Study on center of mass calibration and K-brand ranging system calibration of the GRACE mission
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The twin Gravity Recovery and Climate Experiment (GRACE) satellites were successfully launched on March 17, 2002. The mission goal is to make significant improvement in current measurements of the Earth’s gravity field. The satellites are linked by a K-band ranging system, which measures the range change due to the gravitational and non-gravitational accelerations. The non-gravitational accelerations can be obtained by transforming the accelerometer measurements into the inertial frame of reference based on the star camera observations, and will be used to separate gravitational effects in the range changes. However, the accelerometer’s proof mass offset from the center of mass of the spacecraft must be minimized and the misalignment between star camera frame and accelerometer frame must be known accurately in order to reduce the accelerometer data error. In addition, the phase center of the K-band horn must be known to make antenna offset corrections to the range and range change data. The objective of the center of mass calibration is to determine the proof mass offset, and then, to use the center of mass trim assembly mechanism to eliminate this offset. The main purpose of the K-band ranging system calibration is to determine the phase center of the K-band antenna, which will be used to adjust the satellite attitude orientations and make the antenna offset corrections to the K-band ranging system phase measurements. Furthermore, this calibration allows the misalignment between star camera frame and accelerometer frame to be determined. The calibration maneuvers have been designed for the real mission. Estimation algorithms have been developed and complete simulations have been performed. Finally, the real calibration data have been processed. Analysis shows that the proof mass offset has been determined better than the requirement value of 0.1 mm and trimmed well below this value. The boresight error of the K-band horn’s phase center has been determined better than 0.3 mrad and the resultant antenna offset correction error of range and range rate will be much less than the system resolution (10[mu]m ,1[mu]m/ s) and the frame misalignment parameters have been determined better than (0.04o ,1[delta] ) . Overall, the goal of calibrations has been successfully achieved.