Towards understanding the measurements of the GRACE FO electrostatic accelerometers

Some of the most prominent anomalies and open questions concerning the measurements of the ultrasensitive accelerometers on both GRACE FO spacecraft are related to the activation of attitude thrusters. As a consequence, measurements around thruster activations are replaced by modeled data for the GFO1 accelerometer, while the complete measurements are replaced by modeled or hybrid data for GFO2. Motivated by this unsatisfactory situation, we present results of a detailed study on the characterization of the accelerometer response to thruster impulses, for both the GFO1 and GFO2 instruments. The study describes the impulse response in angular accelerations (for the first time), in linear accelerations as well as in the six electrode control voltages of each instrument. For GFO1, we find a realistic impulse response for some thruster pairs. For other pairs, parts of the impulse response are obscured by switch effects. For GFO2, we see similar features as for GFO1, which are, however, superimposed by heavy jumps and drifts that are triggered by the activation. We demonstrate how the stacking of measurements from many events allows to distinguish relevant details in the impulse response and their changes over time. We expect that the results will help to constrain the sources of the anomalies and lead to a better understanding of the motion and control of the accelerometer test masses.