GRACE Follow-On LRI measurements of linear accelerations produced by attitude control thrust firings

Abstract

The attitude of the GRACE Follow-On satellites is controlled by six pairs of attitude control thrusters.  An ideal thruster system would produce a change only in angular momentum.  The imperfect system, however, also produces undesired linear accelerations that must be removed for orbit analysis and gravity recovery.  On-orbit calibration is performed to determine the linear acceleration response to each of the six thrust events for both satellites.  These thrust calibration models are applied to the ACC1B data product and its variants.  As is standard for L1B data products, a CRN low-pass filter is applied with a cutoff frequency of 35 mHz for the case of the ACC1B datasets.  This cutoff frequency was chosen for orbit and gravity analysis using KBR1B from the beginning of the GRACE mission, considering the balance between dampening  gravitational signals and increasing KBR thermal noises with frequency.  We found spurious acceleration signals, most clearly visible in the form of range acceleration computed from LRI1B data and a prefit dynamic orbit.  These spurious accelerations were in fact LRI measurements of thrust events — all six cases — appearing in a frequency band well above 35 mHz.  We discuss how the LRI measures thruster firings, how we model and remove them, and why this modeling matters for global gravity recovery.