Downlink synchronization for the lunar laser communications demonstration

The Lunar Laser Communications Demonstration (LLCD) is an ongoing project to demonstrate a high-rate, bidirectional optical communications link between a lunar satellite and an Earth-based ground terminal. The optical downlink operates at data rates up to 622 Mbps and employs photon counting array receiver technology. While the use of a 16-PPM modulation format with narrow (200 ps) slots gives rise to challenging time-alignment requirements between the transmit and receive oscillators that are in constant orbital motion with respect to one another, the additional signaling bandwidth also offers the unique opportunity to provide high-resolution, centimeter-class ranging data between the satellite and the ground terminal. We discuss some of the link challenges that necessitated the creation of novel clock recovery techniques, outline the design and analysis of the header detection algorithms used to perform synchronization on the LLCD high-speed optical downlink, and present the hardware implementation and photon-counting communication testbed used to validate the design.