# Shipborne GNSS atmospheric water vapor retrieval for SWOT radiometer validation in the framework of the Wem-SWOT and BioSWOT campaigns

Aurélie Panetier, F Mercier, Pierre Bosser, Clémence Chupin, Jean-Baptiste Roustan, Aurelien Ponte, Pierre Garreau, Franck Dumas, Félix Perosanz

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## ► To cite this version:

Aurélie Panetier, F Mercier, Pierre Bosser, Clémence Chupin, Jean-Baptiste Roustan, et al.. Shipborne GNSS atmospheric water vapor retrieval for SWOT radiometer validation in the framework of the Wem-SWOT and BioSWOT campaigns. SWOT ST 2025, Oct 2025, Arcachon, France. . hal-05322175

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https://hal.science/hal-05322175v1

Submitted on 20 Oct 2025

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# Shipborne GNSS atmospheric water vapor retrieval for SWOT radiometer validation in the framework of the Wem-SWOT and BioSWOT campaigns

A. Panetier * ¹,², F. Mercier ³, P. Bosser ³, C. Chupin ³, JB. Roustan ², A. Ponte ⁴, P. Garreau ⁴, F. Dumas ², F. Perosanz¹

¹ CNES, France

² Shom, France

³ Lab-STICC M3, UMR 6285, ENSTA, Brest

⁴ Ifremer, France

## ► Context

Since 2015, the French Oceanographic Fleet (FOF) has collected geodetic-quality GNSS data aboard its ocean-going vessels. Once analyzed, this data can be used to estimate atmospheric water vapor at sea. This data is valuable for marine climatology and can be used to calibrate satellite altimetry missions, such as the new SWOT satellite, at sea. Here, we present data from two FOF campaigns in the Mediterranean Sea: WemSWOT (SHOM) and BioSWOT (IFREMER), which were conducted aboard L'Atalante in 2023. During its Cal/Val phase, L'Atalante was under one of the SWOT satellite's tracks (Figure 1).

<figure><img src="image_4.png" /><figcaption>**Figure 2.** SWOT radiometer measurement (Source: JPL)</figcaption></figure>

The radiometer on board the altimetry satellite measures water vapor at the center of each of the two beams of its KaRIn interferometer (Figure 2). This measurement is used to correct for the effect of atmospheric water vapor on sea surface height (SSH) measurements. Without correction, this effect can induce an error of around 10 cm.

<figure><img src="image_5.png" /><figcaption>**Figure 1.** L'Atalante and SWOT Cal/Val swaths</figcaption></figure>

**Objective:** Evaluate the tropospheric correction applied to SWOT measurements using GNSS observations from the oceanographic vessel.

## ► WemSWOT and BioSWOT 2023

The GNSS tropospheric corrections shown in Figures 3 and 4 come from the reprocessing of FOF data with GipsyX from 2015 to 2024.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Zenith Troposphere Delay (ZTD) time series from the GNSS antenna (GipsyX, JPL, NASA)</figcaption></figure>

## ► SWOT Cal/Val in Mediterranean Sea

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Tropospheric correction differences between SWOT and ERA5 for each hal-orbit passing above the study zone.</figcaption></figure>

The wet tropospheric correction (WTC) coming from SWOT radiometer measurements agree with the ERA5 reanalysis (ECMWF) with an RMS difference of 6,9 mm (Figure 4).

## ► GNSS - SWOT radiometer comparison

In Figure 5, SWOT shows a bigger RMSD with GNSS of 10.7 mm. This is because the ship is often in an area impacted by the coast when SWOT passes.

Figure 6 shows centimeter-level agreement between GNSS and SWOT data in offshore areas. However, it shows discrepancies in areas close to the coast when comparing SWOT's tropospheric correction with GNSS measurements. These errors can reach several tens of centimeters in some places.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** A comparison of the SWOT WTC with GNSS during its passage over the area is shown for the Cal/Val half-orbits numbered 476 (March 30), 513 (May 4), and 485 (April 8) from top to bottom. A spatial threshold of 50 km was applied to the comparison.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Statistics on the differences in WTC between SWOT radiometer and L'Atalante GNSS, per pass over the study area.</figcaption></figure>

## ► Coastal discrepancies

Figure 7 reveals artifacts in the SWOT WTC correction (from radiometer data) near coastal areas. These are likely caused by land contamination, affecting SWOT SSH measurements by up to ~1 cm on average.

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Local mean variability in SWOT-GNSS WTC difference.</figcaption></figure>

## ► Conclusions and perspectives

* GINS Processing Chains Update at Shom
  → Integration of SPOTGINS consortium [San+25]
  → Kinematic adaptation for vessel GNSS

* SSH estimation from shipborne GNSS measurements to validate KaRIn interferometer SSH measurements over oceans

## ► References

* Hay et al., « Small Scale Variability in the Wet Troposphere Impacts the Interpretation of SWOT Satellite Observations », Geophysical Research Letters, vol. 52, n° 4, p. e2024GL112778, 2025, doi: 10.1029/2024GL112778.

* Panetier, « Shipborne Global Navigation Satellite Systems for Offshore Atmospheric Water Vapor Monitoring », These de doctorat, Brest, ENSTA Bretagne, 2023.

* M. Demol et al., « C-SWOT2023 / WEMSWOT. Two combined campaigns dedicated to the mesoscale dynamics under SWOT swaths in the Mediterranean Sea », JS LEFE/GMMC 2023. https://archimer.ifremer.fr/doc/00847/95923/.

* Doglioli A., Gregori G. (2023) BioSWOT-Med cruise, RV L'Atalante, doi: 10.17600/18002392.

* Contact : aurelie.panetier@ensta-bretagne.org