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 dragfree 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.

The Disturbance Reduction System (DRS) is designed to demonstrate technology required for future gravity missions, including the planned LISA gravitational-wave observatory, and for precision formation-flying missions. The DRS is based on a freely floating test mass contained within a spacecraft that shields the test mass from external forces. The spacecraft position will be continuously adjusted to stay centered about the test mass, essentially flying in formation with the test mass. 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 DRS goal is to demonstrate a level of acceleration noise more than four orders of magnitude lower than previously demonstrated in space. The DRS will consist of an instrument package and a set of microthrusters which will be attached to a suitable spacecraft. The instrument package will include two Gravitational Reference Sensors comprised of a test mass within a reference housing. The position of the test mass will be measured with respect to the reference housing by monitoring changes in capacitance between the electrodes on the reference housing, using the test mass as a virtual ground plane. The spacecraft position will be adjusted using colloidal microthrusters, which are miniature ion engines that provide continuous thrust with a range of 1-20 µN with resolution of 0.1 µN. The DRS will be launched in 2007 as part of the ESA SMART-2 spacecraft. The DRS is a project within NASA's New Millennium Program.

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