# On orbit performance of the GRACE Follow-On Laser Ranging Interferometer

## TL;DR

The GRACE Follow-On Laser Ranging Interferometer successfully demonstrated continuous, high-precision laser range measurements between spacecraft, achieving noise levels significantly lower than microwave systems and maintaining stable operation over 50 days.

## Contribution

This paper reports the first successful laser interferometric range measurements between remote spacecraft in orbit, with autonomous control and active beam pointing, demonstrating improved noise performance.

## Key findings

- Continuous operation over 50 days without phase tracking interruptions
- Range measurement noise level of 1 nm/√Hz at frequencies above 100 mHz
- Biased range measurements comparable to microwave systems with lower noise

## Abstract

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 degree of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wavefront 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 $1\,{\rm nm}/\sqrt{\rm Hz}$ at Fourier frequencies above 100 mHz.