GRACE is different from most Earth observing satellite missions - Terra and Aqua for example - because it doesn't carry a suite of independent scientific instruments on board. It does not make measurements of the electromagnetic energy reflected back to it from the Earth's surface. Instead, the two GRACE satellites themselves act in unison as the primary instrument. Changes in the distance between the twin satellites are used to make gravitational field measurements.
The two identical satellites orbit one behind the other in the same orbital plane at an approximate distance of 220 km (137 miles). As the pair circles the Earth, areas of slightly stronger gravity (greater mass concentration) will affect the lead satellite first, pulling it away from the trailing satellite, then as the satellites continue along their orbital path, the trailing satellite is pulled toward the lead satellite as it passes over the gravity anomaly. The change in distance would certainly be imperceptible to our eyes, but an extremely precise microwave ranging system on GRACE is able to detect these miniscule changes in the distance between the satellites. A highly accurate measuring device known as an accelerometer, located at each satellite mass center, will be used to measure the non-gravitational accelerations (such as those due to atmospheric drag) so that only accelerations caused by gravity are considered. Satellite Global Positioning System (GPS) receivers will be used to determine the exact position of the satellite over the Earth to within a centimeter or less. Members of the GRACE science team can download all this information from the satellites, and use it to construct monthly maps of the Earth's average gravity field.
Now that we have an idea how GRACE works, let's peer "under the hood" of this high-tech wonder and understand some of the component parts of GRACE.
K-band Ranging System (KBR)
Provides precise (within 10 µm) measurements of the distance change between the two satellites needed to measure fluctuations in gravity.
Ultra Stable Oscillator (USO)
Provides frequency generation for the K-band ranging system.
SuperSTAR Accelorometers (ACC)
Precisely measures the non-gravitational accelerations acting on the satellites
Star Camera Assembly (SCA)
Precisely determines the two satellite's orientation by tracking them relative to the position of the stars.
Coarse Earth Sun and Sensor (CES)
Provides omnidirectional, reliable, and robust, but fairly coarse, Earth and Sun tracking. Used during initial acquisition and whenever GRACE operates in safe mode.
Center of Mass Trim Assembly (MTA)
Precisely measures the offset between the satellite's center of mass and the "acceleration-proof" mass and adjusts center of mass as needed during the flight.
BlackJack GPS Receiver and Instrument Processing Unit (GPS)
Provides digital signal processing; measures the distance change relative to the GPS satellite constellation.
Globalstar Silicon Solar Cell Arrays (GSA)
Covers the outer shell of the spacecraft and generates power..