Surface Wind Speed Estimation Over Open Ocean Using Bidirectional Observation by Sentinel-2/MSI and Landsat-8/OLI

摘要

Large stripes, observed on first Sentinel-2/MSI images over ocean, are not due to instrumental artifacts but to the target itself. The same kind of signature can be observed on Landsat-8/OLI. Both MSI and OLI instruments are known for their excellent radiometric quality for land observation. The MSI's focal plane is composed by 12 elements to cover the 300km-swath, respectively 14 elements for a 185km-swath for OLI. For technical reason, elements were slightly shifted forward/backward alternatively in the focal plane. As a consequence, each element has a different viewing angle than the next/previous one, leading for a considered target on the ground to a significant difference in zenith/azimuthal viewing angles. These angular variations, fully acceptable for the land mission, become sensitive for specific targets such as sunglint, a highly directional signal. It was already demonstrated the possibility to retrieve surface wind speed from bidirectional space measurements with POLDER/PARASOL instruments. Indeed, using multiple viewing angles is a good way to better constrain the inversion because it doesn't fully rely on the absolute estimation of a unique measurement. Somehow, MSI or OLI can be seen as bidirectional sensors for targets located between 2 elements of the focal plane. Knowing the exact acquisition geometry, the observed radiometric gap can be directly related to the surface wind speed. Because it is a relative estimation, this inversion becomes more robust to aerosol contamination. Finally, an improved retrieval can be foreseen thanks to the multiple spectral bands provided by MSI and OLI.