Future space based missions for gravitational wave research call for an improved inertial reference sensor with acceleration noise levels of fm/s2. Spherical test masses can enable increased performance by suspension-free operation, contrary to cuboid solutions suffering from cross-coupling of attitude control noise. However, interferometric readout is affected by surface irregularities and test mass tumbling [1]. An accurate surface map for compensation must be established for compensation, either by characterisation a priori or in flight, when optical path length changes due to surface occur in the measurement band. We demonstrate a method for generating a surface map of a spherical body with optical point sensors using a differential method to suppress common mode errors present, taking advantage of the excellent performance of heterodyne interferometry at sub-nanometer levels. A measurement setup is proposed in which two beams of a Nd:YAG Michelson interferometer are used to scan the surface, which is afterwards reconstructed from the differential measurement. Such a method could potentially benefit other research areas, such as the precise determinations of the Avogadro constant [2] or aspheric surface metrology.
With GRACE (launched 2002) and GOCE (launched 2009) two very successful missions to measure earth’s gravity field have been in orbit, both leading to a large number of publications. For a potential Next Generation Gravity Mission (NGGM) from ESA a satellite-to-satellite tracking (SST) scheme, similar to GRACE is under discussion, with a laser ranging interferometer instead of a Ka-Band link to enable much lower measurement noise. Of key importance for such a laser interferometer is a single frequency laser source with a linewidth <10 kHz and extremely low frequency noise down to 40 Hz / √Hz in the measurement frequency band of 0.1 mHz to 1 Hz, which is about one order of magnitude more demanding than LISA. On GRACE FO a laser ranging interferometer (LRI) will fly as a demonstrator. The LRI is a joint development between USA (JPL,NASA) and Germany(GFZ,DLR). In this collaboration the JPL contributions are the instrument electronics, the reference cavity and the single frequency laser, while STI as the German industry prime is responsible for the optical bench and the retroreflector. In preparation of NGGM an all European instrument development is the goal.
ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission and the American-German Gravity Recovery and Climate Experiment (GRACE) mission map the Earth’s gravity field and deliver valuable data for climate research.
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