Characterization of star tracker distortion for the ICESat mission




Wolfenbarger, Natalie Soheila

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Precision attitude determination is essential to the success of many spacecraft missions, particularly those in remote sensing where slight deviations in instrument pointing can yield high measurement errors, such as for the Ice, Cloud, and land Elevation Satellite (ICESat). Characterizing and understanding the nature of star tracker distortion is beneficial to the development of improved models for estimation and correction in data post-processing towards achieving arcsecond pointing accuracy for applications such as geolocation. Using a localized attitude dependent distortion estimation algorithm, star tracker distortion throughout the seven-year ICESat mission lifespan is analyzed to determine how the estimated distortion changes with respect to certain parameters of interest. These parameters include time, apparent motion of stars across the star tracker field of view, region of the celestial sphere observed, frequency and duration of star tracker blinding events, star tracker temperature, and star color. Distortion is estimated for each operational period and for the full mission. An increase in the estimated distortion of approximately half an arcsecond to one arcsecond was observed over the duration of the mission. Regions of high and low distortion were observed to shift depending on the direction stars entered and exited the field of view. The region of the celestial sphere observed was determined to be insignificant relative to the effects of star motion across the field of view. Artificially high distortion observations that followed periods of blinding were confirmed to be the source of the high distortion estimated for certain operational periods of the mission. Star tracker temperature was determined to have no effect on the estimated distortion. Chromatic aberration could not be definitively confirmed. Recommendations are provided for implementation of this method on future missions. CCD aging effects should be considered if a static distortion model yields distortion residuals with a temporal dependence. The incorporation of telemetry from additional star trackers to support attitude determination while the star tracker of interest is blinded would eliminate high distortion observations caused by poor attitude estimates after periods of star tracker blinding. Adjustment of the distortion basis function to include star color dependent coefficients would allow for direct estimation of chromatic aberration. Proper consideration of the results presented in this thesis will yield a more robust distortion estimation and correction process for future missions which require precision pointing.


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