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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 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/GraceMonths/ 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
 

(April 24, 2008)
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. At present, data prior to Jan 1, 2005 are labeled Release-00, and all the data, thereafter, are labeled Release-01. Please see NOTE-1 (Data Status Notes) for special remarks on AOD1B products from June 2006 to Sep 2007.

Level-2 data refer to monthly estimates of spherical harmonic coefficients of the Earth gravity field. Please see NOTE-3 (Data Status Notes) for special remarks on re-processed Level-2 products from June 2006 through September 2007. Data products from all three centers are available, as follows:

Center Release Span
Remarks
UTCSR
(Operational)
All months from Apr ’02 to present; except Jun-Jul ’02 and Jun ‘03

RL-01 Series is now discontinued. See NOTE-2 about July 2007 field.

GFZ
(Operational)
All months from Aug ’02 to present; except Sep ’02, Dec ’02, Jan & Jun ’03 and Jan ’04
The RL-03 fields are not available past Jan 2007.  See NOTE-1 about Jun ’06 to Sep ’07 fields.
JPL
(Validation)
All months from Apr '02 to
Jun '08, except Jun ’03 and Jan’ 04
 

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/
 

  
Data Links
 
Related Links  
   
 
   
   
   
Data Status  
 
  • Feb 29, 2008 (NOTE-3): The influence of the error in AOD1B product, described in NOTE-1, on the Level-2 gravity field products, though small, is not negligible. Therefore all GSM-products between June 2006 and September 2007 have been reprocessed with the correct AOD1B. Click Here for a summary of the principal changes in the Level-2 fields in this duration.    More >>
  • Feb 26, 2008 (SPECIAL NOTE ON GPS-PRN32 and LEVEL-1 READ SOFTWARE): The GPS satellite PRN32 (SVN23) was set "healthy" as of Feb 26, 2008. Tracking of this satellite was therefore enabled on GRACE. The Level-1B data since Feb 26, 2008 contains GPS tracking data from PRN32. Users should download and re-install the entire Level-1 Read software suite (RELEASE_2008-03-20).  More >>

View all notes here >>

  
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.
 

Orbit

  
 
 
Altitude 456 km (above 6,378km, on 3/24/2011)
Inlination 89°
Eccentricity 2.25e-03
Semi-major axis 6873 km
RAAN (Right Ascension of Ascending Node) 277.26°
Argument of perigee 96.2°
True anomaly 156.5°
Mean anomaly 156.4°
Orbit period 5679 seconds
Satellite separation 209 km (on 3.24.2011)
 

For current orbit parameters please see http://www.csr.utexas.edu/grace/operations/configuration.html