Home

Goddard Space Flight Center, Greenbelt, Maryland 20771

ENGINEERING COLLOQUIUM

Monday, March 29, 2004 / 3:30 PM, Building 3 Auditorium

Byron Tapley

"GRACE Mission: Status and Early Results"

ABSTRACT -- The Gravity Recovery and Climate Experiment (GRACE) is a dedicated satellite mission whose objective is to map the global gravity field with unprecedented accuracy over a spectral range from 500 km to 40,000 km. The measurement precision will support gravity field solutions in this frequency range that are between 10 and 1000 times better than our current knowledge. The mission profile calls for a gravity field solution with this accuracy every thirty days. Accurate measurements, with both high spatial and temporal resolution, will allow studies of the gravitational signals associated with the mass exchange between the Earths solid, ocean and atmospheric system components. The primary measurement provided by the High Accuracy Inter-satellite Ranging System (HAIRS) is the range change between two satellites orbiting, one behind the other, at an approximate distance of 200 km. The range change is measured with a precision better than 10 microns over a ten second averaging interval. A highly accurate three-axis accelerometer, located at the satellite mass center, is used to measure the surface force and attitude control induced accelerations. Satellite GPS receivers are used to position the satellites over the earth with centimeter level precision. With this set of measurements, GRACE will provide highly accurate measurements of the global gravity field once every thirty days. The two satellites were launched on March 17, 2002 and were designed to operate for a period of five years. The satellites orbit in coplanar nearly circular and polar orbits, at an altitude varying from 500 at launch to approximately 300 km, at mission end. The two satellites are separated by approximately 200 km along track. The mission is one of the first NASA Earth System Pathfinder Missions, and is implemented through a collaborative arrangement by NASA and DLR. This presentation will review the mission structure, describe the early satellite and instrument performance, the status of the data system development and describe some of the early analysis results.

SPEAKER -- Dr. Tapley, at The University of Texas at Austin, holds the Clare Cockrell Williams Centennial Chair in Engineering and has served as Director of the Center for Space Research since it was established in 1981. His research interests include orbit mechanics, nonlinear parameter estimation, and the uses of methods from these areas in satellite applications to geodesy, geodynamics and oceanography. A fundamental interest has been the development of the field of Precise Orbit Determination, which has been concerned with the issue of computing satellite orbits with accuracies that approach the centimeter level. He is involved in the analysis of satellite altimeter data and in the application of laser ranging and GPS tracking to the determination of the earth's gravity field, and its surface motions and solid body rotations. In 1960, Dr. Tapley received the Ph.D. in Engineering Mechanics from The University of Texas at Austin.

In 1998, he received the Doctor Honoris Causa from Delft University of Technology in The Netherlands. He is a member of the National Academy of Engineering, and he is a fellow member of AIAA, AGU and the American Association for the Advancement of Science. He is the recipient of the NASA Medal for Exceptional Scientific Achievement, the NASA Public Service Medal, the AAS Brouwer Award, the AIAA Mechanics and Control of Flight Award, and the AGU Charles A. Whitten Medal. He is the Principal investigator for the Gravity Research and Climate Experiment (GRACE); the first NASA Earth System Pathfinder to become operational. He is a registered professional engineer in the State of Texas.




Home