Precision optics for global climate change monitoring
The design, testing, and in‑orbit performance analysis of the GRACE Follow-On triple mirror assembly
The GRACE Follow-On mission consists of two satellites orbiting Earth and measuring nanometer changes in their relative distance of around 200 kilometers. These distance changes are detected with technology initially developed for gravitational-wave science. They are used to infer indicators of global warming, such as melting polar ice caps, changing groundwater levels, and global mean sea level rise. This is done by sending laser beams back and forth between the two satellites. Due to the satellite topology, the incoming and outgoing beams must be offset by 60 cm from one another. Therefore, a special assembly of three mirrors that act as part of a large corner reflector is required. A recent study published in Physical Review Applied describes the design, manufacturing, and characterization of the triple mirror assembly, both on ground and in orbit. Researchers at the Max Planck Institute for Gravitational Physics have performed pre-analysis, experiments, and verification of the flight hardware built by SpaceTech Immenstaad, as well as analysis of its in-flight behavior.
Paper abstract
The Gravity Recovery And Climate Experiment (GRACE) Follow-On mission was launched on May 22, 2018, to continue monitoring changes in the gravity field of the Earth by measuring distance variations between two spacecraft that fly 200 km apart in a low-Earth polar orbit. The laser ranging interferometer (LRI), a technology demonstrator onboard GRACE Follow-On, is the first of its kind to perform interspacecraft ranging measurements and has shown noise levels of 1 nm/√Hz at 100 mHz and 200 pm/√Hz at 5 Hz. Its development was shared between parties in Germany and the United States. A key optical component for the LRI’s success is the triple mirror assembly (TMA), which acts as a corner-cube retroreflector and enables the laser link between the two spacecraft. This paper presents the TMA design and characterization from the unit level to measurements in orbit. The in-orbit measurements furthermore provide the far-field intensity distribution of the Gaussian beams exchanged between the spacecraft after traveling 200 km. We address lessons learned that have influenced the design of the next generations of the LRI.
