Editing GRACE and GRACE-FO (section)

=== Instruments ===

The two satellites (nicknamed [[Tom and Jerry|"Tom" and "Jerry"]]) constantly maintain a two-way, [[K band (IEEE)|K-band]] microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. This is made possible due to the onboard Ultra Stable Oscillator (USO) which produces the frequencies for the K-band ranging system.<ref name="grace-craft">{{Cite web |url=http://www.nasa.gov/mission_pages/Grace/spacecraft/index.html |title=Spacecraft |series=GRACE Mission |publisher=NASA |date=6 June 2013 |access-date=10 March 2019}}</ref> The micrometer-sensitivity of this measurement requires accordingly precise measurements of each spacecraft's position, motion, and orientation to be useful.  To remove the effect of external, non-gravitational forces (e.g., [[drag (physics)|drag]], [[solar radiation pressure]]), the vehicles use sensitive Super STAR electrostatic accelerometers located near their respective centers of mass.  GPS receivers are used to establish the precise positions of each satellite along the baseline between the satellites.   The satellites use star cameras and magnetometers to establish [[Spacecraft attitude control|attitude]]. The GRACE vehicles also have [[Corner reflector|optical corner reflectors]] to enable [[satellite laser ranging|laser ranging]] from ground stations using the Center of Mass Trim Assembly (MTA) which ensures the center of mass is modified throughout the flight accordingly.<ref name="grace-craft"/>