Analysis of a ground to satellite optical link with a cooperative satellite beacon

Establishing a free-space optical communications link between a ground station and an orbiting satellite can be aided greatly with the use of a beacon on the satellite. A tracker, or even an adaptive optics system, can use the beacon during communications activities to correct beam pointing for atmospheric turbulence and mount jitter effects. Several models have recently been developed to study the effects of turbulence on a ground-to-geostationary satellite uplink, but published results from these models have not incorporated the effects of tracking. Two effects that need to be considered with a tracking system include a pointing lead-ahead issue and an aperture mismatch issue. The lead ahead problem involves the fact that the uplink beam needs to be pointed at the position the satellite will be when the pulse arrives. This problem exists even for a geostationary satellite. The aperture mismatch involves the sensing of the beacon through an aperture that is smaller, or larger, or even displaced from the uplink transmit aperture. In both cases, the sensed tilt might not be the same as the tilt required for optimal transmission to the satellite. These two issues have been studied separately, to some extent, by researchers in the past, although their combined effects in an optical communications uplink have never been investigated. The present paper applies previously published theories in an initial assessment of the impact of these tracking issues in an optical communications uplink. The analysis considers geosynchronous Earth orbit satellites as well as low Earth orbit satellites.