New observations from the SWIM radar on board CFOSAT: instrument validation and ocean wave measurement assessment

Danièle Hauser (1) , Cedric Tourain (2) , Laura Hermozo (2) , Dunya Alraddawi (1) , Lotfi Aouf (3) , Bertrand Chapron (4) , Alice Dalphinet (3) , Lauriane Delaye (5) , Maeva Dalila (6) , Emmanuel Dormy (7) , Flavien Gouillon (2) , Victor Gressani (4) , Antoine Grouazel (4) , Gilles Guitton (8) , Romain Husson (6) , Alexey Mironov , Alexis Mouche (9) , Annabelle Ollivier (6) , Ludivine Oruba (10) , Fanny Piras (6) , Raquel Rodriguez-Suquet (2) , Patricia Schippers (1) , Céline Tison (2) , Ngan T. Tran (6)

1 SPACE - LATMOS
2 CNES - Centre National d'Études Spatiales [Toulouse]
3 DIRSE - Météo-France Direction Interrégionale Sud-Est
4 LOPS - Laboratoire d'Océanographie Physique et Spatiale
5 ACRI-ST - Analytic and Computational Research, Inc. - Earth Sciences
6 CLS - Collecte Localisation Satellites
7 FRDPENS - Fédération de recherche du Département de physique de l'Ecole Normale Supérieure - ENS Paris
8 OceanDataLab
9 IFREMER - Institut Français de Recherche pour l'Exploitation de la Mer
10 LATMOS - Laboratoire Atmosphères, Milieux, Observations Spatiales

This paper describes first results obtained from the SWIM (Surface Waves Investigation and Monitoring) instrument carried by CFOSAT (China France Oceanography Satellite), which was launched on October 29th, 2018. SWIM is a Ku-Band radar with a near-nadir scanning beam geometry. It was designed to measure the spectral properties of surface ocean waves. First, the good behavior of the instrument is illustrated. It is then shown that the nadir products (significant wave height, normalized radar cross-section and wind speed) exhibit an accuracy similar to standard altimeter missions, thanks to a new retracking algorithm, which compensates a lower sampling rate compared to standard altimetry missions. The off-nadir beam observations are analyzed in details. The normalized radar cross-section varies with incidence and wind speed as expected from previous studies presented in the literature. We illustrate that, in order to retrieve the wave spectra from the radar backscattering fluctuations, it is crucial to apply a speckle correction derived from the observations. Directional spectra of ocean waves and their mean parameters are then compared to wave model data at the global scale and to in situ data from a selection of case studies. The good efficiency of SWIM to provide the spectral properties of ocean waves in the wavelength range [70m-500m] is illustrated. The main limitations are discussed, and the perspectives to improve data quality are presented.

Keywords: Radar measurements; scatterometer; altimeter; speckle noise; sea surface; spaceborne radar; ocean wave spectra

Domains: Ocean, Atmosphere; Oceanography

Danièle Hauser, Cedric Tourain, Laura Hermozo, Dunya Alraddawi, Lotfi Aouf, et al.. New observations from the SWIM radar on board CFOSAT: instrument validation and ocean wave measurement assessment. IEEE Transactions on Geoscience and Remote Sensing, 2021, 59 (1), pp.5-26. ⟨10.1109/TGRS.2020.2994372⟩. ⟨insu-02324383v2⟩