The ULR6b GPS solution is an updated version of ULR6a. That is, ULR6b is aligned in the ITRF2014 reference frame, instead of ITRF2008 for ULR6a. Both UMR6 solutions result from the reanalysis of 19 years of GPS data from 1995 to 2014, carried out within the framework of the 2nd data reprocessing campaign of the International GNSS Service (IGS)
Double-differenced ionosphere-free GPS carrier phase observations from a global network of 756 stations were reanalyzed using GAMIT/GLOBK software version 10.5. The data set covered the period January 1995 to December 2013. Station coordinates, satellite orbits, Earth orientation parameters (EOPs), and zenith tropospheric delay parameters every hour, were estimated. The data analysis strategy (models, corrections...) was compliant with the specifications adopted by the IGS for this reanalysis (more information here). A detailed description of ULR data analysis strategy can be found in the reference below.
The GPS velocities were obtained in the final step of the analysis, which combined the daily GAMIT/GLOBK global network solutions (station coordinates with full co-variances) from the entire data time span into a long-term solution using CATREF software. This first long-term solution (ULR6a) was aligned to ITRF2008 (latest version of the ITRF available at that time) using minimal constraints over a selected set of IGS Reference Frame stations. Later on, this solution was aligned to the ITRF2014 using the official transformation parameters between ITRF2008 and ITRF2014.
To assign more realistic uncertainties on the GPS velocities, the noise content in the position time series was examined by the maximum likelihood estimation (MLE) technique using CATS software (Williams, 2008). Time series were detrended at the CATREF stage. Details on the GPS velocity uncertainty assessment are given in the reference below.
Vertical velocities table
The ULR6b_Vertical-Velocities_Table provides the vertical GPS velocities and uncertainties for the 501 stations fulfilling the criteria of 3 years of minimum length without discontinuities and with data gaps not exceeding 30%.
Daily residual time series
The ULR6b_detrend.zip file contains individual station data files of daily residual position time series in ITRF2014 with respect to the linear long-term combined solution at the reference epoch. These residuals (detrended) are expressed in meters in the local frame (North, East, and Up). The reference position and the 3D velocity in the local coordinate system (East, North, Up) are provided in the header of each file. Both "raw" (wannual) and de-seasoned (woannual) time series are available.
The ULR6b_discontinuities_Table.txt file provides the position offsets that were estimated.
|Analysis center:||ULR||Number of stations:||674|
|Date of publication:||2020||Reference Frame||194|
|Time span:||1995.0 - 2013.9||Others||77|
|Reference Frame:||ITRF 2014|
|Estimated (robust) velocities:||493|
|Average of formal errors:||0.54 mm/yr|
|Median of formal errors:||0.36 mm/yr|
Santamaría-Gómez A., M. Gravelle, S. Dangendorf, M. Marcos, G. Spada, G. Wöppelmann (2017). Uncertainty of the 20th century sea-level rise due to vertical land motion errors. Earth and Planetary Science Letters, 473, 24-32.