行脱粒.  相似文献