Characterization of Two-way Time and Frequency Transfer for Free-space Optical Links Using a Scintillation Playback System

Owing to reduced RF-spectrum availability for current communication networks, greater attention has been given to free-space optical communication (FSOC) as an alternative technology to RF-based approaches for terrestrial and space-based communication applications. FSOC can support high link bandwidths and provide high data security without the spectral constraints associated with RF links. In addition to data communication, FSOC also provides a mechanism for two-way time and frequency transfer (TWTFT) in RF congested or disadvantaged environments for positioning, navigation, and timing (PNT) applications. A principle challenge for FSOC is atmospheric turbulence which causes both spatial and temporal disturbances that can have a significant impact on link performance. While single-mode-fiber coupled implementations in free-space have been used to achieve femtosecond timing precisions, these approaches can require adaptive optics (AO) and tight pointing tolerances which introduce operational complexity, and size, weight, and power (SWaP) burdens for practical FSOC. Furthermore, for many FSOC applications, femtosecond precision is not required. In this work, we present TWTFT performance of off-the-shelf time and frequency distribution hardware coupled to a custom optical-electrical-optical converter (OEO) interface based on large area, high sensitivity avalanche photodiodes (APD). Initial characterization of TWTFT performance using an NRL-developed scintillation playback system will also be discussed.