The Gravity Recovery and Climate Experiment (GRACE) missions use inter-satellite interferometry to measure changes in the Earth’s local gravity. Comparing the measurements over time allows for an understanding of global mass transport systems, and particularly the movement of water around the Earth. This data contributes to approximately one quarter of the fifty-five ‘Essential Climate Variables’ tracked as part of the Global Climate Observing System and are key for ongoing environmental and climate monitoring. 

The next GRACE mission (GRACE-C) is expected to launch in 2028 and will require the addition of a new technique for long term absolute laser frequency knowledge to enable the use of a laser interferometer as the primary instrument. 

This talk will present the work undertaken to develop such a technique based on a dual frequency modulation scheme, which has now been baselined for inclusion on the mission. The technique has been taken from proof of concept to functional prototype unit and undergone thermal ambient testing as well as functional testing in NASA JPL’s flight testbed. High fidelity simulations have also been developed to model the parameter space of the modulation scheme. Finally, a discussion of the system limitations will be presented, highlighting areas that require careful design and consideration for the mission and where there are potential future improvements.