The Copernicus Sentinel-6 satellites reach 66°N and 66°S – a specific orbit occupied by the earlier missions that supplied the reference sea-surface height data over the last three decades. This orbit allows 95% of Earth’s ice-free ocean to be mapped every 10 days. As the next radar altimetry reference mission, Copernicus Sentinel-6 is continuing the long-term record of sea-surface height measurements that were started in 1992 by the French–US Topex Poseidon satellite and then by the Jason series of satellite missions. Copernicus Sentinel-6 comprises two identical satellites launched five years apart. Firstly, Copernicus Sentinel-6 Michael Freilich in 2020 and then Copernicus Sentinel-6B in 2025 to supply measurements until at least 2030.