The ULR4 GPS solution is the result of a reanalysis of 13 years of GPS data from 1996 to 2008. Its vertical velocity field is published in Journal of Geophysical Research. It is expressed in ITRF2005.
Double-differenced ionosphere-free GPS carrier phase observations from a global network of 316 stations were reanalyzed using GAMIT/GLOBK software version 10.34. The data set covered the period January 1996 to December 2008 and 216 stations out of 316 were co-located with a tide gauge. Station coordinates, satellite orbits, Earth orientation parameters (EOPs), and zenith tropospheric delay parameters every 2 hours, were estimated. Details on the GPS data processing strategy (models, corrections,...) can be found in the IGS-like ULR analysis centre form associated with this solution.
The GPS velocities were obtained in the final step of the analysis which combined the weekly 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 long-term solution (ULR4) was aligned to ITRF2005 using minimal constraints over a selected set of IGS Reference Frame stations. A detailed description can be found in the reference below.