Gravity Recovery and Climate Experiment (GRACE) Mission

GRACE was a joint project between the National Aeronautics and Space Administration ( NASA ) and the Deutsches Zentrum für Luft- und Raumfahrt ( DLR ). The mission has been proposed in 1996 jointly by the University of Texas at Austin, Center for Space Research ( UTCSR ), the German Research Centre for Geosciences ( GFZ ) and the Jet Propulsion Laboratories (JPL) in Pasadena. GRACE was selected in 1997 as second mission in NASA's Earth System Science Pathfinder ( ESSP ) program. As an innovation, the Principal Investigator Prof. Byron Tapley (UTCSR) and his team are ultimately responsible for developing the flight mission hardware from selection to a launch-ready condition, for accomplishing the scientific objectives and delivering the proposed measurements to the broader Earth science community and general public as expediently as possible. Co -Principal Investigator of the mission was Dr. Frank Flechtner (GFZ). Project management and systems engineering activities are carried out by JPL.

The primary science objective of the GRACE mission was to measure the Earth’s gravity field and it’s time variability with unprecedented accuracy. The increase in accuracy has been achieved by utilizing two satellites following each other on the same orbital track. To consider precise attitude and non-gravitational forces both satellites were equipped with star cameras and accelerometers. The position and velocity of the satellites were measured using onboard GPS antennae and (for validation purposes) SLR retro-reflectors. Additionally, the twin satellites were interconnected by a K-band microwave link to measure the exact separation distance and its rate of change to an accuracy of better than 0.1 µm/s. These measurements are directly coupled to the seasonal and sub-seasonal variations in the continental hydrological cycle, to ice mass loss in the large glacier systems in Greenland or Antarctica, to sea level rise, to long wavelength ocean circulation processes or to the transport of ocean heat to Earth’s poles. Thus, during more than 15 years of mission operations GRACE has essentially contributed to a better understanding of system Earth. This becomes visible, e.g. in more than 1700 publications in international peer-reviewed journals or more than 5500 registered users at the GRACE archive ISDC (Information System and Data Center). GRACE has lasted three times as long as originally planned for more than 15 years, but came to an end in October 2017 due to battery problems and running out of fuel.

The secondary science objective of the GRACE mission was to obtain about 150 very precise globally distributed vertical temperature and humidity profiles of the atmosphere per day using the GPS radio occultation technique. These profiles have been operationally (24/7) provided to various numerical weather services.

Notes:

The GRACE project was promoted by the Space Agency of the German Research Center for Aeronautics and Space (DLR) with funding of the German Federal Ministry of Economics and Technology (BMWi) based on a resolution of the German Bundestag with support code 50 EE 1027.

GRACE ground segment operations were co-funded by ESA. Therefore, ESA is supporting the continuation of the measurements of mass redistribution in the Earth system.

Gravity Field Results

Time-variable gravity field models on monthly and weekly basis

GFZ’s monthly and weekly time-variable GRACE gravity field models are freely distributed in the form of GRACE Level-2 products, i.e. as sets of spherical harmonic coefficients representing the Earth’s potential field for a dedicated time-period. Different time-series for different releases (along with helpful documentation) are available at the two GRACE archives ISDC and PODAAC. Here you can also find the monthly GRACE SDS newsletter which provides an up to date status of available Level-1 and Level-2 products of the GRACE Science Data System.

Monthly time-series of gravity field models are available for releases 01 till 05. Detailed information about the current release (GFZ RL05) can be found here.

Weekly time-series of gravity field models are derived by solving subsets (aligned to GPS weeks) of the monthly normal equation systems. They offer an increased resolution in time at the expense of decreased resolution in space. Weekly models are only available for releases 04 and 05 and can be downloaded from the ISDC archive.

Static gravity field models

Additionally, GFZ – in cooperation with GRGS – derives static, i.e. long-term mean, satellite-only (from combination of GOCE, GRACE, CHAMP and/or LAGEOS tracking data) as well as combined (with terrestrial gravity data) so-called EIGEN (European Improved Gravity model of the Earth by New techniques) gravity field models. A list of available EIGEN models is given below. All of these models can be downloaded from the ICGEM data base at GFZ.

  • Combined gravity field model EIGEN-6C2 complete to degree and order 1949 from GOCE, GRACE, LAGEOS and surface gravity data, released on December 10, 2012.
  • Satellite-only gravity field model GO_CONS_GCF_2_DIR_R3 complete to degree and order 240 from GOCE, GRACE and LAGEOS data. This model is release 3 of the official ESA GOCE model by means of the Direct Approach and was computed by GFZ and GRGS behalf of ESA within the framework of the GOCE High Level Processing Facility (GOCE-HPF).
  • Combined gravity field model EIGEN-6C complete to degree and order 1420 from GOCE, GRACE, LAGEOS and surface gravity data, released on June 27, 2011.
  • Satellite-only gravity field model EIGEN-6S complete to degree and order 240 from GOCE, GRACE and LAGEOS data, released on June 27, 2011.
  • Combined gravity field model EIGEN-51C complete to degree and order 359 from GRACE, CHAMP and surface gravity data, released on June 22, 2010.
  • Combined gravity field model EIGEN-5C complete to degree and order 360 from GRACE, LAGEOS and surface gravity data, released on September 29, 2008.
  • Satellite-only gravity field model EIGEN-5S complete to degree and order 150 from GRACE and LAGEOS data, released on September 29, 2008.
  • Satellite-only gravity field model EIGEN-GL04S1 complete to degree and order 150 from GRACE and LAGEOS data, released May 24, 2006.
  • Combined gravity field model EIGEN-GL04C complete to degree and order 360 from GRACE, LAGEOS and surface gravity data, released on March 31, 2006.
  • Combined gravity field model EIGEN-CG03C complete to degree and order 360 from CHAMP, GRACE and surface gravity data, released on May 12, 2005.
  • Combined gravity field model EIGEN-CG01C complete to degree and order 360 from CHAMP, GRACE and surface gravity data, released on October 29, 2004.
  • GRACE gravity field model EIGEN-GRACE02S complete to degree and order 150 released on February 13, 2004 to the GRACE Science Team and August 9, 2004 to the public.
  • First GFZ GRACE gravity field model EIGEN-GRACE01S complete to degree and order 140 released on July 25, 2003.

Links to gravity field results of other processing centers or additional help for visualization procedures or software tools for gravity field coefficients manipulation and transformation can be found here.

GRACE GFZ RL05

The GFZ RL05 time-series is being released since March 17, 2012 and has replaced its precursor RL04 after the monthly solution for April 2012. At the time of writing (May 2013), GFZ RL05 solutions cover the time span from January 2003 till February 2013, including 117 monthly models and 494 weekly models, respectively. For the following months no models are available due to missing or anomalous L1B data: June 2003, January 2011, June 2011, May 2012, October 2012 and March 2013.

There have been no changes in information content compared to RL04, i.e. the RL05 models contain gravitational variations caused by hydrology, cryosphere, episodic events such as large earthquakes, glacial isostatic adjustment (GIA) and errors or unmodelled effects of the applied background models. A major difference w.r.t. RL04 is the fact that no rates for the spherical harmonic coefficients C20, C30, C40, C21 and S21 are included anymore, so users do not have to take care of applying any rates before analyzing the RL05 time-series (the reference epoch of each solution is the middle of the data span which has been used).  The maximum degree and order of the RL05 models has been reduced to 90x90 (RL04: 120x120).

For some dedicated months suffering from sparse ground track coverage caused by short-interval repeat orbit patterns (e.g. 4d-repeat peaked in 09/2004, 3d-repeat peaked in 05/2012), the solutions  are stabilized by applying a modified version of the regularization method used for previous releases, which is based on Kaula’s power law (Bettadpur, S (2004), GRACE Mission Status and Gravity Field Product Improvement Plans, Eos Trans. AGU, 85(47), Fall Meet. Suppl., Abstract G23A-01).

Compared to RL04, the current RL05 time-series shows improvements of about a factor of 2 in terms of noise reduction (i.e. less pronounced typical GRACE striping artefacts) and spatial resolution. The latter is displayed in the figure below, where cumulated degree variances of the models’ calibrated errors are shown. The level of mm-geoid accuracy has been improved from ~525km (RL04) to ~350km (RL05).

More details on RL05 can be found in the GFZ GRACE Level-2 Processing Standards Document for Level-2 Product Release 0005 or in the Release Notes for GFZ GRACE Level-2 Products - version RL05.

If any results based on the GFZ RL05 time series are published, users are kindly requested to cite the following reference:

Dahle, Christoph; Flechtner, Frank; Gruber, Christian; König, Daniel; König, Rolf; Michalak, Grzegorz; Neumayer, Karl-Hans (2012): GFZ GRACE Level-2 Processing Standards Document for Level-2 Product Release 0005, (Scientific Technical Report STR12/02 – Data, Revised Edition, January 2013), Potsdam, 21 p. DOI: 10.2312/GFZ.b103-1202-25

GRACE Products

The GRACE Level-0 to Level-2 products are processed and archived in a shared Science Data System (SDS) between the Jet Propulsion Laboratory (JPL), the University of Texas Center for Space Research (UTCSR) and the German Research Centre for Geosciences (GFZ).

All SDS Level-0 to Level-2 products are archived at JPL's Physical Oceanography Distributed Active Data Center (PODAAC) and at GFZ's Integrated System and Data Center (ISDC) where also useful documentation such as the GRACE Product Specification Document, the GRACE Level-1B Data Product User Handbook or the GRACE Level-2 Release Notes can be downloaded. Both archives are harmonized on a routine basis.

The Level-0 to Level-2 products are defined as follows:

Level-0:

The Level-0 data are the result of the data reception, collection and decommutation by the Raw Data Center (RDC) of the Mission Operation System (MOS) located in Neustrelitz/Germany. Using its Weilheim (WHM) and Neustrelitz (NST) tracking antennas, the MOS receives the science instrument and housekeeping data twice per day from each GRACE satellite which are stored in two appropriate files in the Level-0 rolling archive of the RDC. The SDS regularly retrieves these files and extracts and reformats the corresponding instrument and ancillary housekeeping data. Additionally, primarily for operational radio occultation data analysis, the raw data are also received during every pass at the GFZ polar Satellite Receiving Station (SRS) in Ny Alesund (NYA) and forwarded to GFZ in Potsdam.

Level-1:

The Level-1A data products are the result of a non-destructive processing applied to the Level-0 data. The sensor calibration factors are applied in order to convert the binary encoded measurements to engineering units. Where necessary, time tag integer second ambiguity is resolved and data are time tagged to the respective satellite receiver clock time. Editing and quality control flags are added, and the data is reformatted for further processing. The Level-1A data are reversible to Level-0, except for the bad data packets. This level also includes the ancillary data products needed for processing to the next data level.

The Level-1B data products are the result of a possibly destructive, or irreversible, processing applied to both the Level-1A and Level-0 data. The data are correctly time-tagged, and the data sample rate is reduced from the higher rates of the previous levels.

Collectively, the processing from Level-0 to Level-1B is called the Level-1 Processing. This level also includes the ancillary data products generated during this processing and the additional data needed for further processing (such as the Level-1B Atmosphere and Ocean De-aliasing Product (AOD1B).

Level-1 instrument data processing software is developed and operated by JPL.

Level-2:

Level-2 data include the short term (monthly and weekly) and static gravity field derived from calibrated and validated GRACE Level-1B data products. This level also includes ancillary data sets (e.g. mean atmospheric and oceanic mass variations) which are necessary to interpret time variability in gravity field solutions. The Level-2 processing software has been developed independently by all three processing centers. Routine processing is done at UTCSR and GFZ, while JPL is generating Level-2 products for verification purposes.

For alternative GRACE products (outside the SDS) please go to the “Links” page.

GRACE and GRACE-FO Related Publications

The links below lead to a compilation of GRACE and GRACE-FO related publications (no abstracts). 

This list is periodically updated, but may be incomplete. If you are missing a reference please contact Prof. Dr. Frank Flechtner or Dr. Volker Klemann.

Some GRACE-FO-relevant publications can be found also here.

GRACE Related Links

Products and related documentation of the GRACE Science Data System (SDS) can be found at the two GRACE archives:

Alternative GRACE gravity solutions are provided by the following processing centers: 

  • Mascon solutions of the Goddard Space Flight Center (GSFC)
  • 10-daily GRACE and LAGEOS combined models from the Groupe de Recherche de Geodesie Spatiale (GRGS)
  • Daily, monthly and static models of the Institute of Geodesy and Geoinformation (IGG) of the University Bonn
  • Monthly and static models of the Astronomical Institute (AIUB) of the University of Bern
  • Mass transport models of the Delft Institute for Earth-Oriented Space Research (DEOS)

Additional product help can be found at 

  • the International Centre for Global Earth Models (ICGEM) at GFZ which collects available gravity models, offers web based visualization of the gravity field models and their time variation as well as a service for calculating different functionals of the gravity field models,
  • JPL/Tellus which provides user-friendly data grids for the GRACE SDS gravity products (monthly and time-averaged), with additional post-processing, alone or in combination with other data, and with the most up-to-date corrections or
  • the University of Colorado which provides an interactive tool to display fully corrected, de-striped, monthly grids and hydrologic basin averages from GRACE.

Main GRACE Partners:

  • National Aeronautics and Space Administration (NASA)
  • Deutsches Zentrum für Luft- und Raumfahrt (DLR)
  • DLR German Space Operations Center (GSOC)
  • University of Texas Center for Space Research (UTCSR)
  • NASA Jet Propulsion Laboratory (JPL)
  • German Research Centre for Geosciences (GFZ)
  • Astrium GmbH
  • EUROCKOT Launch Service Provider
  • Industrieanlagen-Betriebsgesellschaft (IABG)

German GRACE related research programs:

GRACE Systems

The GRACE Project is divided into five systems. Each of the five systems is shortly described in the following paragraphs.

Launch Vehicle System (LVS)

The LVS included the ROCKOT launch vehicle, a multi-satellite dispenser, and the personnel, test equipment and facilities for preparation, integration and launch of the satellites. The LVS was managed by the DLR Launch Vehicle System Manager and supported by the JPL Project and its contractors. GRACE was launched on March 17, 2002. Read more about “GRACE Launch”

Satellite System (SAT)

JPL led the development of the Satellite System in partnership with Space Systems/Loral (SS/L) and Astrium GmbH (GmbH). Astrium provided major elements of two flight satellites based on an existing small satellite designed for the CHAMP mission. SS/L provided the attitude control system, microwave instrument electronics and system and environmental testing. Read more about “GRACE Satellites”

Science Instrument System (SIS)

The SIS included all elements of the inter-satellite ranging system, the GPS receivers required for precision orbit determination and occultation experiments, and associated sensors such as the Star Cameras. This System also coordinated the integration activities of all sensors, assuring their compatibility with each other and the satellite. Read more about “GRACE Payload”

Mission Operations System (MOS)

The MOS consists of facilities and resources of the German Space Operations Center, tracking antennas at Weilheim and Neustrelitz, and other stations and facilities needed for supporting LEOP (Launch and Early Mission Operation) and contingency operations. These facilities are used to monitor and control the satellite, perform initial processing of the telemetry data, and deliver all data to the SDS for further processing and generating science products. In addition to real-time operations, the MOS function provides the Central Checkout System for ground testing using command and data interfaces. The operations team also monitors satellite performance and health throughout the duration of the mission. Mission operations are conducted at the GSOC control center in Oberpaffenhofen, Germany. Read more about “GRACE Operations”

Science Data System (SDS)

The SDS functions include science data processing, distribution, archiving and product verification. The SDS is a distributed entity and managed in a cooperative approach by JPL and UTCSR in the US and GFZ in Germany. The cooperative approach includes sharing of processing tasks, harmonization of product archives and validation/comparison of products. Data and products to be processed and archived by the SDS include corrected inter-satellite range and accelerometer measurements, GPS orbit and occultation data, orbit, gravity field and GPS occultation products. The SDS also receives, processes and archives ancillary data (e.g. meteorological fields) necessary for data processing and verification. Further information is given in other pages such as “Products” or “Gravity Field Results”.

GRACE Ground Tracks

Based on the predicted orbit for GRACE-A the ground track over Europe is plotted (at 5s spacing) for each month of the GRACE mission to illustrate the ground track coverage. One should note that the actual coverage used for gravity recovery can differ significantly due to larger data gaps in the science instrument data occuring in the particular month. In this way the displayed plots give the "optimum" coverage possible.

GRACE Ground Tracks Plots Europe 2017

GRACE Ground Tracks Plots Europe 2016

GRACE Ground Tracks Plots Europe 2015