Reliable Distance Scaling of AC Magnetic Fields for Space Mission Verification Campaigns

Present and future space missions place ever more stringent requirements on the emission of AC magnetic fields from space hardware, from individual equipment to spacecraft level. In order to ensure that these missions will meet their target sensitivity, test methods are required that allow the verification of these requirements on ground, prior to launch. Because the typical ground environment is noisier than the space environment, a classical approach is to characterise a unit under test at close distances, where the signal can be measured above the environmental noise, and then to extrapolate to the actual distance of interest. However, at close distance, the field complexity can increase considerably, and the reliable distance scaling of magnetic fields is a significant challenge. This spatial complexity applies already for static fields (DC). For time-varying fields (AC), a further complication is that the field oscillates at different positions in space in different directions, and the temporal and spatial link between the near-field measurement and extrapolated positions must be correctly covered. In the frame of a research and development activity in ESA's Technology Research Programme, we have developed and validated a new test method, focusing on magnetic fields of frequencies up to 50 kHz. This method is based on the spherical multipole expansion of magnetic fields, and on the use of an array of magnetometers to map the field around the unit under test. The decomposition of the fields into multipolar orders automatically provides the correct distance scaling law. In addition, the ability to distinguish sources enclosed by the sensor array (the unit under test) from sources outside (the environment) enables the efficient suppression of environmental noise. The activity covered all stages: from the initial requirement review, through the development of extensive analysis tools and test setup design, to hardware selection and procurement, programming of a data processing software library, to manufacturing of support equipment and eventual validation by test with flight representative hardware. We successfully scaled the AC magnetic emissions from real units, measured at close distance, to larger distances. The agreement between theoretical predictions and experimental observations at the larger distance was excellent. For one unit, a reaction wheel, we accurately extrapolated AC magnetic fields of up to octupolar order (these fields scale with distance as 1/R5). Our test method sets a new standard, as far as reliable AC magnetic field distance scaling is concerned, and a measure of its success is that it is already in use for magnetic AC characterisation of units for the JUICE spacecraft. JUICE is the first large-class mission in ESA's Cosmic Vision 2015–2025 programme and is planned to launch in 2022.