Table of Content

20 August 2022, Volume 51 Issue 8

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Collaborative Precision Positioning

Modeling and effects analysis of GNSS precise point positioning ambiguity resolution based on the decoupled clock method

YUAN Yunbin, LIU Shuai, TAN Bingfeng

2022, 51(8):  1669-1679.  doi:10.11947/j.AGCS.2022.20210554

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Precise point positioning with ambiguity resolution can significantly improve the positioning accuracy.However,as an important ambiguity resolution model,there are few literatures focused on the study of the decoupled clock model.Firstly,this paper introduces the product estimation strategy for ambiguity resolution based on the decoupled clock model,analyzes the differences of clock offsets between the conventional ionosphere-free model and the decoupled clock model,derives the satellite clock estimation model with satellite code bias extraction,and deeply investigates the advantages of the decoupled clock model in the convergence speed of satellite clock estimation.Different from other models,the wide-lane ambiguity bias of the decoupled clock model is time-varying in one day and estimated epoch by epoch.Based on this,the intra-day time-varying characteristics of wide-lane ambiguity bias are studied.Finally,the accuracy of the decoupled clocks is evaluated,and the precise point positioning ambiguity resolution experiment is carried out by using the decoupled clock products.The results show that,compared with the satellite clock estimation model with satellite code bias extraction,the decoupled clock model has faster convergence speed,and the clock product is more stable.The wide-lane ambiguity bias is relatively stable in one day,and the decoupled clock product has high accuracy.Compared with the conventional ionosphere-free model,the precise point positioning ambiguity resolution based on the decoupled clock product can significantly improve the positioning accuracy.

Assessment and improvement of the empirical solar radiation pressure models for BDS-3 satellites

LI Xingxing, LI Jie, YUAN Yongqiang, ZHENG Hongjie, HUANG Shi, LIU Chengbo, ZHANG Keke

2022, 51(8):  1680-1689.  doi:10.11947/j.AGCS.2022.20210532

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Solar radiation pressure (SRP) is the largest non-gravitational perturbation for GNSS satellites,and is the main error source of GNSS precise orbit determination (POD).Currently,two empirical SRP models,namely the extended CODE orbit model (ECOM) and the updated ECOM (ECOM2) model,are widely used in GNSS POD.However,these two models are not suitable for BDS-3 satellites,as they are designed for GPS and GLONASS satellites.The five-parameter ECOM model exhibits large orbit discontinuities in low β periods.By introducing the first-order harmonic D_c parameter,the orbit overlapping RMS values of CAST satellites can be reduced by 60%,52%,and 29% in the along-track,cross-track,and radial components,respectively.As for the ECOM2 model,strong correlations are observed between D_2c and D₀ parameters as well as that between D_2s and B_s parameters.Based on the strong correlations,the eight-parameter ECOM2 model and the seven-parameter ECOM2 model are proposed,where the parameters D_2c and D_2c/D_2s are not estimated,respectively.The results show that the radial overlapping RMS values can be reduced by about 18% and 27% for CAST and SECM satellites when using the eight-parameter ECOM2 model.In addition,a further reduction of 5.2%~8.5% in the radial overlapping RMS values can be achieved when applying seven-parameter ECOM2 model.By considering both the accuracy of overlapping orbit and SLR residuals,we conclude that the seven-parameter ECOM2 model without D_2c and D_2s is more applicable for BDS-3 POD.The orbit overlapping RMS values are (5.0,3.4,1.4) cm and (5.4,3.5,1.5) cm for CAST and SECM satellites in the along-track,cross-track,and radial components,respectively.The standard deviation values of SLR residuals are 3.1~3.2 cm and 4.4~4.7 cm for CAST and SECM satellites.

Improved tropospheric delay model for China using RBF neural network and meteorological data

XU Tianhe, LI Song, WANG Shuaimin, JIANG Nan

2022, 51(8):  1690-1707.  doi:10.11947/j.AGCS.2022.20210480

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Single-level meteorological products (ERA5 single-level data and measured meteorological parameters) and multi-level meteorological products (ERA5 pressure level data and COSMIC data) are used to estimate the ZTD of 236 CMONOC stations,namely ERA5S_ZTD,MET_ZTD,ERA5P_ZTD and RO_ZTD,based on the model method and the integration method respectively.Four ZTD estimations are evaluated with the reference of GNSS_ZTD;the results show that the average monthly RMSE are 42.8,53.6,16.1 and 62.3 mm,respectively.The accuracy of ERA5P_ZTD estimated by the integral method is the highest,that of ERA5S_ZTD and MET_ZTD calculated by the model method are the next.Estimating RO_ZTD with the integral method has the lowest accuracy.In order to further improve the accuracy of ZTD estimations,improved model of tropospheric delay is proposed with the RBF neural network in this paper.The calculation results show that:the average monthly RMSE between the ZTD from four improved models and GNSS_ZTD are 23.5,32.1,14.2 and 40.8 mm,which are reduced 43.4%,36.3%,10.0% and 34.4% than raw ZTD estimations.The overall modified effect of the improved model is obvious,and the improvement rate is related to the density of station distribution.

Precise cooperative positioning for vehicles with GNSS and INS integration

LI Bofeng, CHEN Guang'e

2022, 51(8):  1708-1716.  doi:10.11947/j.AGCS.2022.20210471

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Autonomous driving and intelligent transportation require not only the high-precision positioning of vehicles,but also the high-precision cooperative control between vehicles.The current research idea of relying on purely adding sensors to improve the accuracy of vehicle positioning still cannot effectively solve the complex situation of satellite signals losing lock for a long time.This paper proposes a vehicle-based cooperative high-precision positioning method based on GNSS/INS combined sensors.It imposes the spatial relative position constraints between vehicles by sharing their observations,thereby improving the positioning accuracy and reliability.The experimental results show that the GNSS/INS cooperative positioning mode can significantly improve the positioning accuracy in the harsh environment compared to the traditional standalone positioning mode.

Grid VRS technique of real time high-precision location-based service

HUANG Dingfa, FENG Wei, LI Jianfeng, GONG Xiaoying, LI Shuaishuai

2022, 51(8):  1717-1724.  doi:10.11947/j.AGCS.2022.20210435

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In order to ensure the positioning performance of virtual reference station (VRS) based network RTK,reduce the loading impact of massive concurrent users on the solution server,and improve the service quality of VRS,this paper systematically proposed a VRS gridding technique for massive concurrent user services,and established two methods for VRS generation,namely the coordinates and triangulated irregular network (TIN) VRS grid.Considering that the key problem of space-related atmospheric delay correction is the influence of ionosphere delay,the experimental tests are conducted in the CORS convergence of middle and low latitudes respectively.The effective service spacing of gridded VRS under different regional conditions is analyzed and determined,the results show that the effective service spacing limits of grid VRS in the middle latitude and low latitudes are 12 and 7 km,respectively.Compared with the non-grid VRS (built with SPS results),the VRS generation and computing load can be greatly reduced,and the server's service capabilities to massive users can be significantly improved.

Undifferenced and uncombined PPP-RTK:algorithmic models,prototype terminals and field-test results

ZHANG Baocheng, KE Cheng, ZHA Jiuping, HOU Pengyu, LIU Teng, YUAN Yunbin, LI Zishen

2022, 51(8):  1725-1735.  doi:10.11947/j.AGCS.2022.20210465

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The emerging industries like intelligent driving,precision agriculture and the drones require the global navigation satellite system (GNSS) to provide faster and more accurate positioning services.The undifferenced and uncombined PPP-RTK combines the advantages of precise point positioning (PPP) and those of real-time kinematic (RTK) technique,enabling one to process flexibly the multi-frequency and multi-GNSS data,and achieve rapidly the high-precision positioning with wide-area coverage.However,PPP-RTK is still in the stage of technology research and development;its industrial application has not yet completely formed.From theory to practice,this study formulates the code and frequency division multiple access undifferenced and uncombined PPP-RTK models,develops the corresponding real-time processing software,and designs a set of prototype terminals.Based on the precise corrections estimated by the Beijing-Tianjin-Hebei reference network with the inter-station distance of 154 km,we mount these terminals on a drone,an agricultural tractor and a car to carry out real-time positioning experiments.The results show that the times to first fix for three scenarios are 5 s,2 s and 7 s,the success rates of ambiguity resolution are 99.79%,99.14%,and 98.96%,respectively.The horizontal and vertical positioning accuracy are about 1 cm and 4 cm for all three scenarios.The results demonstrate that our PPP-RTK implementations can provide continuous,reliable and accurate positioning services.

High-precision cloud platform timing based on GNSS precise point positioning

LIU Genyou, GAO Ming, YIN Xiangfei, XIAO Gongwei, LÜ Dong, WANG Shengliang, WANG Run

2022, 51(8):  1736-1743.  doi:10.11947/j.AGCS.2022.20210441

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In this paper,based on GNSS precise point positioning (PPP) technology and clock steering technology,a cloud computing platform high-precision GNSS timing system is constructed,and the GNSS timing receiver prototype is developed with integrating multi-system GNSS receiver board,oven controlled crystal oscillator (OCXO) and digital signal processors (DSP).Using the 5 s interval satellite orbit and precise clock products provided by the Cooperative Precision Positioning Platform Analysis Center (Wuhan),PPP is used to calculate the coordinates and clock offset of the GNSS timing terminal in real time,and the OCXO can be steered to achieve long-term high-precision timing capability for sub-ns precision 1 PPS output.Experiments verify that the PPP timing accuracy is better than 1 ns (RMS) through the comparison between two GNSS timing terminals among short baseline and the comparison between UTC and the terminal time output.

Tightly-coupled stereo visual-inertial-LiDAR SLAM based on graph optimization

WANG Xuanbin, LI Xingxing, LIAO Jianchi, FENG Shaoquan, LI Shengyu, ZHOU Yuxuan

2022, 51(8):  1744-1756.  doi:10.11947/j.AGCS.2022.20210503

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Simultaneous localization and mapping (SLAM) technology based on a single sensor has gradually been unable to meet the increasingly complex application scenarios of the intelligent mobile carriers such as mobile robots,unmanned aerial vehicles,and self-driving cars.In order to further improve the localization and mapping performance of the mobile carriers in complex environments,multi-sensor fusion SLAM has become a hotspot of current research.In this contribution,we present a graph-optimization based and tightly-coupled stereo visual-inertial-LiDAR SLAM termed S-VIL SLAM,which integrates the LiDAR observations into a visual-inertial system.In this work,the IMU measurements,visual features,and laser point cloud features are jointly optimized in a sliding window.Moreover,a vision enhanced loop-closure algorithm of LiDAR is designed in this paper by using the complementary characteristics between vision and LiDAR,which further improves the global positioning and mapping accuracy of the multi-sensor fusion SLAM.We perform vehicle-borne experiments in outdoor environments to assess the performance of the proposed approach.The experimental results indicate that the proposed S-VIL odometry outperforms the state-of-the-art tightly coupled visual-inertial odometry (VIO) and LiDAR odometry in terms of pose estimation accuracy.The proposed loop-closure algorithm can effectively detect the loop closure of trajectories in large-scale scenes and achieve high-precision pose graph optimization.The point cloud map after loop closure optimization has good resolution and global consistency.

Geodesy and Navigation

Realization of the international height reference system in the region of Mount Qomolangma

JIANG Tao, DANG Yamin, GUO Chunxi, CHEN Bin, ZHANG Chuanyin

2022, 51(8):  1757-1767.  doi:10.11947/j.AGCS.2022.20210468

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For the first time,the orthometric height of Mount Qomolangma based on the international height reference system (IHRS) is determined and released in the 2020 height measurement campaign of Mount Qomolangma.In order to realize the IHRS in the region of Mount Qomolangma,the scheme of establishing high precision gravimetric geoid model in this area is adopted.Based on the spectral combination approach and data-driven spectral determination method,the gravimetric quasigeoid model in the area of Mount Qomolangma is computed from the combination of airborne and terrestrial gravity data.Optimal reference gravity model,its truncation degree and spherical cap integration radius are chosen through comprehensive test.The validation against highly accurate GNSS leveling measured height anomalies at 61 benchmarks indicates that the accuracy of the gravimetric quasigeoid model reaches 3.8 cm and the addition of airborne gravity data improves the model accuracy by 51.3%.The interpolation method considering height difference correction is proposed for interpolating the height anomaly at the summit from the quasigeoid model.The rigorous formula for geoid-quasigeoid separation considering topographic masses is used for converting the height anomaly to the geoid undulation at the summit.Based on the IHRS defined gravity potential value W₀ and the GRS80 reference ellipsoid,and using the newly observed ground gravity at the summit,the high precision geoid undulation of Mount Qomolangma in the IHRS is determined.

Inversion of global sea level change and its component contributions by combining time-varying gravity data and altimetry data

LI Yang, GUO Jinyun, SUN Yu, YUAN Jiajia, CHANG Xiaotao, ZHANG Hongri

2022, 51(8):  1768-1778.  doi:10.11947/j.AGCS.2022.20210169

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Understanding sea level change is of paramount importance,it indirectly reflects the changes of climate-related factors in earth system.Based on a set of sea level fingerprints and steric empirical orthogonal functions,a joint inversion is applied by combining time-varying gravity data and satellite altimetry data to derive global sea level change from April 2002 to February 2020.In this research,global sea level change was divided into the contributions of polar ice sheets melting,terrestrial glaciers melting,terrestrial water storage change,glacial isostatic adjustment and steric effect.Joint inversion result shows that global mean steric sea level change is 1.08±0.05 mm/a,which is consistent with the results of relevant literature.It was found that the joint inversion which combined altimetry and time-varying gravity data could reduce the underestimation of global mean ocean mass change during GRACE Follow-On period.Regional sea level changes were investigated by joint inversion,in most coastal areas,inversion results are good of quality,which shows that this method can be used to study regional sea level change.

GNSS carrier phase integer equivariant estimator and its application in improving PPP performance

ZHOU Feng, YANG Yuze, WANG Lei, XU Tianhe

2022, 51(8):  1779-1786.  doi:10.11947/j.AGCS.2022.20210371

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Precision point positioning (PPP) technology can provide global high-precision positioning results,but its main technical bottleneck lies in the long positioning convergence time.The carrier phase ambiguity fixing technology is one of the main means to speed up PPP convergence and improve positioning accuracy.The reliability of ambiguity fixing is particularly prominent in PPP positioning,because the quality of the ambiguity-float solutions depends on the quality of the server product,various factors such as the noise characteristics of the receiver and the observation environment,making high reliability PPP ambiguity fixing technology is still full of huge challenges.In order to ensure the reliability of PPP positioning,we introduce the best integer equivariant (BIE) estimator into the PPP ambiguity estimation process.The estimator improves the estimation accuracy of float ambiguities through multiple ambiguity-fixed solution weighting methods,thereby improving the PPP positioning accuracy and shortening the ambiguity convergence time.105 globally distributed MGEX stations are selected to verify the performance of BIE in PPP ambiguity estimation.Experimental results showed that compared with the ambiguity-fixed solutions,using BIE to estimate PPP ambiguities can further improve the positioning performance of east,north and up components of the coordinates,the convergence time of which can be shortened by 37%,28% and 31%,and the positioning accuracy of which is improved by 9%,8% and 3%,respectively.Furthermore,there are fewer glitches and steps in the positioning results derived from BIE-based PPP.

Determination of truncation parameter based on the differences of TSVD parameter estimates for ill-posed problems in geodesy

LIN Dongfang, YAO Yibin, ZHENG Dunyong, LI Chaokui

2022, 51(8):  1787-1796.  doi:10.11947/j.AGCS.2022.20210377

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TSVD is an effective method commonly used in solving ill-posed geodetic problems.In this method,truncation parameter is a critical factor.However,the existing methods can only work out effective truncation parameters rather than identify the optimal truncation parameter.Determining truncation parameters based on the minimum mean square error is a relatively complete theoretical way to determine truncation parameters.But the true values of the model parameters required for the calculation of the mean square error cannot be obtained in practical applications,which makes it difficult to calculate the optimal truncation parameter theoretically.To solve these,this paper develops a method to identify the TSVD truncation parameter on the basis of the variation characteristics of the parameter estimation in the influence of the mean square error (including variance and bias).To achieve this,this paper truncates small singular values in turn based on TSVD to acquire the changes of the variance and parameter estimation and analyzes the changes to determine the effects of the biases,which can avoid the calculation biases in the use of the true value of the parameters.Thereby,based on the theory of the minimum mean square error,the truncation parameter can be determined.The results in numerical experiments and practical applications show that the truncation parameters in the new method can effectively improve the quality of TSVD solution.Therefore,the proposed method is an effective method to determine the truncation parameters.

Cartography and Geoinformation

Personalized city region of interests recommendation method based on city block and check-in data

LIU Jiping, ZHANG Zhiran, YANG Chaowei, XU Shenghua, CHEN Cai, QIU Agen, ZHANG Fuhao

2022, 51(8):  1797-1806.  doi:10.11947/j.AGCS.2022.20210094

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Using the historical user data of social media to analyze the preferences of users'spatial activities and region of interest (ROI) is of great value for city commercial planning,people's urban life and needs.The ROI obtained by existing methods has big ambiguity and vague in content,and lack actual geographical scope and accurate geographical description for users.We propose a personalized ROI mining and recommendation method combing city block and check-in data (CBCD) that introduces the concept of city blocks to solve the problem of vague ROI boundaries.Specifically,the large number of check-in points are mapped into city blocks generated by the road network,followed by modeling of user geographical and categorical preference.Finally,we integrate geographical and categorical activity preference models to recommend ROIs to users.Experiments on real datasets show that this method has high recommendation accuracy and is of great value for mining and recommending city blocks of interest to users.

Traveling trajectory description model considering the point-line spatio-temporal correlation characteristics

LIN Yan, HE Rixing, CHEN Jun, LI Jiatian, ZHANG Wenyu

2022, 51(8):  1807-1816.  doi:10.11947/j.AGCS.2022.20210365

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Associated traveling becomes new research highlight in the fields of urban planning,traffic traveling,infectious disease prevention and controlling,crime investigation etc.Especially,effectively identifying the related behavior of individuals with subjective intentions (such as meeting and waiting) is considered as a difficult problem in the area of spatio-temporal cognition.In this paper,spatio-temporal trajectory of crime is studied.First,the inadequacy of the previous traveling trajectory description methods is analyzed,which indicating that the detailed point characteristics and the complete line characteristics of the traveling trajectory should be considered,based on that a travel trajectory description model considering the point-line characteristics is proposed.Second,a trajectory discrimination method of associated traveling is proposed based on spatio-temporal topological relationship,which can describe four basic associated traveling trajectory modes including meeting,waiting,coexistence and companion,further distinguishing 19 different associated traveling subtypes.Finally,the effectiveness of the method is validated by an experimental comparison.

A fast matching method of coastline changes using single-chain numbering

CHENG Siyuan, ZHANG Ying, ZHAI Liang

2022, 51(8):  1817-1825.  doi:10.11947/j.AGCS.2022.20210291

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Affected by natural environment changes and human development activities in the coastal zone,coastline continues to change dramatically.Detecting coastline changes and matching the coastline change segments before and after changing are critical for analyzing coastline changes.As the traditional tools of spatial analysis lack of direct and effective matching function,this article introduced the single-chain numbering idea to match the coastline change segments quickly.The matching relationships of the coastline change segments were grouped into "one-to-one" and "multi-to-multi" .Besides,we also put forward a set of formula for determining the coastline change types such as position change,type change,disappearance and appearance.The matching and recording method of the coastline changes based on single-chain numbering has been designed and packed using python language.Experiments showed that the method proposed realized coastline changes matching quickly and accurately,which has solved the problem of matching the multi-to-multi coastline change segments.

Summary of PhD Thesis

Monitoring species diversity and measuring vegetation resilience with satellite remote sensing

WU Jinhui

2022, 51(8):  1826-1826.  doi:10.11947/j.AGCS.2022.20200539

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