The Gravity Recovery and Climate Experiment (GRACE) twin satellites, launched 17 March 2002, are making detailed measurements of Earth's gravity field and improving investigations about Earth's water reservoirs, over land, ice and oceans.
GRACE measures gravity by relating it to the distance between the 2 satellites. When there is an increase in gravity ahead of the pair, the front satellite speeds up and the distance between the pair increases, when the increased gravity is between the pair their distance decreases; the opposite occurs when there is decreased gravity ahead of, or between the satellite pair.
The satellites are separated by 220 km and they can detect changes smaller than a micrometer per second in relative velocity. These measurements, in conjunction with other data and models, have provided observations of terrestrial water storage changes, ice-mass variations, ocean bottom pressure changes and sea-level variations.
GRACE is a collaboration of the US and German space agencies (NASA and DLR). The key partners are the University of Texas Center for Space Research (CSR), the GeoForschungsZentrum (GFZ) Potsdam, and the Jet Propulsion Laboratory (JPL).
GRACE originally was planned for a 5 year mission, but is currently operating in an extended mission. It has a non-repeat orbit, but encompasses the entire Earth in about a month.
Since the launch of GRACE the orbit has been slowly decaying due to the atmospheric drag on the satellites. The orbit parameters given in the orbit section are from the beginning of the mission. For the current orbit parameters please visit http://www.csr.utexas.edu/grace/.
In December 2005 the GRACE satellites switched positions so the leading satellite would not undergo the majority of wear and tear throughout the mission. More information about the satellites switching can be found at http://www.csr.utexas.edu/grace/operations/switch_maneuver.html.
Since December 2010 the batteries on GRACE are feeling their age and are not capable of retaining a full charge. To extend the battery life data are not collected when GRACE is eclipsed and cannot collect solar energy; when GRACE is not eclipsed it collects data normally.
Please visit http://grace.jpl.nasa.gov/data/grace-months/ to find out which days are missing data.
To see up-to-date battery status and other mission status, please visit http://www.csr.utexas.edu/grace/operations/mission_status/
GRACE ground segment operations are co-funded by ESA. Therefore, ESA is supporting the continuation of the measurements of mass redistribution in the Earth system.