GRACE: Millimeters and Microns in Orbit

W. Bertiger, Y. Bar-Sever, S. Desai, C. Dunn, B. Haines, G. Kruizinga, D. Kuang, S. Nandi, L. Romans, M. Watkins, and S. Wu

Abstract:	The Gravity Recovery and Climate Experiment (GRACE) was launched March 17, 2002. The experiment, intended to improve models of the Earth’s gravity field, consists of two almost identical spacecraft, separated by approximately 200 km, in a near polar, near co-planar orbits, at about 500 km altitude. Each spacecraft carries four instruments, a GPS receiver, a K/Ka-band ranging system, and star camera (all integrated with a common processor), and a precision accelerometer. The GPS receiver can track up to 14 GPS with dual-frequency data quality comparable to precision geodetic ground receivers. The K/Ka-band ranging system can measure the range (with a bias ) to the micron level. The accelerometer has a precision of 1 nm/s2 and the star tracker measures attitude with a precision of 10 arcsec. The GPS data are processed to (1) contribute to the recovery of long wavelength gravity field, (2) remove errors due to long term on-board oscillator drift, and (3) align K/Ka-band measurements between the two spacecraft to 0.1 ns. Scale and bias parameters for the accelerometer are determined through a combination of GPS data and modeling. This paper will concentrate on the use of GPS for these timing and calibration functions and will not address the recovery of the gravity field. The timing functions of GPS are, of course, intimately connected with precision orbit determination. Orbit accuracies are better than 2 cm in each coordinate. Validation results are presented, that include GPS residuals, orbit overlaps, the K/Ka-band ranging, and Satellite Laser Ranging. All GPS data processing for orbit and clock parameters is accomplished by a data driven, automated system, designed for constellations of spacecraft carrying GPS receivers.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	2022 - 2029
Cite this article:	Bertiger, W., Bar-Sever, Y., Desai, S., Dunn, C., Haines, B., Kruizinga, G., Kuang, D., Nandi, S., Romans, L., Watkins, M., Wu, S., "GRACE: Millimeters and Microns in Orbit," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 2022-2029.
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