伪卫星定位系统中的频率与时间统一     

张涛  徐亚明  黄劲松 《全球定位系统》2011,36(1):28-31,37

在电磁波定位系统中,时间和频率的统一与稳定性非常重要,会直接影响系统的可用性和精度．通过对一个伪卫星定位系统中时间和频率问题的研究,设计出了一套完整的时频统一方案,并编写了程序实现。该方案可以有效提高系统初始化速度,并提高观测质量。  相似文献   

多类用户的混合星座导航系统定位性能分析   总被引：1，自引：0，他引：1  

聂鹏程  姚静  易东云 《测绘科学》2011,36(3):62-64

本文针对地面用户与空间用户在测量环境与观测几何动态性等方面的不同,基于导航卫星可见性与加权精度因子(WDOP)对混合星座导航系统的定位性能进行了分析.给出了不同用户的星座天空视图与可视时长;从理论上讨论了权值选取对WDOP的影响.仿真表明,地面静态用户对不同类型导航卫星的可见性差异很大;而低轨卫星用户的可视时长更短,观...  相似文献   

Nontectonic signals in reprocessed continuous GPS position time series of CMONOC     

Tian  Yunfeng 《地球空间信息科学学报》2011,14(3):207-215

The nearly nine-year continuous GPS data collected since 1 March 1999 from the Crustal Motion Observation Network of China(CMONOC) were consistently analyzed.Most of the nonlinear movements in the cumulative position time series pro-duced by CMONOC data center disappeared;and more accurate vertical terms and tectonic signals were extracted.Displacements caused by atmospheric pressure loading,nontidal ocean loading,soil moisture mass loading,and snow cover mass loading using the National Centers for Environmental Prediction(NCEP) Reanalysis I/II models and Estimation of the Circulation and Climate of the Ocean(ECCO) data can explain most of the vertical annual terms at many stations,while only parts can be explained at Lhasa and southern coastal sites,indicating that there are some deformation mechanisms that are still unknown or not modeled accurately.The remarkable differences in vertical position time series for short-baseline sites reveal that GPS stations can be greatly affected by lo-cal factors;and attention should be paid when explaining observed GPS velocity vectors.  相似文献   

实时精密单点定位中周跳探测与修复的算法研究     

易重海  朱建军  陈永奇  戴吾蛟 《武汉大学学报(信息科学版)》2011,36(11):1314-1319

提出了一种适用于实时GPS精密单点定位的周跳探测与修复的新算法。该算法步骤为：①利用M-W组合和电离层残差组合初步确定没有发生周跳和可能发生周跳的卫星;②利用当前历元与前一（或几个）历元的L1、L2和Lw观测值和第一步得出的没有发生周跳的卫星信息,采用基于历元间差分观测值的周跳处理模型对可能发生周跳的卫星进行周跳探测;③对第二步中周跳处理失败的卫星进行进一步的精化处理,以尽可能修复周跳。实验表明,新算法在实时GPS精密单点定位中可以准确地探测并修复周跳。  相似文献   

盾构下穿既有隧道实时监测及其风险控制研究   总被引：1，自引：0，他引：1  

梅文胜  陈雪丰  周小波  张广伟 《武汉大学学报(信息科学版)》2011,(8):923-927

阐述了测量机器人实时自动化监测系统的组成、结构及关键技术,利用该系统进行盾构下穿期间既有隧道的实时风险控制。结果表明,盾构下穿过程中所有风险控制值都在预警值之内,控制效果良好,但隧道断面结构存在显著拉伸与压缩变形,为保证隧道结构安全,给出了有益的风险控制建议。测量机器人实时监测系统满足盾构下穿既有隧道风险控制的要求,且具有获取结构变形信息全面、精度高、安装简便、可重复利用等优点。  相似文献   

Can a 25-year trend in Soudano-Sahelian vegetation dynamics be interpreted in terms of land use change? A remote sensing approach   总被引：3，自引：0，他引：3  

Agnès Bégué  Elodie VintrouDenis Ruelland  Maxime Claden 《Global Environmental Change》2011,21(2):413-420

This study is based on the premise that, in the Sahel/Sudanian belt of Africa, the main determinants of interannual variation in vegetation dynamics are rainfall and land cover type. We analyzed the spatio-temporal sensitivity of the NOAA-AVHRR 8 km-resolution vegetation index (NDVI) to (i) annual rainfall variability (0.5° × 0.5° resolution) acquired over a 25-year period (1982-2006); and (ii) land use changes in the different eco-climatic regions of the Bani catchment in Mali (130 000 km²). During the period 1982-2006, there was no clear trend in rainfall over the catchment, whereas there was a strong positive trend in the NDVI, both when the NDVI values were corrected using annual rainfall variability and when they were not. We divided the catchment into three eco-climatic regions based on the relationship between the annual NDVI and rainfall. In each region, we analyzed the observed greening in relation to changes in land use after correcting for the effect of annual rainfall on the NDVI. Results show that there is a mixed level of agreement between the land cover changes at the grid cell scale and the spatial pattern of the NDVI trend. Increased cropping does not explain the increase in the annual NDVI, except in the Sahelian part of the catchment. We hypothesize that the natural vegetation dynamics related to the non-linear rainfall patterns during the 25-year study period were responsible for these results.  相似文献   

Changes of P, Ca, Al and Fe contents in fringe marshes along a pedogenic chronosequence in the Pearl River estuary, South China   总被引：2，自引：0，他引：2  

Rong Xiao  Honggang ZhangHaifeng Gao  Xinhui LiuAndreas Wilkes 《Continental Shelf Research》2011,31(6):739-747

Soil and plant samples were collected in four fringe marsh zones (i.e., A, B, C and D zones) along a pedogenic chronosequence in the Pearl River estuary in the Spring of 2009. Samples were subjected to a total digestion technique and analyzed for P, Ca, Al and Fe in order to study the changes of nutrient contents, storages in soils and their bioavailabilities to wetland plants (e.g. Cyperus malaccensis) in four zones. Results showed that soil Ca increased with depth along soil profiles, while P, Al and Fe generally kept constant in soil profiles in four zones. Al and Fe contents in the top 10 cm soils showed significant decreases from D to A zone, while a significant increase in Ca contents (P<0.05). Significant increases along pedogenic chronosequence for P, Al and Fe at 30-40 cm soil horizons were also observed. Ca was mainly accumulated in plant aboveground parts; Al and Fe were accumulated in the belowground parts; while P was homogenously distributed among the tissues of C. malaccensis. C. malaccensis in D zone had lower Bio-concentration factors (BCFs) of P in the shoots and Al and Fe in the roots, and higher values of Ca in the shoots than those in older zones (P<0.05). Compared to Al and Fe, both Ca and P had relatively higher translocation capacities for C. malaccensis, while only lower TFs for P and higher values for Al and Fe in D zone were observed than those in A and B zones (P<0.05). Except for Al with no significant changes, the total BCFs for P and Fe showed an increasing trend with soil ages, while a decreasing trend for Ca. The total bio-storage factors (TBSFs) of P and Ca declined with the pedogenic time, whereas an increase for Fe. The results of this study can contribute to the wetland conservation and management in the Pearl River estuarine region.  相似文献   

Effects of Watershed Land Use and Lake Morphometry on the Trophic State of Chinese Lakes: Implications for Eutrophication Control   总被引：1，自引：0，他引：1  

Wenzhi Liu  Quanfa Zhang  Guihua Liu 《洁净——土壤、空气、水》2011,39(1):35-42

Water quality in lakes is influenced by a large number of watershed and lake characteristics. In this study, we examined the relative effects of watershed land use and lake morphology on the trophic state of 19 lakes in the Yunnan plateau and lower Yangtze floodplain, the two most eutrophic regions in China. Trophic state parameters consisted of total nitrogen, total phosphorus, chemical oxygen demand, chlorophyll‐a, Secchi depth, and trophic state index, while lake morphometric variables included area, maximum depth, mean depth, water residence time (WRT), volume, and length to width ratio. Percentages of forest, grassland, cropland, unused land, built‐up land, and water body in each lake's watershed were extracted from a land use map interpreted from Landsat TM images. A t‐test indicated that lower Yangtze floodplain lakes were shallower and had higher percentages of cropland and built‐up land in watersheds than Yunnan plateau lakes. Pearson's correlation analysis indicated that both watershed land use and lake morphometric variables were significantly related to most of the trophic state parameters. However, stepwise regression analyses demonstrated that the trophic state of the lower Yangtze floodplain lakes was mainly controlled by the percentages of cropland and built‐up land in watersheds, while that of Yunnan plateau lakes was mostly determined by the lake depth and WRT. These results suggest that the relative effects of watershed land use and lake morphology on lake trophic state are dependent on the lake's location. This study can provide some useful information in watershed land use management for controlling eutrophication in Chinese lakes.  相似文献   

A wetting and drying algorithm with a combined pressure/free-surface formulation for non-hydrostatic models     

S.W. Funke  C.C. Pain  S.C. Kramer  M.D. Piggott 《Advances in water resources》2011,34(11):1483-1495

A wetting and drying method for free-surface problems for the three-dimensional, non-hydrostatic Navier–Stokes equations is proposed. The key idea is to use a horizontally fixed mesh and to apply different boundary conditions on the free-surface in wet and dry zones. In wet areas a combined pressure/free-surface kinematic boundary condition is applied, while in dry areas a positive water level and a no-normal flow boundary condition are enforced. In addition, vertical mesh movement is performed to accurately represent the free-surface motion. Non-physical flow in the remaining thin layer in dry areas is naturally prevented if a Manning–Strickler bottom drag is used. The treatment of the wetting and drying processes applied through the boundary condition yields great flexibility to the discretisation used. Specifically, a fully unstructured mesh with any finite element choice and implicit time discretisation method can be applied. The resulting method is mass conservative, stable and accurate. It is implemented within Fluidity-ICOM [1] and verified against several idealized test cases and a laboratory experiment of the Okushiri tsunami.  相似文献   

Study on the S-wave Splitting Characteristics in the Longtan Reservoir Area,Guangxi,China     

Shi Haixi  Zhao Cuiping 《中国地震研究》2011,25(2):213-226

In this paper,the shear wave splitting features of the Longtan reservoir area are studied by adopting the traditional cross-correlation coefficient method and polarization analysis,using the data recorded by the seismic network founded by a project under the National Science and Technology Pillar Program from April 2009 to April 2010.We found that most of polarization directions at seismic stations are consistent with the direction of the overall regional stress field,but local structures and faults may con...  相似文献