13 results on '"Zhang, Kefei"'
Search Results
2. Radiosonde-Based New Spatiotemporal Modelling for the Construction of Temperature Profiles for GNSS Applications
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Li, Longjiang, Shen, Zhen, He, Qimin, Wan, Mofeng, Zhang, Kefei, Wu, Suqin, Angrisani, Leopoldo, Series Editor, Arteaga, Marco, Series Editor, Panigrahi, Bijaya Ketan, Series Editor, Chakraborty, Samarjit, Series Editor, Chen, Jiming, Series Editor, Chen, Shanben, Series Editor, Chen, Tan Kay, Series Editor, Dillmann, Rüdiger, Series Editor, Duan, Haibin, Series Editor, Ferrari, Gianluigi, Series Editor, Ferre, Manuel, Series Editor, Hirche, Sandra, Series Editor, Jabbari, Faryar, Series Editor, Jia, Limin, Series Editor, Kacprzyk, Janusz, Series Editor, Khamis, Alaa, Series Editor, Kroeger, Torsten, Series Editor, Liang, Qilian, Series Editor, Martín, Ferran, Series Editor, Ming, Tan Cher, Series Editor, Minker, Wolfgang, Series Editor, Misra, Pradeep, Series Editor, Möller, Sebastian, Series Editor, Mukhopadhyay, Subhas, Series Editor, Ning, Cun-Zheng, Series Editor, Nishida, Toyoaki, Series Editor, Pascucci, Federica, Series Editor, Qin, Yong, Series Editor, Seng, Gan Woon, Series Editor, Speidel, Joachim, Series Editor, Veiga, Germano, Series Editor, Wu, Haitao, Series Editor, Zhang, Junjie James, Series Editor, Sun, Jiadong, editor, Yang, Changfeng, editor, and Xie, Jun, editor
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- 2020
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3. An Improved Principal Component Analysis Method for the Interpolation of Missing Data in GNSS-Derived PWV Time Series.
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Zhu, Dantong, Zhong, Zhenhao, Zhang, Minghao, Wu, Suqin, Zhang, Kefei, Li, Zhen, Hu, Qingfeng, Liu, Xianlin, and Liu, Junguo
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PRINCIPAL components analysis ,MISSING data (Statistics) ,PRECIPITABLE water ,GLOBAL Positioning System ,TIME series analysis ,ROOT-mean-squares - Abstract
Missing data in precipitable water vapor derived from global navigation satellite systems (GNSS-PWV) is commonly a large hurdle in climatical applications, since continuous PWV is an important prerequisite. Interpolation using principal component analysis (PCA) is typically used to resolve this problem. However, the popular PCA-based interpolating methods, e.g., rank-deficient least squares PCA (RDPCA) and data interpolating empirical orthogonal function (DINEOF), often lead to unsatisfactory results. This study analyzes the relationship between missing data and PCA-based interpolation results and proposes an improved interpolation-based RDPCA (IRDPCA) that can take into account the PWV derived from ERA5 (ERA-PWV) as an additional aid. Three key steps are involved in the IRDPCA: initially interpolating missing data, estimating principal components through a functional model and optimizing the interpolation through an iterative process. Using a 6-year GNSS-PWV over 26 stations and ERA-PWV in Yunnan, China, the performance of the IRDPCA is compared with the RDPCA and DINEOF using simulation experiments based on both homogeneous data (i.e., interpolating ERA-PWV using available ERA-PWV) and heterogeneous data (i.e., interpolating GNSS-PWV using ERA-PWV). In the case of using homogeneous data, the root mean square (RMS) values of the interpolation errors are 3.45, 1.18 and 1.17 mm for the RDPCA, DINEOF and IRDPCA, respectively; while the values are 3.50, 2.50 and 1.55 mm in the heterogeneous case. These results demonstrate the superior performance of the IRDPCA in both the heterogeneous and homogeneous cases. Moreover, these methods are also applied to the interpolation of the real GNSS-PWV. The RMS, absolute bias and correlation of the GNSS-PWV are calculated by comparison with ERA-PWV. The results reveal that the interpolated GNSS-PWV using the IRDPCA is not impacted by the systematic discrepancies in the ERA-PWV and agrees well with the original data. [ABSTRACT FROM AUTHOR]
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- 2023
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4. A new troposphere tomography algorithm with a truncation factor model (TFM) for GNSS networks
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Zhao, Qingzhi, Zhang, Kefei, Yao, Yibin, and Li, Xin
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- 2019
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5. Homogenization of daily precipitable water vapor time series derived from GNSS observations over China.
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Zhu, Dantong, Zhang, Kefei, Sun, Peng, Wu, Suqin, and Wan, Moufeng
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PRECIPITABLE water , *GLOBAL Positioning System , *TIME series analysis , *METADATA , *ANTENNAS (Electronics) - Abstract
Precipitable water vapor (PWV) retrieved from Global Navigation Satellite Systems (GNSS) observations (GNSS-PWV) subjects to non-climatic changepoints (NCCs), mainly due to hardware (e.g., antenna and receiver) changes and geological events, while only the former is documented in station metadata. This study proposed a new strategy to verify the origins of the undocumented NCCs in GNSS-PWV and correct the inhomogeneity based on ERA5 and coordinate time series of GNSS stations. The new strategy initially applies a zero-difference method to detect changepoints in GNSS coordinate time series (coordinate changepoints) and undocumented NCCs in GNSS-PWV with the aid of PWV derived from ERA5 (ERA-PWV) and station metadata. The coordinate changepoints are treated as ancillary to verify the origins of the undocumented NCCs related to geological events in GNSS-PWV since coordinates are also influenced by geological events. After the verification, shifts and directions of all NCCs were robustly estimated and corrected for a final homogenization. The strategy was applied in the GNSS-PWV at 207 stations during 2008–2018 in China. Results showed that among 193 NCCs detected in 110 inhomogeneous GNSS-PWV, only 5 ones (3% of NCCs) were documented in metadata, and 44 ones (23% of NCCs) were related to geological events, indicating the good performance of the new strategy on the identification of the undocumented NCCs. After the homogenization, the RMS and absolute bias between GNSS-PWV and ERA-PWV were decreased by 5% and 22%, and the correlation coefficient of their linear trends was improved to 0.98 from 0.75. [ABSTRACT FROM AUTHOR]
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- 2023
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6. A Novel Method for Monitoring Tropical Cyclones' Movement Using GNSS Zenith Tropospheric Delay.
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Lian, Dajun, He, Qimin, Li, Li, Zhang, Kefei, Fu, Erjiang, Li, Guangyan, Wang, Rui, Gao, Biqing, and Song, Kangming
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TROPICAL cyclones ,PRECIPITABLE water ,GLOBAL Positioning System ,EXTREME weather ,ARTIFICIAL satellites in navigation - Abstract
Precipitable water vapor (PWV) is an important meteorological factor for predicting extreme weather events such as tropical cyclones, which can be obtained from zenith tropospheric delay (ZTD) by using a conversion. A time difference of ZTD arrival (TDOZA) model was proposed to monitor the movement of tropical cyclones, and the fifth-generation reanalysis dataset of the European Centre for Medium-range Weather Forecasting (ERA5)-derived ZTD (ERA5-ZTD) was used to estimate the movement of tropical cyclones based on the model. The global navigation satellite system-derived ZTD and radiosonde data-derived PWV (RS-PWV) were used to test the accuracy of the ERA5-ZTD and analyze the correlation between ZTD and PWV, respectively. The statistics showed that the mean Bias, RMS and STD of the ERA5-ZTD were 6.4 mm, 17.1 mm and 16.5 mm, respectively, and the mean correlation coefficient of the ERA5-ZTD and RS-PWV was 0.951, which indicates that the ZTD can be used to predict weather events instead of PWV. Then, spatiao-temporal characteristics of ZTD during the four tropical cyclone (i.e., Merbok, ROKE, Neast and Hato) periods in 2017 were analyzed, and the result showed that the moving directions of ZTD and the tropical cyclones were consistent. Thus, the ZTD time series over the ERA5 grids around the tropical cyclones' paths were used to estimate the velocity of the tropical cyclones based on the TDOZA model, when the tropical cyclones are approaching or leaving. Compared with the result from the China Meteorological Administration, the mean absolute and relative deviations of the TDOZA model-derived velocity were 2.55 km/h and 10.0%, respectively. These results suggest that ZTD can be used as a new supplementary meteorological parameter for monitoring tropical cyclone events. [ABSTRACT FROM AUTHOR]
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- 2023
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7. Ionospheric responses to typhoons in Australia during 2005–2014 using GNSS and FORMOSAT-3/COSMIC measurements
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Li, Wang, Yue, Jianping, Wu, Suqin, Yang, Yang, Li, Zhen, Bi, Jingxue, and Zhang, Kefei
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- 2018
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8. Extracting an ionospheric phase scintillation index based on 1 Hz GNSS observations and its verification in the Arctic region
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ZHAO Dongsheng, LI Wang, LI Chendong, TANG Xu, and ZHANG Kefei
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monitoring ,gnss ,Physics::Instrumentation and Detectors ,1 hz sampling data ,Physics::Space Physics ,phase scintillation index ,Mathematical geography. Cartography ,GA1-1776 ,Physics::Geophysics ,ionospheric scintillation - Abstract
The ionospheric scintillation, as one of the astronomical disasters occurring frequently in Arctic regions, poses great challenges to GNSS positioning navigation and timing (PNT) services. This calls for an urgent need in studying and effectively monitoring the scintillation to overcome its adverse impact. With the capability of high frequency sampling, ionospheric scintillation monitoring receivers (ISMR) are usually required to monitor the ionospheric scintillation, but the distribution of ISMR restricts the comprehensive monitoring in larger areas (such as the Arctic region). Therefore, based on GNSS observations with 1 Hz sampling, this paper studies the relevant empirical parameters and methods of extracting the ionospheric scintillation signal from the carrier phase observations by using geodetic detrending, precise point positioning and wavelet transform techniques, to construct a new phase scintillation index, which can be used to monitor the ionospheric scintillation. Its effectiveness and accuracy are verified by 188-day observations from 11 stations provided by the Canadian High Arctic Ionospheric Network (CHAIN). The results show that, compared with the commonly used ROTI index, both the scintillation index proposed in this paper and ROTI can effectively detect the occurrence of ionospheric scintillation, but the scintillation index proposed in this paper has a better correlation with the phase scintillation index given by ISMR, especially during periods with strong ionospheric scintillation, indicating that the proposed scintillation index has better ionospheric scintillation monitoring capability.
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- 2021
9. Validating Ionospheric Scintillation Indices Extracted from 30s-Sampling-Interval GNSS Geodetic Receivers with Long-Term Ground and In-Situ Observations in High-Latitude Regions.
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Zhao, Dongsheng, Wang, Qianxin, Li, Wang, Shi, Shuangshuang, Quan, Yiming, Hancock, Craig M., Roberts, Gethin Wyn, Zhang, Kefei, Chen, Yu, Liu, Xin, Hao, Zemin, Cui, Shuanglei, Zhang, Xueli, and Wang, Xing
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GLOBAL Positioning System ,SPACE environment ,GEOMAGNETISM - Abstract
As a frequently-occurred phenomenon in the high-latitude region, ionospheric scintillations affect the stable service of the positioning navigation and timing service of the Global Navigation Satellite System (GNSS), calling for an urgent need of monitoring the scintillations accurately. The monitoring of scintillations usually adopts a special type of receiver, called an ionospheric scintillation monitoring receiver (ISMR), which cannot cover the whole high-latitude region due to its loss distribution. Geodetic receivers are densely distributed, but set at a 30s-sampling-interval usually. It is a controversial issue, namely, the accuracy of the scintillation index extracted from 30s-sampling-interval observations. This paper evaluates the accuracy of two 30s-sampling-interval indices in monitoring scintillations from both the time and space aspects using observations collected in the whole year of 2020. The accuracy in the time aspect is assessed with the phase scintillation index from ISMR as the reference through the following three-pronged approaches, i.e., the accuracy of the daily scintillation occurrence rates in the year 2020, the correlation with space weather parameters, and the variation pattern of the scintillation occurrence rate with the local time and day of the year 2020. The accuracy in space is studied based on the scintillation grid model considering the following two aspects, i.e., the scintillation monitoring performance in a Swarm satellite observation arc, and the statistical scintillation occurrence rate in the whole research region throughout the year 2020. The results of this paper reveal the efficiency of the 30s-sampling-interval scintillation indices in monitoring scintillations and detecting the occurrence patterns in the high-latitude region. The outcome of this paper can provide a basic idea for introducing the widely distributed geodetic receivers to monitor and model the scintillations in the high-latitude region. [ABSTRACT FROM AUTHOR]
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- 2022
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10. An Investigation of Near Real-Time Water Vapor Tomography Modeling Using Multi-Source Data.
- Author
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Tong, Laga, Zhang, Kefei, Li, Haobo, Wang, Xiaoming, Ding, Nan, Shi, Jiaqi, Zhu, Dantong, and Wu, Suqin
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WATER vapor , *GLOBAL Positioning System , *STANDARD deviations , *TOMOGRAPHY , *RADIANCE - Abstract
Global Navigation Satellite Systems (GNSS) tomography is a well-recognized modeling technique for reconstruction, which can be used to investigate the spatial structure of water vapor with a high spatiotemporal resolution. In this study, a refined near real-time tomographic model is developed based on multi-source data including GNSS observations, Global Forecast System (GFS) products and surface meteorological data. The refined tomographic model is studied using data from Hong Kong from 2 to 11 October 2021. The result is compared with the traditional model with physical constraints and is validated by the radiosonde data. It is shown that the root mean square error (RMSE) values of the proposed model and traditional model are 0.950 and 1.763 g/m3, respectively. The refined model can decrease the RMSE by about 46%, indicating a better performance than the traditional one. In addition, the accuracy of the refined tomographic model is assessed under both rainy and non-rainy conditions. The assessment shows that the RMSE in the rainy period is 0.817 g/m3, which outperforms the non-rainy period with the RMSE of 1.007 g/m3. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
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11. Node-Based Optimization of GNSS Tomography with a Minimum Bounding Box Algorithm.
- Author
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Ding, Nan, Yan, Xiangrong, Zhang, Shubi, Wu, Suqin, Wang, Xiaoming, Zhang, Yu, Wang, Yuchen, Liu, Xin, Zhang, Wenyuan, Holden, Lucas, and Zhang, Kefei
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GLOBAL Positioning System ,ALGORITHMS ,TOMOGRAPHY ,WATER vapor ,STANDARD deviations - Abstract
Global Navigation Satellite Systems (GNSS) tomography plays an important role in the monitoring and tracking of the tropospheric water vapor. In this study, a new approach for improving the node-based GNSS tomography is proposed, which makes a trade-off between the real observed region and the complexity of the discretization of the tomographic region. To obtain dynamically the approximate observed region, the convex hull algorithm and minimum bounding box algorithm are used at each tomographic epoch. This new approach can dynamically define the tomographic model for all types of study areas based on the GNSS data. The performance of the new approach is tested by comparing it against the common node-based GNSS tomographic approach. Test data in May 2015 are obtained from the Hong Kong GNSS network to build the tomographic models and the radiosonde data as a reference are used for validating the quality of the new approach. The experimental results show that the root-mean-square errors of the new approach, in most cases, have a 38 percent improvement and the values of standard deviation reduce to over 43 percent compared with the common approach. The results indicate that the new approach is applicable to the node-based GNSS tomography. [ABSTRACT FROM AUTHOR]
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- 2020
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12. An Evaluation of Fengyun-3C Radio Occultation Atmospheric Profiles Over 2015–2018.
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Wei, Jinde, Li, Ying, Zhang, Kefei, Liao, Mi, Bai, Weihua, Liu, Congliang, Liu, Yan, and Wang, Xiaoming
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ALTITUDES ,SAMPLING errors ,STANDARD deviations ,RADIOS - Abstract
Fengyun-3C (FY-3C) is the first Chinese satellite that is capable of using the Radio Occultation (RO) technique to retrieve atmospheric profiles. This research evaluates the quality of FY-3C RO profiles including refractivity, temperature, and specific humidity by comparing with corresponding information from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim) data over the period of 2015–2018. The evaluation is carried out by calculating and analyzing mean systematic differences between FY-3C and ERA-Interim profiles and corresponding standard deviations over a selected spatial and temporal domain. Results show that the FY-3C RO profiles are overall with good agreements with the ERA-Interim data. Global mean refractivity systematic differences are within ±0.2% from 5 to 30 km altitude range with relative standard deviations of less than 2%. Global temperature mean systematic differences vary within ±0.2 K from a 10- to 20-km altitude range with standard deviations of less than 2 K. Global mean specific humidity differences are found to be within ±0.2 g/kg from 2 to 20 km with standard deviations of less than 1 g/kg. FY-3C profiles show visible latitudinal and altitudinal variations, while the seasonal variations are minor. Sampling errors of refractivity and temperature are also found to be larger at higher latitudinal regions due to RO events being less sampled in the polar region. [ABSTRACT FROM AUTHOR]
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- 2020
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13. Statistical study on the characterization of phase and amplitude scintillation events in the high-latitude region during 2014–2020 based on ISMR.
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Zhao, Dongsheng, Li, Wang, Wang, Qianxin, Liu, Xin, Li, Chendong, Hancock, Craig M., Roberts, Gethin Wyn, and Zhang, Kefei
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GLOBAL Positioning System , *SPACE environment , *TERRESTRIAL radiation , *SOLAR radiation - Abstract
A better understanding of the climatology of the ionospheric scintillation in the high-latitude region is beneficial to model the adverse effect of the ionospheric scintillation on the positioning navigation and timing services of Global Navigation Satellite System (GNSS). This paper conducts a statistical study on the characterization of the phase and amplitude scintillation events in the high-latitude region based on scintillation indices provided by ionospheric scintillation monitoring receivers (ISMR) collected during the years of 2014 to 2020. Results of this paper show that phase scintillation dominates in most of the high-latitude regions. The proportion of the phase scintillation with strong magnitude in the middle-high latitude region is greater than that in the high and middle latitude regions, but the duration of the scintillation in the middle-high latitude region is generally shorter, with an average value of about 8 min. The ionospheric scintillation presents seasonal and diurnal dependency due to the effect of the solar radiation and earth orientation. The relationships between the occurrence of ionospheric scintillation and the space weather parameters, Ap and IMF, are also studied, showing that Ap can reflect the intensity of the ionospheric scintillation in the middle-high latitude region, and the ionospheric scintillation occurs more frequently under the condition of the northward IMF. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
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