ST-7 gravitational reference sensor: analysis of magnetic noise sources

A next generation gravitational reference sensor is being developed by Stanford University for the disturbance reduction system (DRS). The DRS will demonstrate the technology required for future gravity missions, including the planned LISA gravitational-wave observatory. The GRS consists of a freely floating test mass, a housing, sensing electrodes and associated electronics. Position measurements from the GRS are used to fly the spacecraft in a drag-free trajectory, where spacecraft position will be continuously adjusted to stay centred about the test mass, essentially flying in formation with it. Any departure of the test mass from a gravitational trajectory is characterized as acceleration noise, resulting from unwanted forces acting on the test mass. The GRS will have an inherent acceleration noise level more than four orders of magnitude lower than previously demonstrated in space. To achieve such a high level of performance, the interaction of the magnetized test mass with the magnetic fields produced by the spacecraft must be considered carefully. It is shown that a new noise source due to the interaction of the time-varying magnetic field gradient and the permanent dipole of the test mass must be added to the noise analysis. A simple current loop model shows that the design of the spacecraft and instrument electronics must be done with attention to the magnetic noise produced.