4269 20190916143757.0 CTALK-2019-0123 Morel, L. Global validity and behaviour of tropospheric gradients estimated by GPS 2019 2019-07-16 Estimation of tropospheric gradients in GNSS data processing is a well-known technique to improve positioning. Today, they are routinely estimated by several global and regional GNSS analysis centres but they are still not yet used for operational meteorology. We discuss the physical meaning of tropospheric gradients estimated from GPS observations recorded by more than 50 permanent stations located all around the world. We estimated Zenith Total Delay (ZTD) and tropospheric gradients using the software GIPSY-OASIS II version 6.4 in order to retrieve gradient estimations, with a focus on regions with significant relief. We observe that gradient directions are stable over the time and pointed toward the relief for most of the stations selected. Based on these results, we formulate why gradients have this particular direction when stations are closed to high mountains. STCE Durand, F. Moudni, O. Pottiaux, E. Follin, J.M. Durand, S. Van Baelen, J. International Union of Geodesy and Geophysics, Montreal, Canada eric.pottiaux@observatoire.be Contributed CTALKCONT