GRACE (the Gravity Recovery and Climate Experiment) maps the Earth's gravity fields 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 the Earth's gravity fields with unprecedented accuracy. The results from this mission have revealed detailed information about the distribution and flow of mass within the Earth and it's surroundings.
The gravity variations that GRACE studies 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 oceans; and variations of mass within the Earth. Another goal of the mission is to create a better profile of the Earth's atmosphere.