Abstract's details
Shipborne GNSS atmospheric water vapor retrieval for SWOT radiometer validation in the framework of the Wem-SWOT / C-SWOT dual campaigns
Event: 2025 SWOT Science Team Meeting
Session: Oceanography: Calibration and Validation
Presentation type: Poster
The Surface Water and Ocean Topography (SWOT) mission allows sub-mesoscale ocean processes to be investigated with great precision. The SWOT KaRin capability to access oceanic scales of about 20 km involves new error sources that were orders of magnitude smaller for classical altimetry. In particular, small-scale atmospheric effects may affect the sea surface height (SSH) product. Water vapor is measured through two radiometers onboard the satellite, measuring at the center of each of the two KaRIn interferometers on each side of the Swot nadir. Hay et al. (2025) highlighted some irregularities in the KaRIn dataset due to the spatial variations of the wet troposphere correction not sensed by the wide footprint and single measurement correction applied to both KaRIn swaths.
This research is conducted within the framework of the dual campaigns WEMSWOT, led by the Shom, and C-SWOT-2023, led by Ifremer, in March and April 2023, involving two research vessels (RVs), each carrying geodetic GNSS antennas. The two RVs were sailing along the KaRIn footprint every day during 20 days of the CalVal period of the SWOT mission. We propose to use the GNSS retrieval of water vapor content with state of the art method to qualify the SWOT nadir measurement of the water vapor used for the correction of the altimetric measurements of SWOT. Indeed, the water vapor sensing of the atmosphere from shipborne geodetic GNSS antennas has been assessed in the literature with an accuracy reaching 2 kg.m-². Assessing the radiometer accuracy in the coastal area, ~50 km from the coast, where the radiometer is less performant, is especially relevant. During the twin campaigns, both ships crossed a small yet intense atmospheric cyclone within the SWOT swath, potentially introducing biases in the SSH product. These effects will be thoroughly documented.
To achieve this, we compare the water vapor sensing from the GNSS with external datasets, such as reanalysis, radiosondes launched from Ile du Levant, and reference GNSS antennas when the RVs are close to harbors. Then, we compare the SWOT radiometer dataset to the shipborne GNSS-retrieved water vapor. Taking into account the time difference between SWOT and in situ GNSS measurements, we evaluate the capabilities of the SWOT radiometer, which could introduce small-scale atmospheric variations into the SSH product.
Back to the list of abstractThis research is conducted within the framework of the dual campaigns WEMSWOT, led by the Shom, and C-SWOT-2023, led by Ifremer, in March and April 2023, involving two research vessels (RVs), each carrying geodetic GNSS antennas. The two RVs were sailing along the KaRIn footprint every day during 20 days of the CalVal period of the SWOT mission. We propose to use the GNSS retrieval of water vapor content with state of the art method to qualify the SWOT nadir measurement of the water vapor used for the correction of the altimetric measurements of SWOT. Indeed, the water vapor sensing of the atmosphere from shipborne geodetic GNSS antennas has been assessed in the literature with an accuracy reaching 2 kg.m-². Assessing the radiometer accuracy in the coastal area, ~50 km from the coast, where the radiometer is less performant, is especially relevant. During the twin campaigns, both ships crossed a small yet intense atmospheric cyclone within the SWOT swath, potentially introducing biases in the SSH product. These effects will be thoroughly documented.
To achieve this, we compare the water vapor sensing from the GNSS with external datasets, such as reanalysis, radiosondes launched from Ile du Levant, and reference GNSS antennas when the RVs are close to harbors. Then, we compare the SWOT radiometer dataset to the shipborne GNSS-retrieved water vapor. Taking into account the time difference between SWOT and in situ GNSS measurements, we evaluate the capabilities of the SWOT radiometer, which could introduce small-scale atmospheric variations into the SSH product.