## 2 Data and Methods  
### GNSS Time Series  
As a public service, the IGS continuously provides the products with the highest quality in support of the Earth science research, multi-disciplinary applications, and the education ([[http://www.igs.org](http://www.igs.org)]([http://www.igs.org](http://www.igs.org))). Until now, it is the IG2 product that provides the most reliable and accurate GNSS results worldwide from January 1994 to February 2015 [11]. For the second reprocessing campaign, IGS ACs used up to 21 years of GPS observation data with daily integrations. During the IG2 product processing, main updates including daily data integrations, GLONASS (GLObalnaya Navigatsionnaya Sputnikovaya Sistema) data processed by some ACs, IGb08 (an update of IGS08)/igs08.atx framework, IERS2010 (International Earth Rotation and Reference Systems Service)Conventions, new yaw attitude models for eclipsing satellites, and a priori modeling of Earth radiation pressure and antenna thrust, are recommended. A total of nine solutions participated (7 Global AC solutions, and 2 Tide Gauge solutions) by reprocessing entire time series in a consistent way using the latest models and methodology. IGS combined the daily SINEX Terrestrial Reference Frame (TRF) and Earth Orientation Parameters (EOP), and these combinations were submitted to the IERS for ITRF2014.Different ACs have different processing strategies; for example, the SIO uses GAMIT software which uses double differencing, while JPL uses GIPSY software, which adopts the precise point positioning (PPP) mode [36; 37; 38]. All processing details, including atmospheric effect, ambiguity fix and cycle slips detection, are clearly listed at: [[http://acc.igs.org/reprocess2.html](http://acc.igs.org/reprocess2.html)]([http://acc.igs.org/reprocess2.html](http://acc.igs.org/reprocess2.html)), and strategy summary for each AC is listed in an \"*.acn\" file at [[ftp://igs.ign.fr/pub/igs/igsscb/center/analysis](ftp://igs.ign.fr/pub/igs/igsscb/center/analysis)]([ftp://igs.ign.fr/pub/igs/igsscb/center/analysis](ftp://igs.ign.fr/pub/igs/igsscb/center/analysis)).  
We adopt IG2 GNSS time series to evaluate the consistency between GNSS-observed and GRACE- and GM-modeled VD data at global IGS core stations. In comparison to the individual institution solutions, the IG2 combined solutions provide more stable and robust results, and the random noise in a single analysis center can be averaged out. One hundred and eight-six global GNSS stations were selected from part of the IGS core stations with high geodetic quality, based on the strategies explained in [35]. The locations of the selected stations are shown in Figure 1. The GNSS coordinate time series of the selected network can be directly obtained from the IGS SINEX files ([[ftp://cddis.gsfc.nasa.gov/gps/products/repro2/](ftp://cddis.gsfc.nasa.gov/gps/products/repro2/)]([ftp://cddis.gsfc.nasa.gov/gps/products/repro2/](ftp://cddis.gsfc.nasa.gov/gps/products/repro2/))). Before performing the analysis, the original coordinate time series were simultaneously detrended and corrected for step discontinuities by using a model which included the linear trend, seasonal signals (annual and semiannual signals), offsets (at epochs corresponding to equipment changes) and coseismic displacements, as reported by ITRF2014 IGS discontinuities file ([[http://itrf.ensg.ign.fr/ITRF_solutions/2014/computation_strategy.php?page=2](http://itrf.ensg.ign.fr/ITRF_solutions/2014/computation_strategy.php?page=2)]([http://itrf.ensg.ign.fr/ITRF_solutions/2014/computation_strategy.php?page=2](http://itrf.ensg.ign.fr/ITRF_solutions/2014/computation_strategy.php?page=2))).