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 highest
possible accuracy. Finally, in Section 4 the main conclusions
are presented.