## 2 Quality Assessment of Water Level through Calibration and Validation
### Global Validation of SARAL/AltiKa Sea Surface Height over Ocean  
Since the beginning, SARAL/AltiKa data shows a high quality and provides new information with respect to the previous missions [5]. At high latitude, thanks to its orbit reaching 82\({}^{\circ}\), the Arctic Ocean and ice caps are better covered. Near the coast, measurements are more numerous and reliable [26] thanks to its high rate of 40 Hz (one point every 125 m along-track) and to its robust tracking mode (DIODE/Median, except for cycles 10 to 17 in autonomous DIODE mode). Furthermore, high frequency structures are better addressed, thanks to its Ka-band technology [27]. Its data availability is largely over the specification requirements, reaching 99.6% over oceans, Safe Hold Mode periods included, compared to 99.3% on Jason-2. Over other surfaces, the coverage is also very homogeneous as shown on Figure 11. Even the sensitivity of the Ka-band frequency to rain has less impact than expected. Indeed, only 5% of measurements may be not achieved due to rain rates \(>\) 1.5 mm/h according to geographic areas [28]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**SARAL/Altika Regional** & **Mean** & **Std** & **Nb of** & **Nb of** **Xover** & **Std over the** \\
**Bias Estimates (mm)** & **(mm)** & **(mm)** & **Cycles** & **Points** & **Xover Points (mm)** \\ \hline Senetosa (cycle 1–35) & \(-84\pm 5\) & 32 & 34 & 7 & 5 \\ Ajaccio (cycle 1–35) & \(-37\pm 6\) & 36 & 33 & 6 & 13 \\ Harvest (cycle 1–35) & \(-19\pm 12\) & 64 & 30 & 4 & 12 \\ Bass Strait (cycle 1–25) & \(-22\pm 11\) & 56 & 24 & 5 & 13 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: SARAL/AltiKa regional bias estimates at each site. In Bass Strait, the in situ data available for this study only covers the period until cycle 25. Figure 10: Altimeter crossover points used for the SARAL/AltiKa regional bias estimate in Corsica (**a**: Ajaccio in blue, Senetosa in orange), Harvest (**b**: green dots) and Bass Strait (**c**: green dots). The SARAL/AltiKa and Jason-2 grounds tracks are shown respectively in yellow and red. The relative SSH bias compared to a zero reference of the TOPEX GMSL in 1993 (Global Mean Sea Level, CMEMS 2018 standards (Copernicus Marine Environment Monitoring Service)) is \(-82.3\) mm for SARAL/AltiKa, whereas Sentinel-3A stands for \(-3.9\) mm, Jason-2 for \(-18.5\) mm and Jason-3 for \(-47.3\) mm. Relative SSH bias and its evolution can also be derived from crossovers with a reference mission (such as Jason ones); even if not based exactly on the same length of the time series, results provided in [5] agree at the centimeter level with the ones presented here. Its long-term stability as well, exceeds the mission requirements, featuring a very consistent mean sea level with respect to the reference record, derived from a combination of TOPEX/Jason-1/Jason-2 time series [29]. SARAL/AltiKa GMSL's shows a \(4.8\) mm/year evolution on this time period, thus keeping pace with Jason-2 (\(4.4\) mm/year). It is a very satisfying statistic knowing that the time series for SARAL/AltiKa is still under 5 years which is the minimum period required to have a significant trend, see Figure 12.  
Figure 11: Map of the percentage of available measurements over land on SARAL/AltiKa’s cycles 1 to 34. Figure 12: SARAL/AltiKa’s Global Mean Sea Level (red) compared to Jason-2 (blue) and the reference one (green). As for crossovers analysis (see details in [5]), it demonstrates excellent performances, with 5.3 cm of standard deviation compared to 5.4 cm for Jason-2, using the radiometric correction [30]. This illustrates SARAL/AltiKa's very good skills to resolve mesoscale signatures. Finally, SARAL/AltiKa's Ka-band frequency enables the resolution of small scales of ocean dynamics (mesoscale). Thanks to its higher rate of 40 Hz and a smaller and statistically more homogeneous footprint, its noise level is lower than Jason-2/3 (Figure 13).