Stellar aberration correction and thermoelastic compensation of Swarm μASC attitude observations: A comment to the Express Letter “Mysterious misalignments between geomagnetic and stellar reference frames seen in CHAMP and Swarm satellite measurements”, by Stefan Maus

The Swarm constellation of three satellites measures the magnetic signal of the Earth using both a Vector Field Magnetometer and an Absolute Scalar Magnetometer. A Micro Advanced Stellar Compass (μASC) mounted on a common, supposedly stable, optical bench precisely determines its inertial attitude. However, comparison of the Inter Boresight Angle shows a relative attitude variation between the μASC Camera Head Units. These misalignments between Camera Head Units and a geomagnetic reference frame cannot be explained by incorrect aberration correction (as theorized by Maus). Herceg et al. found them to be caused by thermal gradient sensitivity of the optical bench system, opposing the underlying assumption of perfect platform stability. The results after applying thermal corrections show significant decrease in root mean square, with Inter Boresight Angle of thermally corrected data being nearly flat and clean from any variation caused by thermoelastic effects.