Performance characterization of the attitude control system for the GRACE mission
The Gravity Recovery and Climate Experiment (GRACE) mission is a breakthrough Earth science mission launched in the spring of 2002 that uses satellite-to-satellite tracking (SST) to map the Earth gravity field. In this framework, the non-uniform gravity distribution is inferred using the range change experienced between two satellites. The range change is measured using a microwave K-band ranging system, and non-gravitational forces are accounted for using accelerometer (ACC) data. The vector-offset between the satellite center of mass (CM) and the K-band phase center represents the correction between measured and modeled ranging data. In addition, the offset between the satellite CM and the ACC proof-mass multiplies the attitude angles, rates, and jitter which in turn add spurious signals to the ACC output. For both of these reasons, proper knowledge and control of attitude behavior is vital to successful mission performance. An examination of the GRACE attitude control system (ACS) is presented in this study.
The GRACE ACS system is composed of a PD control law, star camera sensing as the knowledge source, cold-gas thrusters as primary actuators, and magnetic torque rods as supplementary actuators. The dependencies inherent in the ACS are inferred using a sensitivity analysis performed on a simulation model of the GRACE science mode ACS. The results from this sensitivity study are applicable to the general controller class of which the GRACE ACS system is an exemplar.
In this study, the modeled attitude data quality is most sensitive to star camera measurement noise. It is hypothesized that this is because star cameras are used as the sole knowledge source in the ACS scheme. In contrast, the experimental results associated with magnetometer, thruster, and magnetic torque rod perturbations did not significantly affect attitude quality. However, these perturbations do cause thruster activity to significantly magnify. This results in higher attitude acceleration PSD for the frequency band in which time-variable gravity components are captured. A number of future experiments can be performed to improve both attitude quality performance and frequency-based magnifications. Examples include sensor fusion studies, reaction wheel versus thruster assessment, and gravity field estimation sensitivity in response to attitude quality degradation.