GRACE-FO (Gravity Recovery and Climate Experiment Follow-On) is an active NASA-led Earth-observation program launched on 22 May 2018 in partnership with the German Research Centre for Geosciences (GFZ), with German Space Agency (DLR) participation. Its twin satellites map month-to-month changes in Earth’s gravity and surface mass to track water movement, ice sheets, glaciers, groundwater, ocean circulation, sea level, and solid-Earth processes while demonstrating laser-ranging interferometry.