Uplink tip-tilt pre-compensation using a sodium laser guide star propagation delay

For stable ground-to-satellite optical uplinks, the optical feeder link (OFL) laser beam must be pre-compensated before propagation. The atmospheric optical turbulence leads to speckling and beam wander of the OFL from the high and low order phase aberrations, respectively, both leading to deep signal fades at the satellite. To mitigate this, the downlink (satellite-to-ground) communication beam can be used as a reference source to measure the wavefront aberrations in the direction of the satellite; however, the OFL is pointed ahead of the satellite to take into account its motion and the light propagation time. Therefore, by using the satellite as a reference source, the OFL precompensation suffers from wavefront anisoplanatism. To improve the accuracy of the uplink pre-compensation, a laser guide star (LGS) can be used at the point-ahead angle (PAA), providing a reference source in the pointing direction of the OFL. The tip and tilt modes cannot be directly measured from an LGS, limiting the performance of this method. We present simulation results of the propagation delay (PD) method for indirectly measuring the tip and tilt of an LGS by integrating the measured differential tilts. This method was originally proposed for astronomy, and we adapt this method for the application of optical communications to geostationary satellites. We present results from numerical simulations, comparing pre-compensation of the tip and tilt modes using the satellite downlink tilt with the PD method. PD gives a reduction in the RMS angular beam wander of between 34−44 % for a median atmospheric profile in urban Barcelona, corresponding to gains of +5 to +10 dB.