One of the developed methods, designated GNSSderived
Path Delay (GPD), determines the tropospheric
delay due to the wet component of the troposphere in
points that have an invalid MWR correction, using a combination
of three types of data: MWR measurements at
valid points, wet path delays determined at coastal GNSS
stations and data provided by a meteorological model such
as the one provided by ECMWF. Details about the method
can be found in Fernandes et al. (2010) and in Obligis et al.
(2011). In Fernandes et al. (2010), the application of the
method to a region of the SW Europe is presented. Since
then, several studies have been conducted to improve some
of the adopted procedures and allow a global implementation
of the methodology. One of the innovations of this
method is the use of tropospheric delays determined by GNSS techniques. Since the atmosphere is a non-dispersive
medium at frequencies below 20 GHz (Thayer, 1974), the
zenith total delays which affect the GNSS measurements
(frequencies in the L band, 1-2 GHz) are also valid for
radar altimetry measurements (Ku-band frequencies,
13 GHz).