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

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.

On Orbit Performance of the GRACE Follow-On Laser Ranging Interferometer

The paper "On orbit performance of the GRACE Follow-On Laser Ranging Interferometer" discusses the operational capabilities and successes of the Laser Ranging Interferometer (LRI) aboard the GRACE Follow-On mission. The LRI represents a significant advancement in spacecraft range measurement technology, operating with autonomous control systems to lock laser frequencies and establish inter-spacecraft communications.

Overview

The GRACE Follow-On mission, a bilateral collaboration between US and German agencies, continues the legacy of the original GRACE mission by measuring variations in Earth's gravity field. The addition of the LRI serves both as a technological demonstrator and a precursor to methodologies intended for future missions such as the Laser Interferometer Space Antenna (LISA). The LRI's primary function is to measure inter-spacecraft distances with enhanced precision compared to the microwave approach employed by its predecessor.

Methodology

Key components of the LRI include a laser, optical cavity, laser ranging processor, optical bench electronics, optical bench assembly, and a triple mirror assembly. Built via international cooperation, these systems rely extensively on optical technologies to maintain the integrity and fidelity of range measurements. Utilization of Nd:YAG nonplanar ring oscillators facilitates stable laser operations. These are fortified by optical cavities that stabilize laser frequencies, limiting sensitivity and minimizing noise contamination.

The LRI's beam pointing capability is robust, capable of compensating for spacecraft attitude fluctuations through differential wavefront sensing and fast steering mirrors. The instrument maintains performance across a diverse range of frequencies, demonstrating noise levels substantially below its design threshold, with sensitivity factors capped primarily by laser frequency noise at higher registers.

Results

Experimental data collected from the LRI indicates sustained interaction between satellites, with operational periods extending beyond 50 consecutive days without interruption. The instrument's noise levels are reported as $10\,{\rm nm}/\sqrt{\rm Hz}$ at 40 mHz, with further reductions below at 1 Hz frequencies. Occasional discrepancies in phase tracking were noted, typically coinciding with thruster activities. These anomalies were resolved with no loss of data fidelity.

Initial range measurements align closely with those derived from the Microwave Instrument, confirming the reliability and comparative advantage of the LRI's approach.

Implications

The practical implication of the study underscores the LRI's suitability for high-precision geodetic missions. Furthermore, the successful demonstration of long-distance spacecraft interferometry provides critical insights for the upcoming LISA mission, particularly in the context of engineering robust inter-satellite communication systems that can function autonomously amid complex spatial orientations and relative motion.

Conclusion

The operational accomplishments of the LRI aboard the GRACE Follow-On mission solidify its role as an architect of future satellite-based gravitational studies. With promising noise performance and robust autonomous control systems, it offers a blueprint for the next generation of space-based interferometric instruments. The paper elaborates on the potential extensions of this technology in advancing geodetic science and improving our understanding of planetary gravitational dynamics.