Using observational data of double stars to assess the feasibility of tip-tilt retrieval on the downlink signal for uplink pre-compensation in free space optical communications

Abstract

Free space optical communications with satellites are affected by atmospheric turbulence, typically using adaptive optics to correct for the resulting signal errors. This can be done using laser guide stars as a reference signal for the wavefront sensing, with the exception of tip-tilt, which up to date cannot be reliably measured with laser guide stars. Proposed solutions utilise the downlink signal (sent from satellite to ground station) as a reference source with which to pre-compensate for tip-tilt in the uplink signal. However, the point ahead angle due to the orbital motion of the satellites results in the signals propagating through different regions of the atmosphere. Due to atmospheric anisoplanatism, there is a degree of tip-tilt decorrelation between the laser links. The purpose of this study is to determine if the tip-tilt is sufficiently correlated such that the downlink can still be used as a reference source for adaptive optics applied to the uplink signal. This project developed methods to measure the tip-tilt of double stars and analyse the correlation of these effects between the two component stars, equivalent to the downlink and uplink signals. Observational data of double stars at Mount Stromlo Observatory (Canberra, Australia) at different elevations and separations are analysed to obtain statistics of the differential atmospheric tip-tilt in strong turbulence regimes in the Southern Hemisphere; results are compared with similar studies carried out in the Northern Hemisphere. This work aims to assess the feasibility of using the downlink as a reference signal for atmospheric tip tilt sensing with the goal of improving the performance in the uplink communications.