## 3 Calibration Methodologies & Dataset Used
### Sea-Surface Calibration  
Altimetry calibration relying upon sea-surface heights has been customarily carried out in open seas though for the past few decades also in coastal regions and inland waters [24; 25; 26; 27]. Calibrations usually take place where satellite observations are not contaminated by land. This has, up to now,  
Figure 10: Screenshots of the transponder response movie for the Sentinel-3 Pass No.14 calibration on 8 April 2018. The transponder echo in the altimeters records as it enters (**left**), reaches its maximum (**middle**) and leaving (**right**) the tracking window. Similar echo movies are generated within 24 h after each satellite overpassed the CDN1 transponder Cal/Val site. Figure 9: Timeline of Sentinel-3A Pass No.14 transponder calibrations at the CDN1 Cal/Val site. Green boxes represent successful calibration, whereas red boxes correspond to two calibrations canceled as a consequence of extreme weather conditions (4 January 2017, Cycle No.13, Images above) and site maintenance (8 August 2017, Cycle No.21). been accomplished by setting up Cal/Val facilities on either small infrastructures at sea (oil platforms or calibrating instrumentation in shallow waters) or on coastal or islet sites next to open oceans where altimeters measure. In the first case, small-scale infrastructures do not contaminate the altimeter's signal and are commonly located exactly under the satellite altimeter's ground-track. In the second case, coastal Cal/Val sites need to transfer the in-situ reference measurements to open sea where satellite measurements are valid. The Harvest platform, USA [24] and Bass Strait, Australia [25] belong to the first category, whereas the two European permanent Cal/Val facilities in Corsica, France [26] and Gavdos/Crete, Greece [27] represent the second category. In the Gavdos/Crete Cal/Val facility, the sea-surface calibration has been implemented in sea regions where the Sentinel-3 SRAL and MWR observations are not contaminated by land mass (Figure 11). The groundtrack of Pass No.14 of Sentinel-3A ascends west of Gavdos and continues north to the other transponder Cal/Val site on the mountains of west Crete. This pass has been calibrated using the transponder; however, before it reaches that site, it comes to pass west of the Gavdos Cal/Val site where sea-surface calibration is put into action. Additionally, the other Pass No.335 of Sentinel-3A descends from southwest of Crete to south and intersects the Gavdos calibration site. Calibration values for this descending pass are also presented applying sea-surface calibration and crossover analysis. During the satellite overpass, the instantaneous sea-surface height at the reference Cal/Val location \(k_{0}\) is established from tide gauge measurements and absolute geodetic heights. Given the satellite's velocity (\(\sim\)7 km/s), the local tidal range and the tide gauge sampling (i.e., 6-min, 1-min), hourly values of the tide gauge measurements (centered to the satellite pass over the point-of-closest approach) are used to settle the value for the \(SSH(k_{0})\). Then, this value is transferred to open sea in the calibrating regions using reference models. Detailed and precise models have been constructed for the mean dynamic topography (\(MDT\)) and geoid undulation (\(\mathcal{N}\)) in this region [28]. Models are often being validated as a matter of usual practice by means of dedicated boat campaigns. They are used to transfer \(SSH\) from the Gavdos reference Cal/Val site to open seas [29; 30], by first establishing its sea level anomaly (\(SLA\)):  
\[SLA(k_{0})=SSH(k_{0})-\mathcal{N}(k_{0})-MDT(k_{0})\]  
Figure 11: The shaded greenish areas represent the Sentinel-3A Pass No.14 (**left**, ascending) and Pass No.335 (**right**, descending) calibrating regions at the Gavdos sea-surface Cal/Val site. The sea level anomaly \(SLA(k)\) is also required for altimeter calibration over the sea surface along the satellite's track.