GRACE Follow On Project: Phase Locking explained 

Inter-spacecraft laser interferometers like LISA and GRACE interfere two lasers to form a beat note frequency at the difference frequency. By tracking the phase of this beat note we obtain knowledge of the change in separation between the satellites. When both optical beams are of sufficient optical power this beat note is a sine wave at the difference frequency of the two lasers. What happens though when we have very small amount of optical power in one of the beams? What is the minimum optical power that we can track? By stabilising our lasers, we are able to track incredibly low power beams reliably; We expect to be able to track beams in the range of a few femtoWatts. This technology enables new mission architectures for inter-spacecraft laser interferometers with very large separation.