Analyzing and monitoring GRACE-FO star camera performance in a changing environment

Patel, Chirag Rajnish
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The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission is an Earth-observing pair of satellites whose primary objective is to provide a time-variable map of the Earth's gravity field using inter-satellite ranging, and continue the legacy of the GRACE mission. The main observations that impact the final gravity field estimate are the range measurements, accelerometer readings of external non-gravitational forces, GPS position and velocity, and attitude observations. Precise knowledge of the attitude, obtained from post-processing of data from onboard attitude sensors, is an essential contributor to the mission. Attitude knowledge errors can introduce errors in both the inter-satellite range measurements due to inconsistent knowledge of the distance to the center-of-mass, and the accelerometer measurements due to inaccurate transformations from the spacecraft frame to the inertial frame. The primary attitude determination sensors are the three star cameras on board each GRACE-FO satellite, and errors in their measurements stem from star tracker performance, on-ground measurement of the alignments between them and the spacecraft, and how stable that alignment is over the course of the mission. This study characterizes each of those contributions to the overall attitude solution. Alignment especially is shown to be variable and long-term drift is observed. The star cameras also present measurement gaps and show degradation caused by the sun and moon blinding the field-of-view, whose effects on the single-camera attitude solutions are characterized over the short- and long-term. These effects can be mitigated by creating an attitude solution from multiple star cameras and by introducing angular rate measurements from gyros