GRACE was different from most Earth observing satellite missions - TERRA and Aqua for example - because it didn't carry a suite of independent scientific instruments on board. It did not make measurements of the electromagnetic energy reflected back to it from Earth's surface. Instead, the two GRACE satellites themselves acted in unison as the primary instrument. Changes in the distance between the twin satellites were used to make gravitational field measurements. The two identical satellites orbited one behind the other in the same orbital plane at an approximate distance of 220 km.

As the pair circled Earth, areas of slightly stronger gravity (greater mass concentration) affected the lead satellite first, pulling it away from the trailing satellite, then as the satellites continued along their orbital path, the trailing satellite was pulled toward the lead satellite as it passed over the gravity anomaly. The change in distance would certainly have been imperceptible to our eyes, but an extremely precise microwave ranging system on GRACE was able to detect these very small changes in the distance between the satellites.

A highly accurate measuring device known as an accelerometer, located at each satellite mass centre, was used to measure the non-gravitational accelerations (such as those due to atmospheric drag) so that only accelerations caused by gravity were considered. Satellite Global Positioning System (GPS) receivers were used to determine the exact position of the satellite over Earth to within a centimetre or less. Members of the GRACE science team were able to download all this information from the satellites, and use it to construct monthly maps of Earth's average gravity field.

Science

The GRACE mission was selected as the second mission under the NASA Earth System Science Pathfinder (ESSP) programme in May 1997. Launched in March of 2002, the GRACE mission was accurately mapping variations in Earth's gravity field. Designed for a nominal mission lifetime of five years, GRACE operated in an extended mission phase.