Editing GRACE and GRACE-FO (section)

=== Measurement principle ===

GRACE's key measurement, [[satellite gravimetry]], is not derived from electromagnetic waves. 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 {{convert|220|km|mi|sp=us}} apart, {{convert|500|km|mi|sp=us}} above Earth. The ranging system is sensitive enough to detect separation changes as small as 10 micrometers (approximately one-tenth the width of a human hair) over a distance of 220 kilometers.<ref name="presskit-launch" />  As the twin GRACE satellites circle the globe 15 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 anomalies.