In a cleanroom in Ottobrunn, Germany, the latest Copernicus Sentinel satellite is ready for final testing before it is packed up and shipped to the US for liftoff next year. Designed and built to chart changing sea level, it is the first of two identical Sentinel-6 satellites that will be launched consecutively to continue the time series of sea-level measurements. This new mission builds on heritage from previous ocean topography satellites, including the French–US Topex-Poseidon and Jason missions, previous ESA missions such as the ERS satellites, Envisat and CryoSat, as well as Copernicus Sentinel-3. With millions of people around the world at risk from rising seas, it is essential to continue measuring the changing height of the sea surface so that decision-makers are equipped to take appropriate mitigating action – as is being currently highlighted at the COP-25 Climate Change Conference in Spain.