Jiming Guo

Real-Time GPS Precise Point Positioning-Based Precipitable Water Vapor Estimation for Rainfall Monitoring and Forecasting

GPS-based precipitable water vapor (PWV) estimation has been proven as a cost-effective approach for numerical weather prediction. Most previous efforts focus on the performance evaluation of post-processed GPS-derived PWV estimates using International GNSS Service (IGS) satellite products with at least 3-9-h latency. However, the suggested timeliness for meteorological nowcasting is 5-30 min. Therefore, the latency has limited the GPS-based PWV estimation in real-time meteorological nowcasting. The limitation has been overcome since April 2013 when IGS released real-time GPS orbit and clock products. This becomes the focus of this paper, which investigates real-time GPS precise point positioning (PPP)-based PWV estimation and its potential for rainfall monitoring and forecasting. This paper first evaluates the accuracy of IGS CLK90 real-time orbit and clock products. Root-mean-square (RMS) errors of <; 5 cm and ~0.6 ns are revealed for real-time orbit and clock products, respectively, during July 4-10, 2013. Second, the real-time GPS PPP-derived PWV values obtained at IGS station WUHN are compared with the post-processed counterparts. The RMS difference of 2.4 mm has been identified with a correlation coefficient of 0.99. Third, two case studies, including a severe rainfall event and a series of moderate rainfall events, have been presented. The agreement between the real-time GPS PPP-derived PWV and ground rainfall records indicates the feasibility of real-time GPS PPP-derived PWV for rainfall monitoring. Moreover, the significantly reduced latency demonstrates a promising perspective of real-time GPS PPP-based PWV estimation as an enhancement to existing forecasting systems for rainfall forecasting.

Surface Subsidence Analysis by Multi-Temporal InSAR and GRACE: A Case Study in Beijing.

The aim of this study was to investigate the relationship between surface subsidence and groundwater changes. To investigate this relationship, we first analyzed surface subsidence. This paper presents the results of a case study of surface subsidence in Beijing from 1 August 2007 to 29 September 2010. The Multi-temporal Interferometric Synthetic Aperture Radar (multi-temporal InSAR) technique, which can simultaneously detect point-like stable reflectors (PSs) and distributed scatterers (DSs), was used to retrieve the subsidence magnitude and distribution in Beijing using 18 ENVISAT ASAR images. The multi-temporal InSAR-derived subsidence was verified by leveling at an accuracy better than 5 mm/year. Based on the verified multi-temporal InSAR results, a prominent uneven subsidence was identified in Beijing. Specifically, most of the subsidence velocities in the downtown area were within 10 mm/year, and the largest subsidence was detected in Tongzhou, with velocities exceeding 140 mm/year. Furthermore, Gravity Recovery and Climate Experiment (GRACE) data were used to derive the groundwater change series and trend. By comparison with the multi-temporal InSAR-derived subsidence results, the long-term decreasing trend between groundwater changes and surface subsidence showed a relatively high consistency, and a significant impact of groundwater changes on the surface subsidence was identified. Additionally, the spatial distribution of the subsidence funnel was partially consistent with that of groundwater depression, i.e., the former possessed a wider range than the latter. Finally, the relationship between surface subsidence and groundwater changes was determined.

An Optimal Weighting Method of Global Positioning System (GPS) Troposphere Tomography

The functional model of Global Positioning System (GPS) troposphere tomography consists of three types of equations including the observation equation, the horizontal constraint equation, and the vertical constraint equation. The prerequisite for ensuring the accuracy of troposphere tomography modeling is to determine the optimal weights for the three types of equations. In order to reasonably determine the weights among these equations, this paper proposes an optimal weighting method. Compared to the conventional equal weighting scheme and constant weighting scheme, the method proposed in this paper can adaptively adjust the weights for various equations and enable the posterior unit weight variances for the three types of equations that achieve statistically equal. Numerical results under various weather conditions showed that the proposed method can significantly improve the accuracy of GPS tomography modeling with the GPS PPP-estimated slant tropospheric delay as reference when compared to the other two conventional methods.

Wuhan Surface Subsidence Analysis in 2015–2016 Based on Sentinel-1A Data by SBAS-InSAR

The Terrain Observation with Progressive Scans (TOPS) acquisition mode of Sentinel-1A provides a wide coverage per acquisition and features a repeat cycle of 12 days, making this acquisition mode attractive for surface subsidence monitoring. A few studies have analyzed wide-coverage surface subsidence of Wuhan based on Sentinel-1A data. In this study, we investigated wide-area surface subsidence characteristics in Wuhan using 15 Sentinel-1A TOPS Synthetic Aperture Radar (SAR) images acquired from 11 April 2015 to 29 April 2016 with the Small Baseline Subset Interferometric SAR (SBAS InSAR) technique. The Sentinel-1A SBAS InSAR results were validated by 110 leveling points at an accuracy of 6 mm/year. Based on the verified SBAS InSAR results, prominent uneven subsidence patterns were identified in Wuhan. Specifically, annual average subsidence rates ranged from −82 mm/year to 18 mm/year in Wuhan, and maximum subsidence rate was detected in Houhu areas. Surface subsidence time series presented nonlinear subsidence with pronounced seasonal variations. Comparative analysis of surface subsidence and influencing factors (i.e., urban construction, precipitation, industrial development, carbonate karstification and water level changes in Yangtze River) indicated a relatively high spatial correlation between locations of subsidence bowl and those of engineering construction and industrial areas. Seasonal variations in subsidence were correlated with water level changes and precipitation. Surface subsidence in Wuhan was mainly attributed to anthropogenic activities, compressibility of soil layer, carbonate karstification, and groundwater overexploitation. Finally, the spatial-temporal characteristics of wide-area surface subsidence and the relationship between surface subsidence and influencing factors in Wuhan were determined.

Performance analysis of BDS/GPS kinematic vehicle positioning in various observation conditions

Purpose Global positioning system (GPS) kinematic positioning suffers from performance degradation in constrained environments such as urban canyons, which then restricts the application of high-precision vehicle positioning and navigation within the city. In December 2012, the BeiDou Navigation Satellite System (BDS) regional service was announced, and the combined BDS/GPS kinematic positioning has been enabled in the Asia-Pacific area. Previous studies have mainly focused on the performance evaluations of combined BDS/GPS static positioning. Not much work has been performed for kinematic vehicle positioning under constrained observation conditions. This study aims to analyze the performance of BDS/GPS kinematic vehicle positioning in various conditions. Design/methodology/approach In this study, three vehicle experiments under three observation conditions, an open suburban area, a less dense non-central urban area and a dense central urban area, are investigated using both the code-based differential global navigation satellite system (DGNSS) and phase-based real-time kinematic (RTK) modes. The comparison between combined BDS/GPS and GPS-only vehicle positioning solutions is conducted in terms of positioning availability and positioning precision. Findings Numerical results show that the combined BDS/GPS system significantly outperforms the GPS-only system under poor observation conditions, whereas the improvement was less significant under good observation conditions. Originality/value Thus, this paper studies the performance of combined BDS/GPS kinematic relative positioning under various observation conditions.

Precise Orbit Determination of BeiDou Satellites with Contributions from Chinese National Continuous Operating Reference Stations

The precise orbit determination (POD) for BeiDou satellites is usually limited by the insufficient quantity and poor distribution of ground tracking stations. To cope with this problem, this study used the GPS and BeiDou joint POD method based on Chinese national continuous operating reference stations (CNCORS) and IGS/MGEX stations. The results show that the 3D RMS of the differences of overlapping arcs is better than 22 cm for geostationary orbit (GEO) satellites and better than 10 cm for inclined geosynchronous orbit (IGSO) and medium earth orbit (MEO) satellites. The radial RMS is better than 2 cm for all three types of BeiDou satellites. The results of satellite laser ranging (SLR) residuals show that the RMS of the IGSO and MEO satellites is better than 5 cm, whereas the GEO satellite has a systematic bias. This study investigates the contributions of CNCORS to the POD of BeiDou satellites. The results show that after the incorporation of CNCORS, the precision of overlapping arcs of the GEO, IGSO, and MEO satellites is improved by 15.5%, 57.5%, and 5.3%, respectively. In accordance with the improvement in the precision of overlapping arcs, the accuracy of the IGSO and MEO satellites assessed by the SLR is improved by 30.1% and 4.8%, respectively. The computation results and analysis demonstrate that the inclusion of CNCORS yields the biggest contribution in the improvement of orbit accuracy for IGSO satellites, when compared to GEO satellites, while the orbit improvement for MEO satellites is the lowest due to their global coverage.

Land subsidence in Tianjin for 2015 to 2016 revealed by the analysis of Sentinel-1A with SBAS-InSAR

Abstract. It has been suggested that Tianjin, China, has significant land subsidence due to excessive extraction of water. Although it is presently under control, the land subsidence around Tianjin suburbs in recent years should not be ignored. However, existing research work on land subsidence is based on traditional synthetic aperture radar satellite images in which the research time spans are mainly before 2012. An advanced time-series method, namely small baselines subset (SBAS) technique, is applied to a total of 27 Sentinel-1A images over Tianjin acquired between May 31, 2015, and May 13, 2016, to derive the subsidence magnitude and distribution of Tianjin. Furthermore, the overall and quantitative validations of SBAS-derived results are implemented. First, the overall subsidence distribution derived by SBAS is compared with the annual report of land subsidence in Tianjin 2015, which shows the same subsidence trend and distribution. Then, 44 benchmarks and 2 continuously operating reference station datasets, i.e., CH01 and XQYY, are processed to provide a specific validation of SBAS-derived results of Sentinel-1A. Finally, through investigation, an interpretation from two aspects of groundwater extraction and geological structures of the surrounding Wangqingtuo settlement funnel area is given.

Monitoring and analyzing surface subsidence based on SBAS-InSAR in Beijing region, China

Surface subsidence is the main regional environmental geological disaster in plain area in China. The rapid growth of population, the over-exploitation of groundwater and the rapid development of urbanization impacts the occurrence and development of surface subsidence to some extent. The city of Beijing, located in the Beijing Plain, is one of international metropolis in China that experiences the severe surface subsidence. Because of conventional measurement methods with low spatial resolution, differential interferometric synthetic aperture radar(D-InSAR) is susceptible to signal decorrelation and atmospheric delay, persistent scatterer interferometric synthetic aperture radar(PS-InSAR) is based on a large number of SAR images, but small baseline subset interferometric synthetic aperture radar (SBASInSAR) only needs a small number of images and performs better than PS-InSAR for obtaining nonlinear deformation information, in this paper, SBAS-InSAR was used to obtain the high resolution surface subsidence information in Beijing region, China. A spatial-temporal analysis of the surface subsidence in Beijing region during the years of 2007- 2010 was performed utilizing eighteen C-band ENVISAT ASAR images (from August 1, 2007 to September 29, 2010). The results show that subsidence in Beijing region is severe uneven, subsidence funnels appear in Changping District, Shunyi District, Tongzhou District, Daxing District, etc., and many subsidence funnels are interconnected and have an eastward expansion trend; during the period of 2007 to 2010, the subsidence velocities are in the range of -158.5 mm/year to 12.4 mm/year and the maximum subsidence of subsidence center is over 400 mm; surface subsidence is influenced by groundwater exploitation and urbanization significantly.

FARSE scheme for single epoch GPS solution based on DUFCOM and DC algorithm and its performance analysis

Abstract Fast integer ambiguity resolution for single epoch observation is one of main issues of GPS precise positioning in real time surveying applications. An improved solution of dual frequency correlation method (DUFCOM) and direct calculation method (DC), named as fast ambiguity resolution for single epoch scheme (FARSE), is proposed in this paper. A software based on the proposed scheme for monitoring of construction cranes, Gsertcas, is developed. With the help of Gsertcas, the performance and suitability of FARSE are investigated through simulation experiments. The result of the experiment demonstrates that the success rate for ambiguity resolution is above 97% and the root mean square of the position solution with correct ambiguity resolution is better than 3·8 mm.

A Method to Improve the Distribution of Observations in GNSS Water Vapor Tomography

Water vapor is an important driving factor in the related weather processes in the troposphere, and its temporal-spatial distribution and change are crucial to the formation of cloud and rainfall. Global Navigation Satellite System (GNSS) water vapor tomography, which can reconstruct the water vapor distribution using GNSS observation data, plays an increasingly important role in GNSS meteorology. In this paper, a method to improve the distribution of observations in GNSS water vapor tomography is proposed to overcome the problem of the relatively concentrated distribution of observations, enable satellite signal rays to penetrate more tomographic voxels, and improve the issue of overabundance of zero elements in a tomographic matrix. Numerical results indicate that the accuracy of the water vapor tomography is improved by the proposed method when the slant water vapor calculated by GAMIT is used as a reference. Comparative results of precipitable water vapor (PWV) and water vapor density (WVD) profiles from radiosonde station data indicate that the proposed method is superior to the conventional method in terms of the mean absolute error (MAE), standard deviations (STD), and root-mean-square error (RMS). Further discussion shows that the ill-condition of tomographic equation and the richness of data in the tomographic model need to be discussed separately.