Laser beam steering for GRACE Follow-On intersatellite interferometry.

Area of Science:

• Geodesy and Space Geodesy
• Optical Metrology
• Spacecraft Engineering

Background:

• The GRACE Follow-On mission utilizes a Laser Ranging Interferometer (LRI) for precise inter-satellite distance measurements.
• Maintaining a stable laser link between satellites is critical for inferring geoid fluctuations.
• Challenges include large spacecraft separation and significant attitude jitter.

Purpose of the Study:

• To investigate and demonstrate a novel beam steering method for inter-satellite laser ranging.
• To ensure the maintenance of a stable laser link for the GRACE Follow-On mission.
• To assess the applicability of this method to other inter-satellite laser ranging scenarios.

Main Methods:

• The study employs Differential Wavefront Sensing (DWS) to simultaneously coalign beams.
• A hexapod, a six-degree-of-freedom stage, is commanded using GRACE satellite attitude jitter data.
• Breadboard-level demonstrations validate the beam steering technique.

Main Results:

• The beam steering method achieves coalignment of better than 10 μrad.
• A stability of 10 μrad/√Hz is verified within the GRACE Follow-On measurement band (0.002–0.1 Hz).
• Key characteristics like DWS signals, heterodyne efficiency, and rotation-to-pathlength coupling suppression are analyzed.

Conclusions:

• The developed beam steering method is effective in maintaining stable laser links despite spacecraft jitter.
• This technique is vital for the success of the GRACE Follow-On mission's geoid measurements.
• The method shows promise for broader applications in inter-satellite laser ranging.