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 visit http://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/grace-months/ to find out which days are missing data.
 
To see up-to-date battery status and other mission status, please visit http://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

(December 2, 2014)  
Distribution of Release 05 (RL05) began March 2012.  Errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014 were corrected, AOD1B data in the archive were replaced on 3 October 2014.  The CSR Level 2 RL05 fields from June 2013 to June 2014 based on these erroneous de-aliasing fields were replaced on 6 October 2014.  The GFZ Level 2 RL05 fields were replaced on 26 September 2014.  If you have downloaded any of these products before these dates, please download the replacement files.
 
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		 All months from 4/2002 to present

Due to errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014, these AOD1B fields have been corrected and the CSR level 2 data reprocessed.  If you downloaded any Release 05 CSR data product for June 2013 through June 2014 before 6 October 2014, please download those files again.
GFZ
(Operational)		 05a
Release Notes		 All months from 4/2002 to present

Due to errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014, these AOD1B fields have been corrected and the GFZ level 2 data reprocessed.  If you downloaded any Release 05 GFZ data product for June 2013 through June 2014 before 26 September 2014, please download those files again.
JPL
(Validation)		 05.1
Release Notes		 All months from 4/2002 to present Due to errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014, these AOD1B fields have been corrected but the JPL level 2 data have not been reprocessed.  If you downloaded any Release 05 JPL data product for June 2013 through June 2014, please be aware that those data have known problems and have been replaced by JPL Release 05.1, which uses the corrected de-aliasing fields.

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.