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With the latest CHAMP star camera software update the instrument is now able to perform in-flight the astronomical aberration correction.
The relativistic effect stems from the movement of the observer (star camera on spacecraft) to the apparent position of a star on the firmament. The velocity of the Earth is ~29.8 km/sec which needs to be considered to correct for the annual aberration, and by knowing the satellite velocity (CHAMP: ~7.6 km/sec) from GPS information it is possible to correct for the orbital aberration.
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The above 2 plots depict the inter-boresight angle, IBA, between the two boom-mounted camera heads versus the satellite latitude (declination) on the left and the IBA as time series on the right. The missing aberration correction causes an apparent deflection of the camera heads of up to ±40 arcsec. As a consequence the IBA varies elliptically over an orbit.
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The same kind of plots as above but for a time period where the in-flight aberration correction algorithm was enabled. Now the IBA is constant over an orbit. The flatness of the IBA versus declination demonstrates that there is no deformation of star tracker assembly due to thermal stresses from sun light and eclipses.
The figures on the left are superpositions of 4 consecutive orbits. The reproducibility is remarkable. Features re-appear at the same position along the orbit. The raw data scatter within a band of only 15 arcsec peak-peak. Postprocessing � merging the readings of both heads � will further reduce the noise to a few arcsec.