For the whole set of GNSS stations, plots of the differences between different ZTD data sets were analysed. The results show that, on average, the pattern of the differences between U.Porto and IGS solutions is different from the pattern of the differences between U.Porto and EUREF solutions.
The information provided by IGS and EPN about the processing carried out in these centres, summarized in Section 2.2, helps to understand these results. In the computation of the IGS and U.Porto ZTD the new standards adopted by IGS in November 2006 were used, in particular those for GNSS antenna phase centre variations, while in the EPN products only those generated after November 2006 used the same standards.