The US-German GRACE Follow-On Mission, launched in May 2018, maps the earth's gravitational field with a monthly resolution. The generated data can thus be used to track the redistribution of water on and below the earth's surface. GRACE Follow-On is the first mission to use a laser interferometer for the high-precision measurement of the inter-satellite distance - but only as a technology demonstrator. This instrument is called “Laser Ranging Interferometer” or LRI for short. The main science instrument is a microwave-based system, which was also used on the previous GRACE mission (2002 - 2017).
In the summer of 2020, a German study on a follow-up mission for GRACE Follow-On was carried out with the participation of the DLR Institute for Satellite Geodesy and Inertial Sensing. The title of this mission is "GRACE-I". It has the same main goal as GRACE and GRACE Follow-On: mapping of the earth's gravitational field. However, significantly higher spatial and temporal resolutions should be made possible. It was also proposed to use a laser interferometer based on the LRI as the main science instrument, completely replacing the microwave instrument. This step requires a new redundancy concept that guarantees a reliable operation of the instrument over the entire duration of the mission.
As part of the “GRACE-I Optical Bench” project, which is being carried out at the DLR Institute for Satellite Geodesy and Inertial Sensing, such a novel optical redundancy concept is being tested in the laboratory in the context of the GRACE-I mission. In addition, we are experimentally analyzing and testing the implementation of a new acquisition sensor that simplifies the establishment of the intersatellite laser link that spans hundreds of kilometers and makes this highly critical mission phase more reliable.