## 4 Sentinel-3A SRAL and MWR Cal/Val Results
### Validation of MWR Measurements  
The MWR is a dual frequency (23.8 GHz & 36.5 GHz) radiometer that observes the natural microwave emissions for the Earth and its atmosphere. Its observations are used to estimate, and subsequently correct, the radar altimeter signal delays caused by the atmosphere and, most importantly, its wet troposphere component [4]. The MWR's sea observations are contaminated in a distance up to 20-25 km from any land mass [36]. Thus, it is not possible to use it when the transponder calibration is employed up in the mountains in West Crete. On the contrary, the MWR observations in the open sea are applied regularly when sea-surface calibration is implemented. In this work, tropospheric delays coming from the Sentinel-3A MWR have been cross-examined against those derived by GNSS coastal stations operating continuously next to the Cal/Val sites. The uncertainties of the GNSS-derived zenith tropospheric delays are of the order of some millimeters [36]. Figure 14: Dates when Sentinel-3A Pass No.14 and Jason-3 Pass No.109 overpassed the cross-over location south of Gavdos within a window of \(\pm\)3 days. Figure 13: The cross-over location of Sentinel-3A Pass No.14 and Jason-3 Pass No.109 south of Gavdos Cal/Val site. The dry troposphere and ionosphere delay may be retrieved with accuracy (<1 cm) from global models as their temporal variation is quite small. This is not the case for the wet troposphere delays: these cannot be modeled flawlessly as they change rapidly over time and exhibit values of several cm (e.g., 5-15 cm in wet troposphere over several hours). Hence, the meteorological parameters of temperature, pressure, and humidity are continuously monitored at the PFAC Cal/Val sites. Furthermore, it has been observed that different GNSS processing software (i.e., GAMIT, Bernese, and GIPSY) may produce slightly different values for the troposphere at times, depending on the algorithms applied and their model resolutions (see Figure 15 as a demonstration example). The wet tropospheric corrections (WTC) as produced by Sentinel-3 radiometer, along Pass No.335, have been compared against corrections derived by GNSS processing. Satellite measurements that are flagged as invalid have not been taken into evaluation. The individual radiometer WTC profiles along-track of each cycle of Sentinel-3A have been plotted with respect to distance from the point of closest approach in Gavdos and are shown in Figure 16. The central red line represents a moving median within a window of 15 km of radiometer values as a function of distance. Positive distance denotes north of Gavdos, while negative distance stands for locations south of Gavdos. Then, the wet tropospheric delays, as determined by the S3A radiometer, were compared against those derived from the GNSS monitoring stations in Gavdos. Comparisons have been made between the last valid value of the radiometer, about 15-20 south of Gavdos (depending on the S3A pass), and the value produced at the reference GNSS sites in Gavdos, at the same time. It seems that there is an upward trend in the plot of Figure 17 with a slope of +0.0603 cm/cycle (98% significance). The average value of that difference in the wet troposphere delays comes to \(-\)0.09 cm, a value very close to zero. The standard deviation for its average is \(\pm\)2 mm. Additionally, a certain larger number of differences seem to be negative rather than positive (lower plot of Figure 17, asymmetrical distribution  
Figure 16: Profiles of Zenith Wet troposphere Corrections in cm, as observed by the S3A radiometer for cycles 1–30, along the descending Pass No.335, starting off north of Crete and converging south onto the Gavdos Cal/Val site (Please see Figure 12, S3A descending Pass No.335, right page). Figure 15: An example, of the Zenith Wet troposphere Delay in mm as derived by GNSS processing with GAMIT (**left**) and GIPSY (**right**) scientific software for 12 July 2017 at the transponder CDN1 Cal/Val site. of differences with respect to central location, i.e., median).