Cat and mouse satellites

The GRACE (Gravity Recovery and Climate Experiment) mission, launched by NASA in 2002, makes use of two satellites, Tom and Jerry, in order to map the Earth’s gravitational field. The satellites can measure differences in the Earth’s gravitational field of the order of 1 milligal or about a millionth of the gravitational field strength at the Earth’s surface. The mission uses a distance measuring technique to determine the gravitational field strength. The satellites orbit at a distance of approximately 220 km from each other and small differences in the Earth’s mass distribution slightly perturb their orbits. A microwave ranging system measures the distance between Tom and Jerry with an accuracy of a few microns. Changes in the spacing between the satellites can be used to calculate gravitational anomalies. The data collected by the mission are being used to measure changes to the masses of ice-sheets and to estimate ocean pressures which can help to determine fluctuations in sea levels.