SWOT-KaRIn Level-3 and Level-4 Algorithms and Products Overview

The DUACS system (Data Unification and Altimeter Combination System) produces, as part of the CNES/SALP project, Copernicus Marine Service and Copernicus Climate Change Service, high quality multi-mission altimetry Sea Level products for oceanographic applications, climate forecasting centers, geophysics and biology communities. These products consist in directly usable and easy to manipulate Level-3 (L3; along-track cross-calibrated SSHA) and Level-4 products (L4; multiple sensors merged as maps or time series).

Level-3 algorithms used for nadir altimeters have been extended to handle SWOT’s unique swath-altimeter data: upgrades with state-of-the-art Level-2 corrections and models from the research community, a data-driven and statistical approach to the removal of spurious and suspicious pixels, a multi-satellite calibration process that leverages the strengths of the pre-existing nadir altimeter constellation, a noise-mitigation algorithm based on a convolutional neural network. The objective of this presentation is to present the uniqueness of Level-3 algorithms and datasets and the regular changes made every 6 months with restatements. The changes introduced by version 2 of the L3 products published in December 2024/January 2025 are as follows:

• Geophysical standards changes
  
  • Internal tides model HRET14
  • Quick fix of the SSB/SSHA offset in polar transitions
  • Addition of 5 cm offset on MDT and ADT to be consistent with other L3 products
• Coverage improved :
  • Eclipse data gaps retrieved with good quality
  • Polar and coastal regions
• Cross-calibration improved, especially for coastal areas and polar seas
• Coastline and distance to coast improved
• Addition of surface classification (ice/leads) in editing flag
• Addition of new variables :
  • Unfiltered geostrophic velocities
  • Internal tide model
  • Cross-track distance

2D topography images from SWOT have been added to nadir altimeter data inside mapping algorithms (MIOST, 4DvarNET, 4DvarQG) to produce Level-4 products. The wide swath data provided by the SWOT mission help to reduce mapping errors mainly in energetic ocean currents, to better position oceanic structures (eddies, fronts…) and to have finer resolution in maps.