The Copernicus Sentinel-6 mission comprises two identical satellites launched five years apart. It not only serves Copernicus, but also the international climate community. Since sea-level rise is a key indicator of climate change, accurately monitoring the changing height of the sea surface over decades is essential for climate science, for policy-making and, ultimately, for protecting the lives of those in vulnerable low-lying areas. Copernicus Sentinel-6 is taking on the role of radar altimetry reference mission, continuing the long-term record of measurements of sea-surface height started in 1992 by the French–US Topex Poseidon and then the Jason series of satellite missions.