An experimental satellite navigation receiver station high atop Spain’s Mallorca island has opened up a novel view of the ever-changing face of the sea. By picking up satnav signals from the far horizon as they bounce off ocean waves, the receivers are able to measure sea surface height down to a scale of centimetres. A pair of satnav receivers was positioned in a near-horizontal orientation, 1400 m above sea level atop Mallorca’s highest peak, 4 km from the coast. The aim was to receive signals from Galileo, GPS and other Global Navigation Satellite Systems (GNSS) in a way that isn’t possible using standard commercial receivers. By picking up coherent satnav signals that have reflected off the sea surface at very slanted, nearly horizontal geometries, the campaign can use these results to derive sea surface height and shape to high accuracy.