The quantity determined from GNSS data processing is the total tropospheric delay, or ZTD, at station height while the corresponding quantity required for coastal altimetry is the total delay or the hydrostatic and wet components at sea level. Thus, since the two components have significantly different height dependence, their separation is a crucial step and the height reduction to sea level shall be performed separately for each component. In Section 3, a study is presented on methods to estimate the hydrostatic component, aiming to achieve a separation of the total delay in its hydrostatic and wet components with the high-est possible accuracy. Finally, in Section 4 the main conclusions are presented.