SOFIE jitter analysis

SOFIE (Solar Occultation for Ice Experiment) is a 16-channel radiometer that was launched into a polar orbit on NASA's Aeronomy of Ice in the Mesosphere (AIM) spacecraft on 25 April 2007. An in-depth jitter analysis was performed to verify that the spacecraft could meet the pointing requirements. The analysis was based on an integrated modeling capability which combines structural dynamics with dynamic ray tracing to determine the motion of the boresight on the focal plane array (FPA) in the presence of disturbances. Two approaches were used and compared: a frequency-based analysis and a time-based analysis. For the frequency approach, the spacecraft provider determined the peak amplitude of the disturbance motions within 10% of each SOFIE modal frequency. The transmissibility factor Q between disturbance motion input and boresight motion output was determined for each degree of freedom and modal frequency. The disturbance amplitudes were then multiplied by each Q and summed over all frequencies and degrees of freedom. For the time-based analysis, the disturbance time histories were applied directly to the integrated model to generate the motions of the boresight ray on the FPA. The resulting motions were input to the sun sensor simulation to determine if the sun tracking algorithm could stay in fine track mode, or lose lock and jump to coarse track mode. As expected, the jitter from the frequency-based analysis was worse than the time-based analysis due to the implied assumption that the disturbance frequencies lined up exactly with the modal frequencies. Even so, the worst-case result met the requirement of 35 arcsec peak-peak jitter. The sun sensor simulation showed that the algorithm would still remain in fine-track mode and not lose lock even under the worst-case condition. Actual on-orbit data is presented that verifies the validity of the analysis.