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.