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The lake processing chain, initially developed at LEGOS (Laboratoire d'Etudes en Géophysique et Océanographie Spatiales), is based on the use of altimetry measurements from the CTOH database (Center for Topography of the Oceans of LEGOS), and includes Envisat GDRs. This database includes, in addition, some enhanced corrections that were not in the first data sets of institutional suppliers (Archiving, Validation and Interpretation of Satellite Oceanographic data (Aviso), ESA), such as for the wet and dry tropospheric as well as for the ionospheric corrections. As part of the online version of HYSOPE (HYdrométrie Spatiale OPErationelle) which underpins the C3S Lakes Service, these tailored corrections were replaced by their corresponding standard corrections included in the operational input products. In recent mission products, the geophysical corrections have been regularly updated by agencies which improves the quality of the estimates and makes it unnecessary to replace them. The main source of uncertainty in atmospheric corrections comes from the wettropospheric correction, which, depending on the region, varies from 2-3 cm. In addition, a module for calculating the height of the geoid has been developed to improve assessment of water levels. Moreover, the overall uncertainty on water surface height depends mainly on the length of the track over the lake and comes mainly from retracking. It varies from 8-10 cm for large lakes (with longer transects) to 1m for small and narrow lakes.
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