Marie-Laure Frery (CNES, France); Bruno Picard (Fluctus, France); Laiba Amarouche (CLS, France); Hélène Roinard (CLS, France); François Bignalet-Cazalet (CNES, France); Gérald Dibarboure (CNES, France); Matthias Raynal (CNES, France)

The aim of the SWOT mission is to measure ocean topography and hydrological water level at high precision and spatial resolution. It carries a Ka-band wide-swath interferometric altimeter (KaRIn) and a conventional Ku and C-band nadir altimeter (Poseidon-C). SWOT also includes two microwave radiometers (MW) to correct the measurements of the two altimeters KaRIn and Poseidon-3C for the delay due to the water vapor content in the troposphere. This is known as the Wet Tropospheric Correction (WTC) and it is necessary for accurate sea surface height measurements. Unlike conventional nadir-looking radiometers, the SWOT radiometer features two beams oriented on both sides of the nadir track. This configuration was specifically developed to match the wide swath coverage of the KaRIn altimeter. The WTC applied along both the nadir altimeter track and across the KaRIn swath is then derived through spatial interpolation of WTC measurements from these two off-nadir beams by two distinct interpolation algorithms designed respectively by CNES/CLS and JPL.
Comparisons with different independent references, either from geophysical models (ECMWF) or other instruments such as GPM/GMI (Global Precipitation Measurement Microwave Imager) or Sentinel-6 AMR-C, indicate that the interpolated SWOT Wet Tropospheric Correction delivers accurate and stable global correction over open ocean, consistent with mission requirement, and better than the Wet Tropospheric Correction computed with the operational model provided by ECMWF, especially in water vapor front zones.
However, several degraded situations have been identified and analyzed. These include data loss in the interpolated WTC where the radiometer measurements are rejected on one beam (due either to precipitation, presence of land, or presence of ice), increased noise in coastal areas due to land contamination, and a lack of sensitivity to fine-scale spatial variations in atmospheric water vapor.
These limitations highlight the challenges of retrieving reliable WTC in complex environments for new altimeter geometries and suggest directions for future improvements in data processing or radiometer instrumental design for future altimetry missions implying wide-swath altimeter.
We propose in this presentation to summarize the main results of this work.