Optical Design of the Laser Ranging Interferometer on GRACE Follow-On

The GRACE Follow-On twin satellites were launched into space on May 22nd, 2018, in order to measure Earth’s gravity field, in particular monthly variations of it. The mission is a continuation of the original Gravity Recovery and Climate Experiment (GRACE; 2002-2017), which tracked variations in the ~200 km inter-satellite distance with a Microwave Ranging Instrument (MWI). These variations are used to recover gravity field information and corresponding mass transport on Earth, for instance, due to ice melting. The GRACE FO satellites are equipped with a MWI, but also with a novel Laser Ranging Interferometer (LRI). The LRI, developed by a US-German collaboration, is the first optical interferometer in space between orbiters. In this talk, we provide an overview on the LRI and focus on the optical subsystem. Although the optical layout of the LRI is composed of only ~10 components per satellite, it exhibits various appealing features. We report on simulations, development, and characterization of the LRI, on-ground and using in-flight data. The LRI is an important milestone towards the future space-based gravitational wave observatory LISA (Laser Interferometer Space Antenna).