Rising seas are one of the biggest threats to society. Satellite altimeters play an important role in monitoring global and regional changes in sea level. However, making sure we understand how well satellite altimeters are working requires continuous work and very specific technology on the ground. The ESA Fiducial Reference Measurements for Satellite Altimetry Project works with the international satellite altimetry community to develop a standardised approach to validating measurements made from space, such as those from the Copernicus Sentinel-3 mission. This short video explains how measurements are taken from the top of a mountain to make sure users get the best data on sea height from satellite altimetry.