"The Gravity Recovery and Climate Experiment (GRACE) satellite mission, in operation from March 2002 to June 2017, was the first remote sensing mission to provide temporal variations of Terrestrial Water Storage (TWS).
GRACE consisted of two identical spacecraft that flew about 220 kilometres (137 miles) apart in a polar orbit 500 kilometres (310 miles) above Earth. GRACE mapped Earth's gravity field by making accurate measurements of the distance between the two satellites, using GPS and a microwave ranging system. It provides scientists from all over the world with an efficient and cost-effective way to map Earth's gravity field with unprecedented accuracy. The results from this mission are yielding crucial information about the distribution and flow of mass within Earth and its surroundings.
The gravity variations studied by GRACE include: changes due to surface and deep currents in the ocean; runoff and ground water storage on land masses; exchanges between ice sheets or glaciers and the ocean; and variations of mass within Earth. Another goal of the mission is to create a better profile of Earth's atmosphere. GRACE results are making a huge contribution to the goals of NASA's Science Mission Directorate, Earth Observation System (EOS) and global climate change studies." (NASA 2014)