C波段转发测轨系统测站偏差的标校

C波段转发测轨体制在GEO卫星测定轨中有突出优势,但其测量系统偏差的标校精度对定轨结果影响显著。基于L波段载波相位多星定轨结果,提出一种基于标准轨道拟合残差的系统偏差标校方法。该方法能够同时标校测量偏差及时标偏差,有利于弥补激光测距标校方法观测量较少的缺陷。利用国内监测站对中国导航卫星实测数据分析结果表明,经过标校后的C波段转发测轨精度可优于5 m,测量偏差及时标偏差具有较好的稳定性。     

Composition, spatial distribution, and environmental significance of water ions in Pumayum Co catchment, southern Tibet

The chemistry of major cations (Mg2+, Ca2+, Na+, and K+) and anions (HCO3 ?, SO4 2?, and Cl?) in the water of Lake Pumayum Co and its inflow river was studied, revealing the obvious ionic difference among various inflow rivers and the lake. The chemical type of the lake water was Mg2+-Ca2+-HCO3 ?-SO4 2+, but the major ions of the main inflow rivers were Ca2+-Mg2+-HCO3 ?. In the lake inlet of Jiaqu River, the main inflow river, there was significant variance of water chemistry within the depth less than 2 m. However, it was almost homogeneous at other area of the lake. Therefore, with the evidence of distribution of water chemistry and oxygen isotope of lake water, a conclusion can be outlined that Jiaqu River had a distinct effect on the hydrochemistry of the water on the submerged delta, whereas this is not the case for other rivers. The Gibbs plot revealed that the dominant mechanism responsible for controlling chemical compositions of the lake water was rocks weathering in the drainage area. Ion ratios and ternary plots further explored the main processes controlling the water chemistry of the catchment, i.e., carbonate weathering, pyrite weathering, and silicate weathering. The different hydrochemistry characteristics between river water and lake water may result from the CaCO3 precipitation. The findings will benefit the explanation of the environmental significance of carbonate in paleolimnological studies in the lake.     

基于沉积物与文献记录的茈碧湖水文波动与近现代生态环境变化

云南高山湖泊对区域气候变化和流域人类活动十分敏感,部分水体同时面临着污染物增加、生物群落快速演替、生态系统功能降低等问题。位于滇西北高山地区的茈碧湖历史资料表明,该湖近现代以来经历了明显的水文波动与人类活动影响。本研究通过对茈碧湖沉积物进行年代序列测定、元素和粒度等多指标分析,重建了近300年来（对应于钻孔顶部18 cm深度样品）茈碧湖环境变化历史,并结合对硅藻的分析,识别了气候变化和人类活动对生物群落变化的驱动影响。结果表明,沉积物粒度记录了茈碧湖水文变化的长期波动,水库修建与水文调控的增强使得湖泊水位波动明显,与文献记录的水文事件基本一致。硅藻群落结构以浮游种为主,1860年前硅藻组合以对水体扰动力有较强耐受性的Cyclostephanos dubius为主要优势种;1860年以来小型硅藻Cyclotella ocellata占据主要优势,同时Cyclotella comta成为次优势种;1990年开始,底栖硅藻Fragilaria spp.属相对丰度增加并出现明显波动,喜营养盐的Fragilaria crotonensis出现。多变量数理分析显示,气候变暖以及湖泊营养水平变化的共同作用是驱动茈碧湖硅藻组合长期演替的重要因素,分别独立解释了群落变化的27.5%和21.3%;其中湖库改造工程导致的外源物质输入增多,使得营养水平总体上升,是导致20世纪90年代以来硅藻组合出现明显变化的诱因。因此,在气候变暖的背景下,对云南高山湖泊的保护需要进行流域开发和水文调控的综合评价。

     

星地跟踪网的导航卫星精密轨道确定及预报

针对区域跟踪网不能覆盖导航卫星全弧段从而导致卫星定轨精度低的问题,简述了导航卫星和低轨卫星联合定轨模型,然后利用星地跟踪网观测数据同时确定了低轨卫星和导航卫星精密轨道,并根据实验结果详细分析了低轨卫星在联合定轨中所起到的作用。计算结果表明,引入低轨卫星之后,全球网和区域网定轨精度分别平均提高了20.0%和44.3%,区域网6h和24h的轨道预报精度分别优于10cm、13cm,利用星地跟踪网观测数据联合定轨方案是一种提高定轨精度并削弱对地面站依赖性的有效方法。     

北斗卫星ECOM光压模型参数选择策略分析

以ECOM经验光压模型为基础,利用北斗卫星3年的精密星历进行轨道拟合,获得了ECOM光压参数的变化规律,给出了北斗3类卫星的ECOM光压参数选择策略。从北斗卫星姿态控制模式出发,通过卫星星体受照分析,指出在北斗卫星地影期零偏航状态下,由于太阳对卫星帆板的不正照,导致与动态偏航姿态相比,光压摄动力存在与轨道周期相关的分量,需要在ECOM 5参数的基础上增加D向周期分量进行吸收。通过MGEX全球网数据定轨试验,本文提出的方法可使零偏段定轨重叠段位置精度提高50%~80%。     

北斗卫星轨道预报精度分析及改进

针对北斗GEO、IGSO、MEO的3种卫星类型和动态偏航、零偏航两种姿态控制模式,进行了以ECOM光压模型为基础的轨道预报精度分析。确定了北斗3类卫星的短期、中期、长期预报光压参数选择策略。采用光压参数修正法,通过对北斗卫星光压参数长期变化规律建模,有效提升了地影段轨道长期预报精度。研究结果可同时服务于北斗卫星轨道确定及历书参数生成。     

直角与直线元素数字化的数据处理

结合地图数字化为GIS采集空间数据的工作实际,对直角、直线数字化的数据处理的平差模型以及相应的精度评定方法进行了研究。作者认为,通过平差和评定精度,可以提高数字化的精度,并有助于实现对GIS数据采集的质量控制。     

基于ArcGIS Engine的地图模板化输出

地图的模板化输出就是利用模板技术思想将制图专家的地图设计知识和实践经验模板化、可视化,对于实现地图设计知识和实践经验的传输与共享,提高地图制图的效率具有重要的理论研究意义和实际应用价值。介绍ArcGIS制图中3种不同的模板技术,分析在.NET平台下采用C#语言基于ArcGIS Engine组件实现地图模板化输出的原理方...     

Monitoring interannual dynamics of desertification in Minqin County,China, using dense Landsat time series

ABSTRACT

Minqin County in northwestern China is highly affected by desertification. Campaigns have been initiated in recent decades to combat desertification in Minqin. To assess the effectiveness of these campaigns, this study used a dense Landsat time series from 1987 to 2017 to investigate the interannual dynamics of vegetation coverage and greenness over the past 31 years. First, this study applied an advanced technology to reconstruct a high-quality Landsat annual time series. Specifically, one image in the vegetation-peak season was selected as the base image in each year, and then problematic pixels were interpolated by the neighborhood similar pixel interpolator using ancillary images in the same year. Second, the land cover map and the enhanced vegetation index (EVI) were derived from all reconstructed images. Third, the change of vegetation coverage and EVI values over the 31 years were analyzed. The results show that the total vegetation coverage and greenness increased during the 31 years. Linking this change trend to other factors suggests that vegetation in Minqin County is highly affected by agriculture and groundwater supply rather than by climate. To mitigate desertification in a sustainable way, agriculture should be well managed to avoid overconsumption of natural resources such as underground water.     

多系统广播电离层模型精度评估

电离层误差是导航定位主要的误差源之一,广播电离层模型为单频用户修正电离层延迟提供了简便有效的方法。本文采用CODE提供的GIM产品作为评估的基准,对4大广播电离层模型进行了多方面精度评估,旨在为后续的模型改进及应用提供参考。结果表明：各个模型北半球改正率高于南半球,白天改正率高于晚间;NeQuick模型在中低纬度服务性能一般,但在高纬度地区明显高于BDSK8、GPSK8模型,改正率高出BDGIM模型约5%,BDSK8、GPSK8、BDGIM模型在中低纬度改正率较高,高纬度带改正率稍差;BDGIM模型性能总体优于其他模型,全球范围改正率可达76.91%。