Observability of Star Tracker / Gyro Based Attitude Estimation Considering Time-Variant Sensor Misalignment

The fusion of measurements from star trackers and gyroscopes within optimal estimators/filters is a common approach for spacecraft attitude determination. For applications where filter tuning is not sufficient to account for unmodelled deterministic errors, state augmentation is often the method of choice. So also here, where the focus is on deterministic time-variant misalignment between star tracker and gyroscope unit as this often occurs in missions with repetitive ecplipse and sun phases. Based on the derived filter dynamics and measurement equations, an observability analysis is performed. Different practical cases are distinguished to analyze: (1) which filter states are observable, (2) which only in linear combination and (3) which not at all.