## 6 Conclusions
Relative calibration of HY-2A against Jason-2 with crossover analysis at sea shows that HY-2A displayed a performance that looks like a linear downward trend in bias, starting from +60 cm at Cycle 23 and reaching about \(-\)60 cm at Cycle 98. This kind of behavior might be explained byrealizing it was generated by a satellite clock defect (oscillation) onboard HY-2A. Comparable bias results for HY-2A have been verified with cross over analysis at sea north of western Crete (Figure 31). * SARAL/AltiKa displays a bias in sea surface height determination which was larger than that of Jason-2. Various error sources may be involved in and contribute uncertainties to delivered results for altimetry calibration. These are related to instrument type, measurement kind, measuring procedures and conditions, applied approximations, environmental conditions, etc. To maintain a comprehensive uncertainty budget for altimetry calibration in support of reliable, long-term, consistent calibration products, the new established standard of fiducial reference measurements for altimetry [50; 51] has to be adopted for the future. Uncertainties in altimetry calibration using sea-surface techniques may arise from constituents related to site location (e.g., sea local effects, specs, etc. ), conditions and settings (e.g., tide gauges, instrument types, offsets, measuring water methods, thermal expansion, harbor  
Figure 36: A synoptic presentation of all derived biases and their distribution for Jason-1, Jason-2, Jason-3, and Sentinel-3A with box plot diagrams. These two satellites have been calibrated and compared with sea-surface and transponder calibrations at different locations, instrumentation and procedures. \begin{table}
\begin{tabular}{c c c c c} \hline
**SSH Bias (cm)** & **Satellite** & **Jason-1** & **Jason-2** & **Jason-3** & **JA3 \(-\) JA2 Offset** \\ \hline \multirow{6}{*}{Sear-Surface Height} & Product & GDR-E & GDR-D & GDR-D & \\  & Cycles & 70–100 & 2–303 & 1–80 & \\  & No. 18 (descending) & +4.7 cm \(\pm\) 1.3 cm & +0.63 cm \(\pm\) 0.3 cm & \(-\)0.74 cm \(\pm\) 0.4 cm & \(-\)1.37 cm \\  & No. 109 (ascending) & +3.5 cm \(\pm\) 1.3 cm & +0.33 cm \(\pm\) 0.2 cm & \(-\)0.50 cm \(\pm\) 0.4 cm & \(-\)0.83 cm \\  & Average Bias & +4.10 cm & +0.48 cm & \(-\)0.62 cm & \\ \hline \multirow{6}{*}{Transponder Bias (cm)} & Product & GDR-D & GDR-D & \\  & Cycles & 267–303 & 1–80 & \\ \cline{1-1}  & Desending & \(-\)1.20 cm \(\pm\) 0.6 cm & +0.76 cm \(\pm\) 0.4 cm & +1.96 cm \\ \hline \end{tabular}
\end{table} 
Table 5: A concise presentation of absolute calibration results for Jason-1, Jason-2, Jason-3, Sentinel-3A, and HY-2A satellites based on sea-surface and transponder measurements at Gavdos and western Crete Cal/Val facilities. local conditions, etc. ), tide gauges (e.g., zero-reference point of measurement, different measuring techniques, trends, gaps, etc. ), local reference surfaces (e.g., connection with ellipsoid, mean sea surface, open sea and satellite measuring relation, etc. ), and measuring strategies (e.g., averaging for an observation, sampling rates, simultaneity, quality control, time-tagging, etc.). The list may be endless, but the contribution generated by each constituent should be commensurate with its relative contribution to the final result in altimetry calibration. According to the FRM4 ALT principle, ground Cal/Val facilities, instrumentation and procedures, set up for calibration and validation of observations, and products in satellite altimetry shall follow well-documented procedures and protocols. Thus, uncertainty budgets have to be built upon metrological standards and capable of being traced to Systeme international units [51]. For the sake of demonstration, we provide an indicative example in Table 6 of how this uncertainty is to be determined for sea-surface calibration. This illustrating example has followed, at this initial stage, the methodology described in a reference document by the Joint Committee for Guides in Metrology [62].