Towards Laser Ranging in Future Gravimetric Missions

The next generation of gravimetric satellite missions will likely  utilize a single laser-based instrument to track distance variations between the satellites in a pair. These variations are used to derive the monthly average of Earth’s gravity field. Mission studies and technology developments are ongoing at NASA/USA, DLR/Germany and at the European Space Agency (ESA) in order to advance the successful technology demonstrator aboard GRACE-FO, the Laser Ranging Instrument (LRI), to a primary instrument with appropriate redundancy. The new instruments should of course incorporate learned lessons from the development as well as in-orbit operation of the instrument on GRACE-FO.

The new generation of instruments is expected to have similar noise requirements as in GRACE-FO, since laser ranging observations are usually not limiting the monthly gravity field maps. Design changes in the future LRI are carefully assessed in order to ensure that the actual in-flight precision can reach the same level as in the LRI aboard GRACE-FO, which has shown at high frequencies a noise of 200 pm/√Hz, i.e. is able to resolve changes in the 200 km distance as small as single atoms over short time scales. Efforts focus in particular on an improved knowledge of the LRI scale factor, i.e. the absolute laser frequency, because the current approach of correlating KBR and LRI range can not be employed in future missions.

In this presentation we address the LRI technology and some of the trade-offs that have been performed in the design of future instruments in the context of the above studies. Moreover, we discuss the limiting performance aspects for tone errors and noise and summarize the learned lessons and their potential relevance for future missions.