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GRACE-FO twin satellites as seen in an artist's rendering
Credit: NASA

The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, a joint partnership between NASA and the German Research Centre for Geosciences (GFZ), launched on 22 May 2018. It uses twin satellites to accurately map variations in the Earth's gravity field and surface mass distribution. It is designed as a successor to the Gravity Recovery and Climate Experiment (GRACE) mission.

Conceptually very similar to the original GRACE mission (2002 – 2017), GRACE-FO consists of two identical satellites flying in formation around Earth at an initial altitude of approximately 490 kilometers and a nominal separation distance of 220+/-50 kilometers. Instruments on board the satellites precisely measure changes in the distance between them due to orbital perturbations caused by geographical and temporal variations in Earth's gravity field.

GRACE-FO will expand GRACE's legacy of scientific achievements. These include tracking mass changes in Earth's polar ice sheets and mountain glaciers (which impacts global sea level); estimating total water storage on land (from groundwater changes in deep aquifers to changes in soil moisture and surface water); inferring changes in deep ocean currents, a driving force in climate; and even measuring changes within the solid Earth itself, such as postglacial rebound and the impact of major earthquakes. 

  • KBR – K-Band Ranging is the key science (and heritage) instrument of the GRACE mission (it is also referred to as the MWI (Microwave Instrument). K-band has a radio frequency of about 24 GHz and Ka-band is near 32 GHz. The GRACE K- and Ka-band frequencies are in an exact 3-to-4 ratio on each satellite. The KBR system can measure the range (with a bias) to the µm level.
  • LRI – The most significant difference to the GRACE mission is the additional inclusion of an experimental Laser Ranging Interferometer as a technology demonstrator. LRI is the first spaceborne laser interferometer to measure distance variations between remote spacecraft, with significant implications for other missions using the same basic measurement such as LISA (Laser Interferometer Space Antenna).
  • ACC – Accelerometer (of SuperSTAR heritage on GRACE) is an improved accelerometer developed by ONERA, France. ACC is designed to measure the non-gravitational accelerations (such as those due to atmospheric drag)
  • SCA Star Camera Assembly is of GRACE and CHAMP (Challenging Minisatellite Payload) heritage. The objective is the precise measurement of satellite attitude. SCA consists of three DTU (Technical University of Denmark) star camera assemblies.
  • TriG-RO – NASA/JPL (Jet Propulsion Laboratory) is developing a next-generation GNSS space science receiver, the Tri-GNSS (GPS+ Galileo+GLONASS) Radio Occultation receiver. The receiver will upgrade the capabilities offered by the current state of the art BlackJack/IGOR GPS science receivers in order to meet NASA's decadal survey recommendations. This includes the ability to track not only GPS, but additional GNSS signals, including Galileo, CDMA GLONASS and Compass.
How it works

GRACE-FO's raw data is a series of measurements showing how the distance between the two satellites varies as they orbit Earth. The twin GRACE or GRACE-FO satellites follow each other in orbit around the Earth, separated by about 137 miles (220 km). They constantly send microwave signals to each other to track the distance variation (down to about one-millionth of a meter) between them.

As the pair circles the Earth, areas of slightly stronger gravity (greater mass concentration) affect the lead satellite first, pulling it away from the trailing satellite. As the satellites continue, 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 the extremely precise microwave ranging system on GRACE-FO detects minuscule changes in the distance between the satellites. A highly accurate measuring device known as an accelerometer, located at each satellite’s center of mass, measures 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 determine the exact position of the satellite over the Earth to within a centimeter or less. All this information from the satellites is used to construct monthly maps of the Earth’s average gravity field, offering details of how mass, in most cases water, is moving around the planet.