Louise Yu (CNES, France); Pierre Dubois (CLS, France); Gwenaël Jestin (LEGOS, France); Sara Fleury (LEGOS, France); Laiba Amarouche (CLS, France); Thomas Moreau (CLS, France); Fanny Piras (CLS, France); François Boy (CNES, France); Matthias Raynal (CNES, France); Nicolas Picot (CNES, France)

The launch of the Surface Water Ocean Topography mission (SWOT) in December 2022 represented a major breakthrough in satellite altimetry. Its primary instrument is the Ka-band Radar Interferometer KaRIn, which uses synthetic-aperture radar (SAR) and interferometry to provide a fine resolution both along and across-track. KaRIn data are processed in two different chains: low-rate (LR), which is applied everywhere and provides one measurement every 2-km-side square tiles (250-m-side for the “unsmoothed” product), and high-rate (HR), which provides ~30-m-resolution pixel clouds (PIXC) on specific 60-km-side square tiles. The HR mask is primarily designed for hydrology and thus covers mostly the continents. However, a seasonal winter mask has been implemented in the arctic ocean, allowing for the characterization of sea ice from both LR and HR data. Although sea ice exploration is not a priority of the SWOT mission, initial results show that it still provides detailed images of both the topography and scattering properties of the ice floes and leads (the fractures between the floes). This is promising for the future Copernicus mission Sentinel-3 Next Generation Topography (S3NG-T) that will provide altimetry measurements over the whole globe almost up to latitude 82°. With both a nadir and a wide swath (with LR and HR modes) altimeter, it will therefore be able to provide enhanced observations of the polar oceans, including sea ice monitoring.

Ensuring the continuity of sea level measurements as well as quantifying the time-varying surface and height of sea ice in polar regions has been a continuous challenge. Since in-situ explorations are difficult in such hostile regions, dedicated space missions (Cryosat-2, SARAL/AltiKa, Sentinel-3, IceSat-2) have recently improved our knowledge and given a first estimate of the sea level and the thickness of the ice [1]. With its unique 2D measurement capabilities, SWOT can provide unprecedented information on the surface classifications of the ice covered oceans (mainly leads and floes) as well as the topography of both the water and ice floes. This was shown in various investigations last year [2, 3], and a classification product computed from SWOT LR “unsmoothed” data is currently being implemented following these studies.

In this work, we propose to show the advantages and limitations of HR data on sea ice. The improved resolution and posting rate allow for the characterization of finer details, but the pre-summing of the raw data and the nature of the surface (ice, snow, dark water) significantly increases the noise, which impacts the topography measurement performance. Learning from the capabilities of SWOT in sea ice is essential to the success of future missions such as S3NG-T. We propose to show comparisons of SWOT HR and LR “unsmoothed” data on sea ice, as well as simulations of both S3NG-T and SWOT, to show the impact of the different instrumental specificities on the performance. Overall, swath altimetry opens a new era for the global understanding and monitoring of sea ice, also contributing to the enhancement of sea level measurements in polar oceans, which is one of the key mission objectives.

References:
[1] Fleury et al. (2024). 30 Years of Sea Ice Thickness and Volume over Arctic and Antarctic from Satellite Altimetry. 30 Years of Progress in Radar Altimetry Symposium
[2] Jestin et al. (2024). First Sea Ice Topography Measurements Using SWOT. 30 Years of Progress in Radar Altimetry Symposium
[3] Kacimi et al. (2025). SWOT observations over sea ice: A first look. Geophysical Research Letters, 52(10), 10.1029/2025GL116079.