Static and Time-Variable Gravity Determination from GRACE Satellite Data

The Gravity Recovery and Climate Experiment (GRACE), is a twin satellite system launched in March 2002. GRACE is a joint partnership between the NASA in the United States and the Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR) in Germany. Dr. Byron Tapley of The University of Texas Center for Space Research (UTCSR) is the Principal Investigator (PI), and Dr. Christoph Reigber of the GeoForschungsZentrum (GFZ), Potsdam, is the Co-Principal Investigator (Co-PI). The GRACE satellites are the instrument that measures the Earth's gravity field with unprecedented accuracy.

The twin GRACE satellites were launched on March 17, 2002, into coplanar, circular, polar orbits (inclined at 89 degrees) at an initial height of 500 kilometers. The satellites themselves, which fly in the same orbit about 200 km apart, comprise the "instrumentation"; additional instruments aboard the satellites (accelerometers, GPS receivers, and star cameras) measure the nongravitational effects (e.g., atmospheric drag) and obtain accurate orbital positioning and spacecraft orientation information.

The main data come from a K-band microwave intersatellite ranging system. The two spacecraft experience the Earth's gravity field at different positions, so they are differentially accelerated or slowed as they pass over the Earth's massive mountain ranges or deep ocean trenches. The ranging system precisely measures distance changes between the twin satellites with time. Combined with other known measurements and models in a complex chain of data processing, the ranging measurements yield a map of the Earth's gravity field.

The most computationally intensive data processing for the GRACE system is carried out on the Longhorn (IBM Power 4) system at TACC, which enables the GRACE science team to calculate an accurate monthly model of the gravity field. They and their collaborators worldwide use this model in conjunction with climate and ocean models to learn about the world's water budgets, ocean currents, and climate change.