Maintenance of Millimeter-level Geodetic Reference Framework

The high-precision terrestrial reference frame,as the spatial benchmark for geodesy,is an important national infrastructure.However,due to the influence of nonlinear factors related to geophysical phenomena,the overall maintenance accuracy of the ITRF framework is still at the centimeter level.Therefore,accurately characterizing the true trajectories of linear motion,nonlinear motion,and geocentric motion of the reference station is the key to achieve the construction and maintenance technology of a millimeter level terrestrial reference framework.Based on long-term global and regional GNSS observation data,more Chinese geodesy scientists devoted much efforts to the maintenance of millimeter-level geodetic reference framework.The main contributions of this work included the followings:①Dynamic maintenance of millimeter-level terrestrial reference frame;②Research progress on the method of maintenance of regional reference frame based on GNSS;③The progress of CGCS2000 frame maintenance in millimeter level accuracy;④Reprocessing and reanalysis of two-decade GNSS observation in continental China;⑤Research on current GNSS velocity field model and deformation in Chinese mainland;⑥The preliminary realization and evaluation of CTRF2020.

Realization of an Optimal Dynamic Geodetic Reference Frame in China:Methodology and Applications

China Geodetic Coordinate System 2000(CGCS2000)has been used for several years as a formal published reference frame.The coordinates of all global navigation satellite system(GNSS)stations in China need to be corrected to align with the CGCS2000 frame.Different strategies can be adopted for the realization of an optimal reference frame.However,different strategies lead to different results,with differences as great as several decimeters when GNSS station coordinates are transformed into CGCS2000-defined coordinates.The two common methods for the coordinate correction of a GNSS station are quasi-stable adjustment under CGCS2000 and plate movement correction,and the differences between their results can be greater than 10 cm.In this study,a statistic method called"supervised clustering"is applied to the selection of GNSS reference stations;a new scheme named"partition spacing"for the grouping of all processed GNSS stations is proposed;and the plate movement correction method is used to correct the coordinates of all GNSS stations from the GNSS epoch to the CGCS2000 epoch.The results from the new partitioning method were found to be significantly better than those from the conventional station-blocking approach.When coordinates from the stations without grouping were used as the standard,the accuracy of all the three-dimensional coordinate components from the new partitioning method was better than 2 mm.The root mean squares(RMSs)of the velocities in the x,y,and z directions resulting from the supervised clustering method were 0.19,0.45,and 0.32 mm∙a
1,respectively,which were much smaller than the values of 0.92,0.72,and 0.97 mm∙a
1 that resulted from the conventional approach.In addition,singular spectrum analysis(SSA)was used to model and predict the position nonlinear movements.The modeling accuracies of SSA were better than 3,2,and 5 mm in the east(E),north(N),and up(U)directions,respectively;and its prediction accuracies were better than 5 mm and 1 cm for the horizontal and vertical domains,respectively.

Update China geodetic coordinate frame considering plate motion

China Geodetic Coordinate System 2000(CGCS2000),as the formal national coordinate reference frame,has been used for 20 years.The coordinates of all Global Navigation Satellite System(GNSS)stations in China need referring to this system.To this end,the first step is to align the coordinates of all stations,usually included in a regional GNSS network,with a given International Terrestrial Reference Frame(ITRF),then these coordinates are corrected to the CGCS2000 in consideration of plate movement.For a better alignment result,regional control stations are needed and their coordinates were estimated from the combination of constraint-free normal equation systems provided by several International GNSS Service(IGS)analysis centers.The effect in using these refined coordinates,which deter-mine a regional coordinate datum,on the alignment result should be evaluated by the coordinate corrections of the regional control stations to the regional coordinate datum,i.e.smaller corrections mean better alignments of the two associated frames.The test results show that the refined coordinates are more accurate than the ones calculated from the station’s velocity,and are well aligned with the ITRF2005.Moreover,for obtaining the coordinates of GNSS stations in an updated CGCS2000 frame,a gridded linear velocity field based on the estimated velocities at 1025 CGCS2000 stations was generated for China's Mainland using the optimal interpolation method,the inverse distance weighting,which is selected from five interpolation methods.The overall precisions of the constructed velocity field at all stations in the East(E)and,North(N)directions are 0.78 mm/a and 0.95 mm/a,respectively.For evaluating the accuracy of the updated CGCS2000 frame,monthly solutions for the coordinates of some CGCS2000 CORS stations in the ITRF2014 during the period from 2000.0 to 2018 were obtained and the Root Mean Square(RMS)of the differences between the coordinates corrected to the CGCS2000 and the known coordinates at these stations are about 2-3 cm.