Two Satellites

GRACE is the first Earth-monitoring mission in the history of space flight whose key measurement is not derived from electromagnetic waves either reflected off, emitted by, or transmitted through Earth's surface and/or atmosphere. Instead, the mission uses a microwave ranging system to accurately measure changes in the speed and distance between two identical spacecraft flying in a polar orbit about 220 kilometers (137 miles) apart, 500 kilometers (311 miles) above Earth. The ranging system is so sensitive it can detect separation changes as small as 10 micrometres (about one-tenth the width of a human hair) over a distance of 220 kilometers. ^[8]

As the twin GRACE satellites circle the globe 16 times a day, they sense minute variations in Earth's gravitational pull. When the first satellite passes over a region of slightly stronger gravity, a gravity anomaly, it is pulled slightly ahead of the trailing satellite. This causes the distance between the satellites to increase. The first spacecraft then passes the anomaly, and slows down again; meanwhile the following spacecraft accelerates, then decelerates over the same point.

By measuring the constantly changing distance between the two satellites and combining that data with precise positioning measurements from Global Positioning System (GPS) instruments, scientists can construct a detailed map of Earth's gravity.

The two satellites (nicknamed "Tom" and "Jerry") constantly maintain a two-way microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. As a cross-check, the vehicles measure their own movements using accelerometers. All of this information is then downloaded to ground stations. To establish baseline positions and fulfill housekeeping functions, the satellites also use star cameras, magnetometers, and GPS receivers. The GRACE vehicles also have optical corner reflectors to enable laser ranging from ground stations, bridging the range between spacecraft positions and Doppler ranges.