The Optical to Orion Time of Flight Ground Processing System

Free space optical communication will increase data rates and science returns from deep space. The Optical to Orion (O2O) project will demonstrate these high-rate links on the Artemis-II mission, showcasing this technology for the first time on a crewed mission from cis-lunar space. This new paradigm of optical communication necessitates a re-evaluation of other core functionalities of remote spacecraft operation, including guidance, navigation and control. Existing RF-based ranging methods can exploit the structure of the communication signal in order to infer the range and range-rate of the spacecraft (known as synchronous two-way ranging), but optical communication utilizes a totally distinct communication signal format. The CCSDS is in the process of standardizing an optical-based method, which has its origins in the technique pioneered on the Lunar Laser Communication Demonstration (LLCD). A variant of this technique is used in O2O's Time of Flight (ToF) system, enabling highly accurate, real time ranging capabilities for the Artemis-II mission. In this paper we describe the ground signal processing implementation of O2O's synchronous two-way ranging scheme with centimeter-class accuracy. In contrast to the technique used in LLCD, the O2O Time of Flight system utilizes a hardware architecture based on a high dynamic-range Time to Digital Converter (TDC)-based receiver. We describe the architecture of the Time of Flight capture system, as well as the hardware and software necessary to extract range and range-rate information from the downlink and uplink signals. We also describe a novel calibration scheme that enables highly accurate compensation of the delays within the ground station without the explicit need to measure individual path lengths. We conclude the paper with simulation and experimental results validating the implementation.