Localized distortion estimation and correction for the ICESat star trackers
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ICESat attitude determination is based on sensor data from three star trackers and a hemispherical resonator gyro. Methods for estimating and correcting star tracker distortion using on-orbit data are studied here. Two methods for modeling distortion, one based on an estimate of the star tracker attitude and the other independent of attitude, are described. Two methods for structuring the least squares estimation of distortion are also described, one based on a patchwork of smoothly joined local estimates and the other using a single global estimate. A new method is introduced for the iteration of attitude and distortion estimation when using the attitude dependent distortion model. Analysis of ICESat's star trackers demonstrates the effects of significant distortion in one of the three star trackers, relative to the other two. These effects include a qualitatively different distribution of residual errors (the differences between observed and computed direction vectors), small scale localized structures in the distortion caused by hardware errors such as bad pixel columns, and discontinuities in attitude estimation corresponding to stars entering and leaving the star tracker field of view. Finally, application of these methods to a conventional camera as part of the ICESat ground calibration and validation program is described in the appendix.