Calibration of a Novel Three-Axis Fluxgate Gradiometer for Space Applications

The importance of magnetic cleanliness and inflight compensation to account for fluctuations in magnetic field is a recognized concern within the space industry. Their effects must be considered, minimized, and where necessary compensated throughout the development of any spacecraft, their subsystems, and while in flight. This is just as true within the context of the laser interferometer space antenna pathfinder mission, where sensitive gravitational wave telemetry systems are vulnerable to magnetic disturbances. In collaboration with the Science and Technology Facility Council, Rutherford Appleton Laboratory, Bartington Instruments, and the National Physical Laboratory (NPL), a compact low-noise gradiometer was developed under European Space Agency contract reference AO/1-6085/09/NL/AF. Such a versatile device will be used on the ground to assess the magnetic cleanliness of spacecraft and also inflight, to monitor changes in the local magnetic field. This paper introduces the capabilities developed by NPL, specifically for the calibration of the compact low-noise fluxgate gradiometer. The procedures used to test and calibrate the novel gradiometer will be discussed in detail, including a range of Helmholtz coils, gradient coils (used to generate known diagonal and off-diagonal gradients), and shielding techniques. These technologies were combined to establish key parameters, such as low frequency noise, offset, gain and gradient sensitivity, with specific attention given to the off-diagonal terms of the gradient tensor matrix.