Near-magnetic-field scaling for verification of spacecraft equipment

Abstract. Magnetic-field measurements are essential to the success of many scientific space missions. Outside of the earth's magnetic field the biggest potential source of magnetic-field contamination of these measurements is emitted by the spacecraft. Spacecraft magnetic cleanliness is enforced through the application of strict ground verification requirements for spacecraft equipment and instruments. Due to increasingly strict AC magnetic-field requirements, many spacecraft units cannot be verified on the ground using existing techniques. These measurements must instead be taken close to the equipment under test (EUT) and then extrapolated. A traditional dipole power law of −3 (with a field fall-off proportional to r⁻³) cannot be applied at these close distances without risk of underestimating the field emitted by the EUT, but we demonstrate that a power law of −2 is too conservative. We propose a compromise that uses a power law of −2 up to a distance equal to 3 times the unit size, beyond which a dipole power law can be applied. When extrapolating from a distance of 0.20 m to 1.00 m from the centre of a 0.20 m wide EUT, we demonstrate that this method avoids an under prediction of the field, and is at least twice as accurate as performing the extrapolation with a fixed power law of −2.