Abstract
The GRACE Follow-On mission will monitor fluctuations in Earth's geoid using, for the first time, a Laser Ranging Interferometer to measure intersatellite distance changes. We have investigated the coupling between spacecraft rotation and the intersatellite range measurement that is incurred due to manufacturing and assembly tolerances of the Triple Mirror Assembly (TMA), a precision retroreflector to ensure alignment between in- and outgoing laser beams. The three TMA mirror planes intersect in a virtual vertex to which satellite displacements are referenced. TMA manufacturing tolerances degrade this ideal vertex, however, a Point of Minimal Coupling (PMC) between spacecraft rotation and displacement exists. This paper presents the experimental location of the PMC under pitch and yaw rotations for a prototype TMA. Rotations are performed using a hexapod, while displacements are monitored with heterodyne laser interferometry to verify the PMC position. Additionally, the vertex of the three TMA mirror planes is measured using a Coordinate Measuring Machine and compared to the PMC position. In the pitch and yaw axes, the biggest deviation between TMA vertex and PMC was 50 ± 64 μm. Thus, within the measurement uncertainties, no difference between TMA vertex and PMC could be observed. This is a key piece of information for integration of the TMA into the spacecraft: It is sufficient to use the readily-available TMA vertex location to ensure minimal rotation-to-displacement coupling during the mission.
Highlights
Since launch in 2002, the Gravity Recovery and Climate Experiment (GRACE, see e.g. [1,2,3]) has been delivering valuable data about the spatial and temporal variations of Earth’s gravity field, proving the feasibility of low-orbit satellite-to-satellite tracking
We have investigated the coupling between spacecraft rotation and the intersatellite range measurement that is incurred due to manufacturing and assembly tolerances of the Triple Mirror Assembly (TMA), a precision retroreflector to ensure alignment between in- and outgoing laser beams
The second retroreflector is selected as a Ball-Mounted Retroreflector (BMR), where its vertex lies at the center of a spherical housing which may be measured by the Coordinate Measuring Machine (CMM)
Summary
Since launch in 2002, the Gravity Recovery and Climate Experiment (GRACE, see e.g. [1,2,3]) has been delivering valuable data about the spatial and temporal variations of Earth’s gravity field, proving the feasibility of low-orbit satellite-to-satellite tracking. For the TMA flight model, the misalignment of each dihedral angle between the TMA mirrors is required to be smaller than 10 μrad Under these conditions, analysis shows that the PMC shall differ from the vertex by less than 5 μm in each axis. Placement along the spacecraft roll axis is less critical, coupling rotations quadratically into the range measurement, a looser placement requirement of ±10 mm is permitted These tolerances are compatible with using knowledge of the TMA vertex position (instead of the PMC position) for integration of the TMA into the spacecraft. The TMA under test was designed and fabricated by a consortium led by the Centre of Gravitational Physics at The Australian National University (ANU) in the framework of a TMA prototype study for future geodesy missions (see Fig. 2) It consists of three mirrors mounted perpendicular with respect to each other on a rigid ceramic spacer bar. Procedure and properties of this TMA are similar to another TMA prototype fabricated during the same TMA prototype study using a Carbon-Fiber-Reinforced Polymer (CFRP) tube as spacer between the mirror mounts [16–18]
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