An altimeter-based assessment of SWOT KaRIn spectral error requirements

The launch of the Surface Water and Topography Mission (SWOT) on 16 December 2022 represented a major breakthrough in satellite altimetry. SWOT new Ka-band radar interferometer (KaRIn) provides for the first time two-dimensional sea surface height measurements at the km resolution over a swath of 120 km, enabling the possibility to observe ocean processes at spatial scales well below those from the current nadir altimeter constellation. The novelty of KaRIn measurements lead to a main paradigm shift also in terms of mission error requirements which, for the first time, were specified in a spectral form in order to resolve ocean signals down to 15-30 km wavelengths. Assessing such requirements represents a major challenge since it requires simultaneous and colocalised independent SSH observations over up to 1000 km along the SWOT swath. Here, we combined KaRIn, SWOT nadir and Sentinel-3 sea surface height observations to provide four different but complementary estimations of KaRIn error spectra. Since all approaches are based on the comparison of observations that are not completely simultaneous or colocalised, the contribution of temporal or spatial natural ocean variability was also assessed and removed from our estimates. The results from the four approaches are consistent with each other, indicating that KaRIn error is below the mission requirements for wavelengths > 100 km. For shorter wavelengths, while the performance remain in line with the expectations, it remains unclear if they are strictly below the requirements. Additional comparisons with in-situ observations from dedicated SWOT field campaigns are required to complement our assessment and better characterize KaRIn performance and the nature of the ocean topography signal observed at scales < 100 km.