The First Validation of Sentinel-3 OLCI Integrated Water Vapor Products Using Reference GPS Data in Mainland China

The integrated water vapor (IWV) products collected from June 1, 2019 to May 31, 2020 from the ocean and land color instrument (OLCI) sensor, onboard the Sentinel-3 satellites, are evaluated against reference water vapor data estimated from ground-based 214 global positioning system (GPS) stations in the Mainland China. This is the first time to thoroughly evaluate the quality of Sentinel-3 OLCI IWV products by
in situ
GPS-measured IWV data from such a large spatial coverage as China. The validation results show that, under cloud-free conditions, the OLCI IWV measurements agree very well with the ground-based GPS water vapor data, with a root-mean-square error (RMSE) of 3.03 mm for Sentinel-3A satellite and 3.13 mm for Sentinel-3B satellite. The dependence of OLCI IWV on various parameters was also analyzed. Analysis showed that the accuracy of inland OLCI IWV products was superior to that in coastal areas and that OLCI tended to overestimate IWV value in lower elevation and underestimate IWV value in higher elevation. The accuracy of OLCI IWV measurements increased as IWV decreased. Solar zenith angle analysis showed that the OLCI IWV product had a higher accuracy at a larger solar zenith angle. In spring and winter, the OLCI IWV observations had higher accuracy than those in summer and autumn. OLCI IWV tended to underestimate IWV value in most land cover types. Except for the polar climatic zone, the Sentinel-3 OLCI IWV products tended to overestimate IWV value. The validation results against previous studies were also discussed in this work.