Using FOAM project facilities for SWOT CAL/VAL (FOAMS)

Principle Investigator: Pascal Bonnefond (Observatoire de Paris - SYRTE)

Co-Investigator(s): Olivier Laurain, Damien Allain, Sylvain Biancamaria, Jean-François Crétaux, Florent Lyard, Fabrice Papa, Adrien Paris, Pierre Exertier, Alvaro Santamaría-Gómez, Sarah Baizeau, Valérie Ballu, Xavier Bertin, Laurent Testut, Cédric Brachet, Michel Calzas, Christine Drezen, Lionel Fichen, Antoine Guillot, Stéphane Renouf, Daniel Moreira, Rodrigo Abarca Del Rio, Alice Fassoni-Andrade, Ayan Santos Fleischmann

---

In collaboration with the CNES and NASA oceanographic projects (T/P and Jason), the OCA (French Riviera observatory) developed a verification site in Corsica since 1996 and LEGOS installed sites in Kerguelen in 1993 and at Vanuatu in 1999 for oceanic validation and at Lake Issykul and along the Amazon river for continental waters. More recently, new coastal sites in France (île d’Aix and Perthuis Charentais, Arcachon Bay, Gironde Estuary) have been developed to deal with the littoral context. CALibration/VALidation embraces a wide variety of activities, ranging from the interpretation of information from internal-calibration modes of the sensors, the determination of the algorithm performing best in picking up the range in the radar echoes in various contexts, to validation of the fully corrected estimates of the heights, whether it is sea level or inland water stage using in situ data. Now, Corsica is, like the Harvest platform (NASA side), an operating calibration site able to support a continuous monitoring with a high level of accuracy: a ’point calibration’ which yields instantaneous bias estimates with a 10-day repeatability of around 30 mm (standard deviation) and mean errors of 3-4 mm (standard error). Moreover, The Corsica facilities has been extended to monitor SWOT over the full swath width (Science Phase, pass 001). The inland sites are also unique in the scientific community. They provided the only estimates of bias for non-oceanic retracking algorithms commonly used for continental waters or for ranges issued from ocean-like waveforms in a context of very low sea-state bias, as it is frequently encountered over lakes.

In-situ calibration of altimetric height (SSH for ocean surfaces) is usually done at the vertical of a dedicated CAL/VAL site, by direct comparison of the altimetric data with in-situ data. This configuration leads to handle the differences compare to the altimetric measurement system at the global scale: the Geographically Correlated Errors at regional (orbit, sea state bias, atmospheric corrections…) and local scales (geodetic systematic errors, land contamination for the instruments, e.g. the radiometer, land contamination for the radar echoes, e.g. tracking/retracking concerns). We intend to continue and ensure the experiments already conducted through the FOAM project since 2008, funded in the framework of the Jason mission (but includes a multi-mission approach), at various sites (Corsica, Vanuatu, Kerguelen, île d’Aix, Perthuis Charentais, Arcachon Bay, Gironde Estuary, lakes and rivers…) where the local conditions are different from each others but where permanent instruments and infrastructures already exist and have been and will be reinforced. The FOAM project has been the place of a reflection about CAL/VAL methodologies and in particular about building dedicated instruments. This last point, which insures consistent procedures and equipment among all the team members is crucial for the CAL/VAL of SWOT. The different sites and approaches have been be adapted to SWOT to insure calibration and validation over larger areas than single passes like for nadir altimetry. Consequently, the team members listed in this proposal are naturally involved in other proposals responding to the SWOT ROSES/TOSCA call that include a potential CAL/VAL activity.

Details on the current FOAM project submitted through the OSTST call (2025-2028) can be found at: https://share.obspm.fr/s/5aEdY8pQFgzeJSm