Abstract
An attitude reconstruction algorithm has been developed and tested for the ALEXIS spacecraft, whose dynamics include significant thermal/flexible motions due to structural damage that occurred during launch. The ALEXIS science mission needs the reconstructed attitude to do off-line mapping of soft x-ray photons from telescopes' image planes aboard the spinning spacecraft into inertial coordinates. The attitude reconstruction algorithm takes inputs from a fine sun sensor and a horizon crossing indicator and processes them using a square-root-information filter/smoother. The filter/ smoother model includes Euler dynamics and randomwalk torques to approximate thermal/flexible effects. The launch damage left a solar array paddle free to flap under the influence of thermal inputs, which causes spin rate and direction changes. Despite the complexity and uncertainty of the dynamic model, the algorithm achieves a 0.25° 1-cr accuracy over 30-min. periods of orbit night, when attitude data is unavailable.
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