The NASA/German Space Agency (DLR) Gravity Recovery And Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions consist of two identical satellites that act in unison as the primary instrument. The two satellites comprising each mission orbit one behind the other in the same orbital plane at an approximate distance of 220 km (137 miles). As the satellite pair circles Earth, areas of slightly stronger gravity affect the lead satellite first, pulling it toward Earth and away from the trailing satellite. As the satellites continue along their orbital path, the trailing satellite passes over the gravity anomaly and is pulled toward the lead satellite. An extremely precise microwave ranging system aboard both GRACE and GRACE-FO detects miniscule changes in the distance between the satellite pairs.