Amongst these studies, two are presented in this paper: (1) a global assessment of zenith total delays (ZTD) determined at interna-tional data centres such as EPN (EUREF Permanent Network) and IGS (International GNSS Service) by comparison with ZTD solu-tions computed at the University of Porto (U.Porto) using state-of-the-art methodologies and ZTD estimated from ERA Interim, the latest reanalysis dataset from ECMWF (European Centre for Medium-Range Weather Forecasts), (2) evaluation of the accuracy of the hydrostatic component of the tropospheric delay (zenith hydrostatic delay, ZHD) estimation from different sources of surface pressure.