Triple mirror assembly in the GRACE Follow-On laser ranging interferometer

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/ <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msqrt> <a:mi>Hz</a:mi> </a:msqrt> </a:math> at 100 mHz and 200 pm/ <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msqrt> <c:mi>Hz</c:mi> </c:msqrt> </c:math> 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.