Claire Maraldi (CNES, France); Ourania Altiparmaki (Delft University of Technology, Netherlands); Thomas Moreau (CLS, France); Marine De Carlo (CLS, France); Romain Husson (CLS, France); Lotfi Aouf (Météo-France, France); Lionel Zawadzki (CNES, France); Nicolas Picot (CNES, France)

For more than three decades, satellite altimeters have been characterizing waves by measuring the significant wave height (SWH) across all open oceans. In a previous study, we described how the FF-SAR altimetry modulation of CryoSat-2, Sentinel-3, and Sentinel-6 radargrams can be related to the swell spectral properties [1, 2]. We showed that long swell wavelengths (> 200 m) could be reliably retrieved in a favorable sea state, although wave-direction estimates remain affected by left-right and 180° ambiguities. We described the two leading sea-state-dependent limitations (or cut-offs) that limit the resolution of the technique, both in azimuth and in range, and the complementarity of the other satellite missions with swell characterization capabilities [3].

Since then, we implemented a more refined approach, using cross-spectra to increase the signal-to-noise ratio [4]. By sublooking the FF-SAR illumination time, we can correlate sea states separated by ~ 1 s, observed with the same instrument, with a slightly different along-track look angle. This technique gives us information about the swell direction of propagation and disambiguates the 180° uncertainty.

In this study, we present the improved spectral characterization allowed by the Sentinel-6 tandem phase on test cases with well-defined swell features. The ~ 30 s separation between the two satellites, typically exceeding a full swell period, allows for a more accurate estimation of swell wavelength and significantly reduces speckle noise, as has been demonstrated using power density spectra of the Sea Level Anomaly during the Sentinel-3 tandem phase [5]. The tandem configuration also opens sensitivity to longerscale wave phenomena, such as wave groups. Additionally, the ~ 800 m cross-track offset between the platforms theoretically disambiguates the left-right symmetry inherent to nadir altimetry.

Finally, we exploited the extended RAW mask on Sentinel-6B and compared it to the colocated RMC mask on Sentinel-6A to illustrate the benefits of enhanced range resolution for swell monitoring [6, 7].

These advances reveal the rich oceanic information embedded in nadir altimetry and demonstrate its growing potential to meet user demand for detailed wave observations.

References:

[1] Altiparmaki, O. et al. (2022). SAR altimetry data as a new source for swell monitoring. Geophysical Research Letters, 49(7), e2021GL096224.

[2] Rodet, L. et al. (2024). Swell Characterization from Sentinel-3 and Sentinel-6MF SAR altimetry data. 30 Years of Progress in Radar Altimetry Symposium

[3] Rodet, L. et al. (2025).  Spectral swell characterization from satellite observations. EUMETSAT Meteorological Satellite Conference 2025

[4] Kleinherenbrink, M et al. (2024). Cross-spectral analysis of SAR altimetry waveform tails. IEEE Transactions on Geoscience and Remote Sensing.

[5] Rieu, P. et al. (2020). Exploiting the Sentinel-3 tandem phase dataset and azimuth oversampling to better characterize the sensitivity of SAR altimeter sea surface height to long ocean waves. Advances in Space Research

[6] Altiparmaki, O. et al. (2024). Advancing Sea-State Characterization with Fully-Focused SAR Altimetry: Insights and Innovations. 30 Years of Progress in Radar Altimetry Symposium

[7] Altiparmaki, O. (2025). Towards a new era of wave observations from nadir-looking SAR altimeters: Why Sentinel-6 RAW data matter, OSTST 2025