Design, Test, Calibration, and Data Processing of the Vectorized Scalar Magnetometer Onboard COSMO

The Compact Spaceborne Magnetic Observatory (COSMO) CubeSat mission is a 6U CubeSat, orbiting in low-Earth orbit, which aims to provide measurements of the Earth’s magnetic field for the next-generation World Magnetic field Model (WMM). COSMO is equipped with two optical rubidium scalar magnetometers within a three-axis coil system. Modulation fields applied to the cube in all three axes enable extraction of the vector components of the Earth’s magnetic field. Currents of a few mA are applied to the cube to create modulation fields at designated frequencies. This vectorization approach is based on the triaxial modulation system developed for SWARM's Absolute Scalar Magnetometer as a technology demonstration. A series of tests were conducted on our vectorized magnetometer at the NASA Goddard Space Flight Center Magnetic Testing Facility, which includes a three-axis 42 ft Braunbek coil system that can zero out the Earth’s magnetic field and generate a designated magnetic field with high accuracy and precision in both magnitude and direction. Tests were conducted in this facility to assess and characterize the heading error, vector accuracy, linearity, and spacecraft biases, which enables the calibration of the in-flight COSMO data. Furthermore, two star trackers are included in the payload system, which provides attitude information for us to transfer from the instrument coordinate system to an Earth-centered coordinate system. The accuracy and precision of the star trackers play a role in the accuracy of the magnetic field direction. Therefore, tests have been conducted under the planetarium and night sky to assess the star tracker performance, and to measure the absolute alignment of the star tracker relative to the magnetometer.