The Copernicus Sentinel-3 mission is part of the first generation of Copernicus satellites and was set out to deliver operationally high quality measurements over ocean, land and atmosphere. The European Space Agency is now studying concepts for the Next Generation Sentinel-3 Topography mission (S3NGT) mission that would launch in the 2032+ time period. In order to meet the primary objectives of the S3NGT mission requirement document a complex analysis of river and lake targets is required to size the satellite mass memory and downlink system.