Optical Payload for Lasercomm Science

The flight system (depicted on the right) has three major parts, the sealed container, the optical gimbal transceiver, and the Flight Releasable Attachment Mechanism (FRAM).[10]

The sealed container houses the electronics, avionics, the communications laser, and a custom power board pressurized to 1 atmosphere with air to keep the electronics cool.[7][10] The laser uses a light wavelength of 1,550 nanometers with 2.5 watts of power[11][12] and has a 2.2 centimeter diameter aperture.[9][6] The laser was routed through fiber to the gimbal transceiver where it was transmitted with 1.5 milliradian beam divergence.[12]

OPALS under construction Image credit JPL/Caltech

The optical gimbal transceiver holds the uplink camera and the laser collimator on a 2-axis gimbal.[10] Due to laser safety considerations, the gimbal may not shine on anything on the ISS.[7] To avoid this, the gimbal is designed with mechanical stops and electromechanical limit switches so that its field of regard (the area where it can point) is limited to 36° wide in elevation and 106° in azimuth, where the azimuthal axis is generally in the direction of motion of the ISS.[7] Because of the gimbal field of regard geometry, the flight system can only perform downlinks when the ISS is north of the ground station.

Because of the fast-changing viewing geometry during passes, the direction in which the gimbal must point throughout the pass must be pre-computed.[13] The list of directions for the gimbal to point was calculated based on the ISS GPS state vector and attitude quaternion.[13] The need for this list to be accurate was very important because of error in ISS orientation predictions and because the gimbal lacked any encoders, so all gimbal movement had to be done through dead reckoning.[13] Once the flight system detects the beacon from the ground system, it tracks beacon with the gimbal.[13]

The FRAM is the interface between OPALS and the ISS.[10] It was not designed by the OPALS team, but was an existing part designed by the ISS team at Johnson Space Center.[14]

The ground system is what receives the signal from the flight system's laser downlinks.[7] Most frequently, the Optical Communications Telescope Laboratory (OCTL) in Wrightwood, California was used as the ground station, but other international stations were used as well. The observatory has a 1-meter mirror through which all of the laser downlinks are performed.[13] The telescope has the capability of tracking objects that are in Low Earth Orbit.[13] The ground system's function is to indicate to the flight system where to point the laser and then to receive that signal. It indicates where the laser must point by illuminating the ISS with a 976 nanometer laser.[7] Signal is received through a 3 nanometer bandpass 1550 nanometer spectral filter in front of an indium gallium arsenide acquisition camera and an avalanche photodiode detector, which keeps the receiver from being overwhelmed by sunlight backscattered by Earth's atmosphere during day passes.[13]