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.
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. Time series were detrended and deseasoned (annual cycles) at the CATREF stage. Details on the GPS velocity uncertainty assessment will be given in the below-mentioned paper.
Vertical velocities table
The ulr4_vertical-velocities_table provides the vertical GPS velocities and uncertainties for the 275 stations fulfilling the criterias of 2.5 years of minimum length without discontinuities and with data gaps not exceeding 30%.
Weekly time series
The ULR4_neu.zip file contains individual station data files of weekly position time series in ITRF2005 with respect to the position at the reference epoch. These positions 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. Theulr4.kmz file shows a Google Earth presentation of the above weekly data files by cliking on a station symbol. The associated IGS-like sitelog can be accessed too, as well as a plot of the position time series in the vertical.
The ulr4_discontinuities_table.txt file provides the position offsets that were estimated.
The long-term solution ULR4 in SINEX format (47 Mo) can be downloaded too. However, we recommend its use only to those familiar with SINEX files, and having a good geodetic background and experience.
A. Santamaria-Gomez, M-N. Bouin, G. Wöppelmann (2012) : Improved GPS data analysis strategy for tide gauge benchmark monitoring. International Association of Geodesy Symposia 136, Geodesy for Planet Earth, pp. 11-18, doi:10.1007/978-3-642-20338-1_2.