Achieving Operational Two-Way Laser Acquisition for OPALS Payload on the International Space Station

The Optical PAyload for Lasercomm Science (OPALS) experiment was installed on the International Space Station (ISS) in April 2014. Developed as a technology demonstration, its objective was to experiment with space-to-ground optical communications transmissions from Low Earth Orbit. More than a dozen successful optical links were established between a Wrightwood, California-based ground telescope and the OPALS flight terminal from June 2014 to September 2014. Each transmission required precise bi-directional pointing to be maintained between the space-based transmitter and ground-based receiver. This was accomplished by acquiring and tracking a laser beacon signal transmitted from the ground telescope to the OPALS flight terminal on the ISS. OPALS demonstrated the ability to nominally acquire the beacon within three seconds at 25deg elevation and maintain lock within 140 μrad (3(sigma)) for the full 150-second transmission duration while slewing at rates up to 1deg/sec. Additional acquisition attempts in low elevation and weather-challenged conditions provided valuable insight on the optical link robustness under off-nominal operational conditions.