“A pair of satellites 220 km apart has demonstrated a record-high precision in distance determination, which will be useful for mapping out Earth’s gravitational field. The used for this measurements the laser interferometry LRI. The LRI has operated continuously without breaks in phase tracking for more than 50 days.”
Original research paper:
In-Orbit Performance of the GRACE Follow-on Laser Ranging Interferometer
Klaus Abich et al.
Phys. Rev. Lett. 123, 031101 – Published 19 July 2019
The Laser Ranging Interferometer (LRI) instrument on the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has provided the first laser interferometric range measurements between remote spacecraft, separated by approximately 220 km. Autonomous controls that lock the laser frequency to a cavity reference and establish the 5 degrees of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wave front sensing compensates spacecraft attitude fluctuations. The LRI has operated continuously without breaks in phase tracking for more than 50 days, and has shown biased range measurements similar to the primary ranging instrument based on microwaves, but with much less noise at a level of 1 nm/√Hz at Fourier frequencies above 100 mHz.