Ground Optical Communication Array Terminal

The US Space Development Agency is deploying a Low-Earth-Orbit (LEO) satellite constellation for  US Government Defense Department communication.  Free-space optical communication terminals are important for both cross-links and downlinks. However, at present, there is an extremely limited infrastructure of optical ground terminals. Relative Dynamics is developing the Ground Optical Communication Array Terminal (GOCArT), a scalable-aperture, deployable ground  terminal with significant user operability to provide Space Development Agency Tranche 1 satellite-ground communication links. Relative Dynamics will investigate the following innovations for GOCART: Telescope array transmitter with novel Lateral Transfer Retroreflectors to facilitate boresight alignment. High-sensitivity new-material avalanche photodiode (APD) detectors Microlens and APD array receiver Fly’s eye receiver with either optical (fiber optic) or digital (commercial transceiver) signal combining In addition, GOCART will leverage commercial hardware to reduce cost wherever possible. A method for using commercial pluggable transceivers with integral APD detectors is a low-cost solution. A newly developed GaAsSb APD could provide significant (5-10 dB) receiver improvement over commercial InGaAs APDs. A telescope array is one method to reduce the effects of atmospheric turbulence, particularly at low-elevations. Analysis by us and others shows that: a telescope array provides only minimal improvement for direct detection receiver systems that are thermal noise limited and use Forward Error Correction codes. a telescope array provides significant improvement for direct detection receiver systems that are close to shot-noise limited. A small APD array (e.g., 2 x 2)  in tandem with a microlens array is a viable solution to mitigate atmospheric turbulence blur circle and beam wander effects. A fly’s eye receiver with a fiber optic combiner and a multimode fiber preamplifier is an option.  The performance needs to be demonstrated in a prototype. Reasonable-cost ground-terminal high-performance telescope and gimbal systems are commercially available that are suitable for free-space communications. A small array of small aperture (0.1-0.2 m) transmitter telescopes is required to mitigate the effects of atmospheric turbulence on the uplink. The GOCArT performance, architecture, algorithm, technology and cost trades are balanced for a cost-effective solution. RDI will  provide prototype key hardware, documentation and test results and provide detailed designs and plans for the gimbal and receiver telescope(s) and the overall optical ground terminal. This is in preparation for a post-Phase II GOCArT Critical Design Review.