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Developing and Testing Mitigation Strategies

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Environmental conditions create significant obstructions to completing Satellite to Ground Links (SGL) with OGS (Optical Ground Links).  Multiple potential approaches exist to negate environmental effects and maximize transmission efficiency. Those approaches may be considered in two basic categories: 1) methods to improve the reliably of the FSO (Free Space Optics) link and 2) methods to bypass a faulty link in favor of a reliable alternate link.

Aperture averaging is a technique used to mitigate the effect of atmospheric turbulence by increasing the size of the receiver aperture averaging out fluctuations caused by small eddies.  Studies suggest that aperture averaging may be advantageous only for satellite downlink (not uplink). However, an increase in the receiver aperture size will also increase the amount of background noise collected by the receiver; adding to the complexity of balancing design decisions.  

Adaptive Optics are systems built into the OGS (Optical Ground Links) that pre-correct the beam in real time by converting the wavefront measurement into actuator commands to minimize the wavefront error before transmitting it into the atmosphere.  This approach in part mitigates the effect of atmospheric turbulence.

Re-transmission protocols such as automatic repeat request (ARR) may be employed wherein if the receiver does not acknowledge receipt of a transmitted packet within a specified time frame, the packet is re-transmitted.  Variants include selective repeat ARQ (Automatic Repeat Query) (SR-ARQ), hybrid-ARQ (H-ARQ), cooperative diversity with ARQ (Automatic Repeat Query) (C-ARQ), modified cooperative diversity with ARQ (Automatic Repeat Query) (MC-ARQ) and Rateless Round Robin.

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