GRACE | Gravity Recovery and Climate Experiment 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.
GRACE Launch and Orbit Decay
Both GRACE satellites were launched simultaneously with a ROCKOT launch vehicle from Plesetsk/Russia on March 17, 2002. The ROCKOT launch vehicle services are provided by EUROCKOT Launch Services GmbH, a joint venture company of DaimlerChrysler Aerospace and Khrunichev State Research and Production Space Center. ROCKOT is a three-stage launch vehicle, comprising an adaption of the SS-19 for the first and second stages, and the maneuverable and re-ignitable BREEZE third stage. With a launch mass of 107 tonnes, ROCKOT can place up to 1900 kg payload into a low Earth orbit.
The GRACE orbit is almost polar (i=89.0°) and near circular (e<0.005) and started with an initial altitude of 500 km. The anticipated lifetime for scientific operations was 5 years. The near-polar orbit was chosen to get a homogeneous and global coverage of the Earth's sphere which is necessary for a precise estimate of the gravitational geopotential. The advantage of the 89.0° orbit vs. a dawn-dusk sun-synchronous orbit is the local time variation of the satellite's ground tracks which is essential for the separation of constituents of periodic phenomena like tides or day-night variations of the atmosphere. An initial altitude of 500 km was chosen (a) in order to guarantee multi-year mission duration even under severe solar activity conditions and (b) to get a good compromise between gravity field solutions, which desire an even lower altitude, and atmospheric/ionospheric applications, which in contrast would benefit from a higher altitude.
Due to atmospheric drag the altitude of the GRACE satellites decrease over the mission duration. As GRACE has been launched around the last solar activity maximum, the predicted natural decay depends on the magnitude of the actual solar activity cycle and has amounted to about 34 km (status August 2010).
During science data collection the satellites are nominally Earth oriented. For precise pointing of the two K-band ranging system antennae both satellites are pitched by about -1°. This causes drag force differences which result in along-track separation changes. Therefore regular station-keeping maneuvers are carried out (1-2 per year) to keep the two satellites at their nominal separation of 220 ± 50 km. To ensure the uniform exposure and aging of the K-band antennae the leading and the trailing satellite have been exchanged in December 2005.
GRACE Satellites
The two GRACE satellites were developed by Astrium GmbH and Space Systems/Loral (SSL) . Astrium GmbH has build - under contract by JPL - major elements of the two flight satellites using heritage from the CHAMP mission. SSL provides the attitude control system, microwave instrument electronics and system and environmental testing.
Both satellites are completely identical and differ only in the S-band radio frequencies used for communication with the ground and in the K-band frequencies used for the inter-satellite link. The satellite design features a simple and robust structure design, mainly based on Carbon Fiber Reinforced Plastic (CFRP) sandwich panels with aluminum core and edge profiles for low thermal distortion. The outer shape of the satellite is optimized with respect to its aerodynamic behavior. This requires a symmetrical form with the center of pressure always in a single plane. Because the spacecraft center of mass is on the same level as the center of pressure, the disturbances due to air drag and solar pressure are minimized.
All electrical units, the harness, the GN2 tanks, as well as the piping to the thrusters are located on both sides of the central equipment panel. The two main solar arrays are symmetrically canted to the equipment panel with two additional solar array panels on the satellite roof. The satellite body is closed at its front and aft side by sandwich panels of the same composite as the solar array panels. The aft panel carries the occultation GPS antenna, while the front panel has the cut-out for the Ku/Ka-band horn. The nadir and zenith S-band antennae are mounted on brackets to minimize gain disturbance.
The following figure describes the physical layout of and provides a view inside the GRACE satellites.
After successful instrument integration, which has been finished in March 2000, both satellites were shipped to IABG for environmental testing.
The Attitude and Orbit Control (AOCS) System is composed of the necessary sensors, actuators, electronics, and software to provide adequate knowledge of spacecraft attitude during all phases of the mission, to generate on-board signals to accurately maintain spacecraft attitude, and to provide necessary orbital control to satisfy the GRACE mission requirements. The AOCS is comprised of the following elements:
- a cold gas propulsion system for attitude control and orbit change maneuvers,
- a set of three magnetic torque rods for attitude control in support of the cold gas system,
- interfaces to star sensors providing the inertial attitude,
- interfaces to a GPS receiver that will provide on-board orbital position,
- a course Earth -sun sensor to provide attitude measurements with respect to Earth and sun,
- a three-axis Inertial Reference Unit used to measure angular rates,
- a three-axis magnetometer mounted in the S-band antenna boom and
- the AOCS flight software.
The GRACE Payload
The key science instrument for GRACE is the JPL K-Band Ranging (KBR) Instrument Assembly. Its components include the Ultra Stable Oscillator (USO), the microwave assembly, the horn, and the Instruments Processing Unit (IPU). The JHU/APL USO serves as the frequency reference. The microwave assembly is used for up-converting the reference frequency to 24 and 32 GHz; down-converting the received phase from the other satellite; and for amplifying and mixing the received and the reference carrier phase. The horn is used to transmit and receive the carrier phase between the satellites. The IPU is used for sampling and digital signal processing of not only the K-Band carrier phase signal, but also the signals received by the GPS antenna and the star cameras. Each satellite transmits carrier phase to the other at two frequencies, allowing for ionospheric corrections. The transmit and receive frequencies are offset from each other by 0.5 MHz in the 24 GHz channel, and by 0.67 MHz in the 32 GHz channel. This shifts the down-converted signal away from DC, enabling more accurate measurements of the phase. The 10 Hz samples of phase change at the two frequencies are down-linked to the ground from each satellite, where the appropriately decimated linear combination of the sum of the phase measurements at each frequency gives an ionosphere-corrected measurement of the range change between the satellites.
The non-gravitational accelerations acting on the satellite are measured using the ONERA SuperSTAR Accelerometer (ACC), mounted at the CG of each satellite. The ACC consists of a sensor unit (SU), electromagnetic exciting unit (EEU), an interface control unit (ICU) and a harness. The SU consists of a metallic proof mass, suspended inside an electrode cage of gold-coated silica. The proof mass motion is servo controlled using capacitive sensors, and is a measure of the non-gravitational accelerations acting on the satellite. The mass and electrode cage core is enclosed by a sole plate and housing in which vacuum is maintained using a getter. The SU vacuum unit is surrounded by analog electronics. The EEU is used to deliver a 10 mg acceleration, and is used only in case of an SU start-up problem. The ICU supplies power to the SU and EEU, and operates the accelerometer through a micro-controller board.
The orientation of the satellite is sensed using two DTU Star Camera Assemblies (SCA), with a field of view of 18° by 16°. These are rigidly attached to the accelerometer, and view the sky at 45° angle with respect to the zenith, on the port and starboard sides.
The GPS signals for navigation and occultation applications are received using three antennas and the JPL Black Jack GPS Receiver. The main zenith crossed dipole antenna is used to collect the navigation data. In addition, a backup crossed dipole antenna and one helix antenna on the aft panel are used for back-up navigation and atmospheric occultation data collection, respectively. This system is capable of simultaneously tracking up to 24 dual frequency signals. In addition, this system provides digital signal processing functions for the KBR and SCA instruments as well.
The Laser Corner-Cube Retro-Reflector (LRR) has been provided by the German Research Centre for Geosciences (GFZ) and is mounted on the bottom side of the spacecraft to permit the orbit verification activities from the terrestrial Laser tracking network.
GRACE Mission Operations
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
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.
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.
GRACE Systems
The GRACE Project was 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 LVS Manager and supported by JPL 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 (Astrium). 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 precise orbit determination and radio occultation experiments, and associated sensors such as the two star cameras. Within the SIS also necessary integration activities of all sensors were coordinated, assuring their compatibility with each other and the satellites. Read more about "GRACE Payload".
Mission Operations System (MOS)
The MOS consisted of facilities and resources of the German Space Operations Center (GSOC) in Oberpfaffenhofen, 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 were used to monitor and control the satellites and instruments, perform initial processing of the telemetry data, and delivered all these data to the Science Data System for further processing and generation of science products. In addition to real-time operations, the MOS function provided the Central Checkout System for ground testing using command and data interfaces. Read more about “GRACE Operations”.
Science Data System (SDS)
The SDS functions included science data processing, distribution, archiving and product verification. The SDS was a distributed entity managed in a cooperative approach by JPL (Jet Propulsion Laboratory) and UTCSR (University of Texas, Center of Space Research) in the US and GFZ in Germany. The cooperative approach included sharing of processing tasks, harmonization of product archives and validation/comparison of products. Data and products to be processed and archived by the SDS included, e.g., corrected inter-satellite range and accelerometer measurements, GPS orbit and occultation data, or gravity field products. The SDS also received, processed and archived ancillary data (e.g. meteorological fields) necessary for data processing and verification. Further information is provided on 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
