Development and Results of a Lasercom Testbed for the CLICK B/C CubeSats and Future Missions

The expansion of interest in small satellite constellation networks underscores the need for precise timing synchronization and reliable high-bandwidth communication between spacecraft. The CubeSat Laser Infrared CrosslinK (CLICK) mission is being developed by the Massachusetts Institute of Technology, the University of Florida, and NASA Ames Research Center. The first phase of the mission, CLICK A, was launched on July 14, 2022, aboard SpaceX’s CRS-25 and put into orbit from the International Space Station, where it successfully demonstrated the downlink to Earth. The second phase of the mission (CLICK B/C) will additionally demonstrate a crosslink between two 3U CubeSats (B and C) that each host a 1.5 U laser communication payload. The terminals will demonstrate full-duplex spacecraft-to-spacecraft communications and ranging capability using commercial-off-the-shelf components at low size, weight and power (SWaP). As part of the mission, CLICK will demonstrate two-way time transfer for chip-scale atomic clock (CSAC) synchronization and data transfer. This data transfer will use pulse-position modulation (PPM) at rates between 20 Mbps and 50 Mbps over separation distances ranging from 25 km to 580 km. A time-transfer precision of < 200 ps between the spacecraft is targeted. CLICK B/C is scheduled to launch in 2025. The University of Florida hosts a testbed to support CLICK developments. Its goal is to enable testing of the optical data- and timing-transfer chain on ground. This encompasses the vital components of the CLICK hardware for both TX (transmission) and RX (receiving). For TX, the electronics and laser system to generate optical pulses are included, with the latter consisting of a micro-integrable tunable laser assembly as seed laser and a semiconductor optical amplifier as shutter. In turn, the RX side consists of an avalanche photodetector (APD) to capture the pulses, electronics to condition and convert the analog signal into the digital domain (time-to-digital and analog-to-digital), and a field-programmable gate array as DSP (digital signal processing) platform. The DSP implements the algorithm to decode the PPM scheme and extract timing information. In between the optical TX and RX, an electrical variable optical gain amplifier is placed to simulate varying distances between satellites and the associated change in received power. The final setup is envisioned to use separate hardware platforms for TX and RX to test the timing transfer between independent CSACs. Here we present the status of the testbed and the associated development of CLICK hardware and DSP, in particular the APD and PPM decoder, along with results of the lasercom testing, showing initial tracking of test data.