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GRACE

Mission Specification

The Gravity Recovery and Climate Experiment (GRACE) twin satellites, launched 17 March 2002, are making detailed measurements of Earth's gravity field and improving investigations about Earth's water reservoirs, over land, ice and oceans.
 
Gravity Recovery and Climate Experiment (GRACE) twin satellitesGRACE measures gravity by relating it to the distance between the 2 satellites.  When there is an increase in gravity ahead of the pair, the front satellite speeds up and the distance between the pair increases, when the increased gravity is between the pair their distance decreases; the opposite occurs when there is decreased gravity ahead of, or between the satellite pair.  
The satellites are separated by 220 km and they can detect changes smaller than a micrometer per second in relative velocity.  These measurements, in conjunction with other data and models, have provided observations of terrestrial water storage changes, ice-mass variations, ocean bottom pressure changes and sea-level variations.
 
GRACE is a collaboration of the US and German space agencies (NASA and DLR). The key partners are the University of Texas Center for Space Research (CSR), the GeoForschungsZentrum (GFZ) Potsdam, and the Jet Propulsion Laboratory (JPL).
 
GRACE originally was planned for a 5 year mission, but is currently operating in an extended mission.  It has a non-repeat orbit, but encompasses the entire Earth in about a month.
 
Since the launch of GRACE the orbit has been slowly decaying due to the atmospheric drag on the satellites.  The orbit parameters given in the orbit section are from the beginning of the mission.  For the current orbit parameters please visithttp://www.csr.utexas.edu/grace/.
 
In December 2005 the GRACE satellites switched positions so the leading satellite would not undergo the majority of wear and tear throughout the mission.   More information about the satellites switching can be found at http://www.csr.utexas.edu/grace/operations/switch_maneuver.html.
 
Since December 2010 the batteries on GRACE are feeling their age and are not capable of retaining a full charge.  To extend the battery life data are not collected when GRACE is eclipsed and cannot collect solar energy; when GRACE is not eclipsed it collects data normally. 

Please visit http://grace.jpl.nasa.gov/data/GraceMonths/ to find out which days are missing data.
 
To see up-to-date battery status and other mission status, please visithttp://www.csr.utexas.edu/grace/operations/mission_status/ 
 
GRACE ground segment operations are co-funded by ESA. Therefore, ESA is supporting the continuation of the measurements of mass redistribution in the Earth system.

Guide to Available Data

(May 25, 2012)  
Distribution of Release 05 (RL05) began March 2012.  Until the reprocessing of RL05 is complete RL04 will be distributed simultaneously.  Currently RL05 are available for 2005 to 2010.  The rest of the reprocessing should be complete by next year.  2011 data will be released first.
 
The newest Level-1B and Level-2 products are made available at approximately monthly intervals.  Level-1B data refer to the collection of ranging, accelerometry, attitude and related ancillary data. All products have been created by the NASA/JPL element of the SDS. 
 
Level-2 data refer to monthly estimates of spherical harmonic coefficients of the Earth gravity field. 

Data products from all three centers are available, as follows:
 

Center Release Span Remarks
UTCSR
(Operational)
05
Release
Notes
Availablesoon
All months from 2004 to 2012

The CSR-GSM products have been replaced for the following four months: July-2004; October-2004; March-2005 and February-2006. The GSM products for these four months have been updated with the refined data editing. While the GAC/GAD products for these four months are unchanged, they have been updated as well, for consistency. If you downloaded the data products for these four months prior to July 17, 2012, please download the replacement products. No other products previously delivered are so affected.

GFZ
(Operational)
05
Release
Notes
All months from 2005 to 2010  
JPL
(Validation)
05
Release
Notes
All months from 2004 to 2012  

If you require gridded/mapped GRACE data or do not want to generate the gravity fields from the spherical harmonics please visit the GRACE Tellus website at http://grace.jpl.nasa.gov/

Instruments
  • ACC – Super STAR Accelerometer is in both satellites and measures non-gravitational forces acting on the satellites, such as solar radiation pressure, air drag and attitude control activator operation.
  • GPS – Black Jack Global Positioning System Receiver is used for navigation and collects atmospheric occultation data.
  • SCA – Star Camera Assembly provides orientation references for both satellites.
  • KBR – K-Band Ranging System uses K and Ka-band to measure the separation change between the two satellites.
  • LRR – Laser Retro Reflector provides orbit verification from the terrestrial Laser tracking network.  It is also used for precise orbit determination.