Design of a magnetic measurement system for small satellites

Abstract

ADCS magnetometers on board of satellites suffer from the disruptive magnetic field produced by the satellite itself. For CubeSats and PocketQubes this magnetic field is usually not quantified. If the largest magnetic field sources are known and if there is freedom in the magnetometer placement, the magnetometer is simply placed as far from the largest sources as possible. The magnetometer is not calibrated for the remaining error due to the satellite magnetic field. The research objective of this thesis is, ’to develop a measurement system which is capable of characterizing the satellite magnetic fields of CubeSats and PocketQubes close to the satellite body itself’. The measurement system is specifically designed for PocketQubes and is designed using the steps of the engineering design process. The design and manufacturing process was conducted separately for three subsystems; the magnetometers, the supporting structure and the data acquisition and processing. The measurement system features four magnetometers, which can be placed in various locations around the satellite with an accuracy of 0.6 mm. In the functional validation several magnetometer configurations were tested using the Delfi-PQ PocketQube as a test case. The functional validation showed that the magnetometers measure the magnetic field produced by a PocketQube with an uncertainty of 0.2 µT for each axis. The magnetic field of the test case, Delfi-PQ, ranges from approximately 5 µT close to the battery pack to 0.5 µT close to the bottom of the satellite. The measurement system can detect dynamic components of the field up to a limiting frequency of 18 Hz, this can in turn be used to trace magnetic noise contributions back to their source.
The uncertainty of 0.2 µT for each axis allows for calibration of on board ADCS magnetometers up to this uncertainty. For an ADCS magnetometer placed at the bottom of Delfi-PQ, this means a reduction in measurement uncertainty from 0.5 µT to 0.2 µT. This reduction would reduce the pointing error introduced by the satellite magnetic field disturbance and EMF model uncertainty from 0.6 to 0.3 degrees for the magnetic inclination and 1.5 to 0.7 degrees for the magnetic declination. The measurement system can also be used to select the optimal location for placing ADCS magnetometers, based on where the satellite magnetic field is the lowest and least dynamic.
The measurement system is not designed to conduct measurements on Delfi-PQ with the antennae deployed, this is a missed input requirement and a recommendation for future work. Additionally, the calibration of the magnetometers features a residual linearity error, the reduction of which is recommended as the subject of future work.