Coupling of Pointing Angles and Angular Rates in the GRACE Follow-On Laser Ranging Interferometer

The Laser Ranging Interferometer (LRI) of the GRACE Follow-On (GFO) mission has successfully shown its capability of continuously measuring the inter-satellite biased range with higher precision than the established microwave ranging system. The instrument behaviour is already well understood. For Fourier frequencies below 30 mHz, the largest error source of the LRI is the so-called tilt-to-length (TTL) coupling, which means that satellite pointing jitter couples into the measured range. We have modelled the TTL error and estimated the model parameters using satellite rotation maneuvers, the so-called center-of-mass calibration (CMCal) maneuvers.

We report here that not only the pointing angles (roll, pitch, yaw) couple into the LRI range, but also the rate of change of one of the angles, namely the yaw angle of the main S/C. We give a theoretical model, which predicts this effect qualitatively and quantitatively. Based on a combined model for TTL and yaw rate coupling, we have re-analyzed the CMCal maneuvers, the results of which we present here.

From the TTL coupling parameters, one can derive nadir and cross-track components of the center-of-mass (CM) positions with respect to the LRI reference point. These will also be shown here, and we can conclude that the LRI is capable of providing accurate tracking of CM movement over time.