Ground optical communication array terminal (GOCArT)

The Ground Optical Communication Array Terminal (GOCArT) is designed as a scalable-aperture, deployable ground terminal with the following innovations under investigation 1) telescope array transmitter with Lateral Transfer Retroreflectors for boresight auto-alignment, 2) low-excess-noise high-sensitivity avalanche photodiode (APD) 2.5 Gbpscapable detectors 3) large-area 2.5 Gbps-capable APD detectors 4) “fly’s-eye” array receiver using single-detector. We present our analysis and design of a ground terminal that is compatible with the Space Development Agency (SDA) standard for 2.5 Gbps space-ground free-space optical communication links. Building on this SDA Phase II SBIR effort, Relative Dynamics’ end goal is to provide operational GOCArT capability for the SDA's Layered Network of Military Satellites, known as the Proliferated Warfighter Space Architecture (PWSA). Two important free-space optical communication ground terminal design drivers are receiver sensitivity and atmospheric turbulence mitigation. We compare approaches to achieve high receiver sensitivity for 2.5 Gbps OOK. We report results for GaAsSb/AlGaAsSb APDs for use at telecom C-band (1550 nm) with extremely low-excess noise factor. We report results for InGaAs/InAlAs APDs for use at telecom C-band (1550 nm) with large detection area (> 100 micron diameter) to improve GOCArT performance in atmospheric turbulence. We present analysis showing that a telescope array provides only minimal improvement for direct-detection receiver systems that are thermal-noise limited and use forward error correction codes. We introduce a fly’s eye receiver with a multimode fiber-optic combiner as an option. We show analysis, design, and tests of a fly’s eye receiver prototype system to assess the performance. GOCArT uses a small array of small aperture transmitter telescopes to mitigate the effects of atmospheric turbulence on the uplink.