Editing GRACE and GRACE-FO (section)

== Spacecraft ==
[[File:GRACE schematics.png|thumb|left|Diagrams illustrating the systems and instruments aboard the GRACE spacecraft]]
{{multiple image |align=right |direction=vertical
 |image1=Global Gravity Anomaly Animation over LAND.gif
 |image2=Global Gravity Anomaly Animation over OCEANS.gif
 |footer=Global gravity anomaly animations over land and oceans by GRACE
}}

The spacecraft were manufactured by [[Astrium]] of Germany, using its "Flexbus" [[satellite bus|platform]]. The microwave [[radio frequency|RF]] systems, and attitude determination and control system algorithms were provided by [[Space Systems/Loral]]. The star cameras used to measure the spacecraft attitude were provided by [[Technical University of Denmark]]. The instrument computer along with a highly precise BlackJack GPS receiver and digital signal processing system was provided by [[Jet Propulsion Laboratory|JPL]] in Pasadena. The highly precise accelerometer that is needed to separate atmospheric and solar radiation pressure effects from the gravitation data was manufactured by [[ONERA]].

=== 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.

=== 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"/>

=== Data products ===

CSR, GFZ, and JPL process observations and ancillary data downloaded from GRACE to produce monthly [[geopotential model]]s of Earth.<ref name="PODAAC">{{cite web |title=GRACE PO.DAAC |url=http://podaac.jpl.nasa.gov/GRACE |website=JPL Physical Oceanography and Distributed Active Archive Center |access-date=11 June 2015}}</ref>  These models are distributed as [[spherical harmonics|spherical harmonic]] coefficients with a maximum degree of 60.  Degree 90 products are also available.  These products have a typical latency of 1–2 months.  These geopotential coefficients may be used to compute [[geoid]] height, gravity anomalies, and changes in the distribution of mass on Earth's surface.<ref>{{cite journal |last1=Wahr |first1=John |last2=Molenaar |first2=M. |last3=Bryan |first3=F. |s2cid=140194666 |title=Time variability of the Earth's gravity field: Hydrological and oceanic effects and their possible detection using GRACE |journal=J. Geophys. Res. |date=1998 |volume=103 |issue=B12 |pages=30205–30229 |doi=10.1029/98JB02844 |bibcode=1998JGR...10330205W|doi-access=free }}</ref>  Gridded products estimating changes in mass in units of liquid water equivalent thickness are available at JPL's GRACE Tellus website.