## 5 Conclusions and Future Plans  
The Sentinel-3A satellite altimeter is the first \"offspring\" of the Sentinel-3 family of the Copernicus program. It is crucial to validate this altimeter's performance in an indisputable way following internationally agreed standards and reporting the results along with their uncertainty employing metrology guidelines. The Permanent Facility for Satellite Altimeter operating in west Crete, Greece has been employed to serve as the fiducial facility for monitoring Sentinel-3A altimeter performance. Two major permanent Cal/Val sites that incorporate diverse calibration methodologies were used: the European Space Agency's Sentinel-3 Altimeter Transponder Calibration Site on the mountains of Crete (\"CDN1 Cal/Val\") and the Gavdos sea-surface Cal/Val facility that provides calibration services for all altimetry missions continuously since 2004. The Cal/Val results for Sentinel-3A, as presented in this work, are in line with those determined by other research teams. In [42], the absolute bias of S3A for sea surface height was estimated to be of the order of +2 cm \(\pm\) 2.5 cm at Bass Strait, Australia [43]. At the Corsica Cal/Val site in France [44], bias for sea-surface heights amounts to +0.9 \(\pm\) 0.6 cm. Moreover, transponder observations processed by other groups arrived at +0.6 cm in range bias for S3A [45]; all within the uncertainty limits and specifications of \(\pm\)3 cm for this Sentinel-3A satellite altimeter. The uncertainty budget analyses presented for the transponder and sea-surface calibrations constitute the first effort to write down the error sources, characterize them according to the International Standards Organization Guide for Measurement Uncertainty and provide a simplified and realistic approach for the uncertainty of each altimeter calibration. Work has already been conducted by this team to implement more advanced statistical tools and methods which will certainly improve and safeguard this uncertainty budget analysis. Satellite altimetry results have been given, along with their uncertainty, attaining the European Space Agency standard of fiducial reference measurements based upon a series of independent, fully characterized, and traceable ground measurements. Conceptualization, S.P.M., C.D. and C.M. ; Data curation, X.F., C.K. and G.V. ; Funding acquisition, S.P.M. ; Investigation, S.P.M., C.M., D.G. and I.N.T. ; Methodology, S.P.M. and C.D. ; Project administration, S.P.M., C.D., P.F., C.M., A.T. and T.G. ; Software, D.G., X.F., C.K. and G.V. ; Supervision, S.P.M., P.F. and C.M. ; Visualization, X.F. ; Writing--original draft, S.P.M. and A.T.; Writing--review & editing, S.P.M. This research was funded by European Union and the European Space Agency grant numbers [400017101/16/1/BG and 4000122240/17/I-BG] and the APC was funded by the Technical University of Crete. This work has been primarily supported and funded by the European Union, the European Space Agency, and the Centre National d'Etudes Spatiales, France. Many thanks to [PERSON] and [PERSON], University of Porto, Portugal, for helping us better understand the wet troposphere delays and for verifying the produced results. [PERSON] of the Danish Space Center has provided the Mean Sea Surface models for the region. The authors declare no conflict of interest. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Bias [mm]** & **Sentinel-3A** & **Ascending No.14** & **Descending No.335** & **FRM Uncertainty** \\ \hline \multirow{4}{*}{Range Bias} & Product & PB 2.27, NTC & PB 2.27, NTC & \multirow{4}{*}{\(\pm\)4.1 cm (68\%)} \\  & Cycles & 3–32 & 1–31 & \\ \cline{1-1} \cline{3-5}  & Transponder & +0.60 cm \(\pm\) 0.3 cm & & \(\pm\)4.1 cm (68\%) \\ \cline{1-1} \cline{3-5}  & S\(\mathrm{\ddot{a}}\)v\(\mathrm{\ddot{o}}\)dos Cal/Val & \(-\)0.12 cm \(\pm\) 0.4 cm & \(-\)1.20 cm \(\pm\) 0.5 cm & \(\pm\)3.60 cm (68\%) \\ \cline{1-1} \cline{3-5}  & S\(\mathrm{\ddot{a}}\)A-J\(\mathrm{\ddot{a}}\)3 & +4.00 cm \(\pm\) 8 cm & & \\ \hline \hline \end{tabular}
\end{table}