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.