In-Orbit Calibration of Vectorized Rubidium Magnetometer onboard COSMO

The University of Colorado Boulder is scheduled to launch the Compact Spaceborne Magnetic Observatory (COSMO), a 6U CubeSat mission at LEO designed to provide high-resolution measurements of the Earth's magnetic field in support of the next-generation World Magnetic Model (WMM), in March 2026. The payload, known as the Vectorize Rubidium Magnetometer (VRuM), is designed to be less than 1U in size and consists of two optical rubidium scalar magnetometers integrated within a triaxial Braunbek coil system and two star trackers. The Braunbek coils are stimulated with modulation currents at distinct frequencies to generate modulation magnetic fields along each axis, allowing for the vector extraction of the Earth’s magnetic field. In combination with the optical scalars and the Braunbek coil system, the VRuM instrument can be self-calibrated and can also measure the vector magnetic field. After the commissioning phase, in-orbit calibrations and tests will be performed. The in-orbit calibrations include vector calibration, heading error calibration, spacecraft bias characterization, and mounting quaternion determination. The vector calibration aims to determine the non-orthogonality of the coils and the magnitude of modulation fields. The heading error calibration characterizes the heading error of the scalars due to the non-alignment between the measured magnetic field and the cell within the scalars. The spacecraft bias test is designed to determine the static magnetic field created by the small amounts of magnetic material around the payload; this bias field has been estimated on the ground as less than 10 nT. The mounting quaternion is a parameter that transfers the coordinates between the coil system and the star trackers. To obtain the most accurate attitude information, the Uncented Kalman filter is applied. This paper outlines the mission design, magnetometer vectorization technique, calibration methods, and plans for in-orbit calibrations in combination with first observations from space.