General Description

GRACE is 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 GeoForschungsZentrum Potsdam (GFZ), the Jet Propulsion Laboratories (JPL), Space Systems/Loral (SSL), the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), and Astrium GmbH. GRACE was selected in 1997 as second mission in NASA's Earth System Science Pathfinder project (ESSP). 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 is Prof. Ch. Reigber of the GeoForschungsZentrum Potsdam.

The primary objective of the GRACE mission is to provide with unprecedented accuracy estimates of the global high-resolution models of the Earth's gravity field for a period of up to five years. The temporal sequence of gravity field estimates will yield the mean Earth gravity field, as well as a time history of its variability. An secondary science objective of the GRACE mission is to provide several hundred globally distributed profiles each day of excess delay or bending angle of the GPS measurements due to the ionosphere and atmosphere using limb sounding. These can be converted to total electron content and/or refractivity in the ionosphere and troposphere,  respectively.

GRACE will succeed the  CHAMP  mission in the area of Earth gravity field measurements.  The anticipated increase in accuracy will be achieved by utilizing two satellites following each other on the same orbital track. These satellites are interconnected by a K-band microwave link to measure the exact separation distance and its rate of change to an accuracy of better than 1 µm/s.  To consider precise attitude and non-gravitational forces both satellites will be equiped with star cameras and accelerometers. The position and velocity of the satellites will be measured using onboard GPS antennae. The International Laser Ranging Service (ILRS at http://ilrs.gsfc.nasa.gov) provides tracking from its global network of laser ranging stations to support the project.

GRACE configuration

The GRACE project is organized such that the responsibilities are divided between the members of the GRACE team. The corresponding organization chart is shown in the following figure. Here the principal US/German implementation elements and the assignment of these elements to members of the management team are identified.
 
 

GRACE project organization during phase C/D

Corresponding to above figure, the GRACE project is divided into five major systems:
 

1. Satellite System (SAT) developed by Astrium GmbH and Space Systems/Loral (SSL). Astrium will build - in contract of 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.
2. Science Instrument System (SIS) including all elements of the inter-satellite microwave tracking system, the GPS receivers required for orbit determination and occultation experiments, the accelerometer used for non-gravitational force measurement and the star camera assembly needed for precise attitude information. Additionally GFZ will provide a Laser retro-reflector for each satellite.
3. Science Data System (SDS) which is a distributed entity and managed in a cooperative approach by JPL, UTCSR and GFZ. This includes sharing of processing tasks, harmonization of product archives and validation/calibration of products.
4. Launch Vehicle System (LVS) including the ROCKOT launch vehcle , the multi-satellite dispenser, and personnel, test equipment and facilities for preparation, integration and launch of the satellites. The LVS is managed by DLR and supported by JPL and its contractors.
5. Mission Operation System (MOS) consisting of facilities and resources of the German Space Operations Center (GSOC), tracking antennae in Weilheim and Neustrelitz and other stations needed for supporting LEOP and contingency operations. These facilities are used to monitor and control the satellite, reception and short-term archiving of the telemetry data and deliver all data to the SDS for further product generation. In addition to real-time operations, the MOS function provides the Central Checkout System (CCS) for for ground testing using command and data interfaces. The operations team at GSOC operations center also monitors satellite performance and health throughout the mission. The following figure gives an overview about the MOS:

MOS Overview