The Gravity Recovery and Climate Experiment (GRACE) twin satellites, which orbited Earth from 2002 to 2017, made detailed measurements of Earth's gravity field and improved investigations about Earth's water reservoirs, over land, ice and oceans.
GRACE measured gravity by relating it to the distance between the two satellites. When there was an increase in gravity ahead of the pair, the front satellite sped up and the distance between the pair increased. When the increased gravity was between the pair, their distance decreased; the opposite occurred when there was decreased gravity ahead of, or between, the satellite pair.
The satellites were separated by 220 km, and they could detect changes smaller than a micrometer per second in relative velocity. These measurements, in conjunction with other data and models, provided observations of terrestrial water storage changes, ice-mass variations, ocean bottom pressure changes and sea-level variations.
GRACE was a collaboration of the US and German space agencies (NASA and DLR). The key partners were the University of Texas Center for Space Research (CSR), the GeoForschungsZentrum (GFZ) Potsdam, and the Jet Propulsion Laboratory (JPL).