Bastien Courcol (CLS, France); Gaspard Minster (CLS, France); Cristina Martin-Puig (EUMETSAT, Germany); Deborah Hazan (CNES, France); Gerald Dibarboure (CNES, France)

Since the early 1990s, more than sixteen satellite altimetry missions have been measuring sea level, collectively enabling the construction of a consistent multi‑decadal database. This long‑term record is essential for preserving climate continuity, particularly for monitoring the global mean sea level (GMSL) and its associated trends.

The transition to a new reference mission constitutes a critical stage for the global altimetry programme, as each satellite carries its own instrumental signature. The main challenge lies in the accurate transfer of this reference, which underpins the cross‑calibration of all other missions at Level‑3. Such a transition introduces a point of vulnerability for the integrity of the multi‑decadal sea‑level record, making it essential to avoid any operational or climate‑scale discontinuity in order to preserve long‑term climate consistency. This transition period also provides a major opportunity to compare multiple satellites flying side‑by‑side, enabling the identification of error sources that previously remained invisible when observed from a single perspective, a crucial cross‑view for detecting, quantifying, and correcting instrumental or geophysical biases.

Also, achieving these objectives requires harmonised geophysical and processing standards, together with a sufficiently long tandem phase to characterise and understand differences between successive reference missions. Ultimately, this process relies on strong international coordination to ensure the robustness and continuity of the global altimetry record.

Within the frame of the Cooperation Agreement between EUMETSAT and CNES concerning cooperation on Sentinel-6 B, we present here the upcoming inclusion of Sentinel‑6B which is expected to become the new reference mission for ensuring the long‑term stability of the global mean sea level (MSL), although this role will be analysed via comparisons to Sentinel-6 MF data.