This paper investigates tilt decorrelations due to atmospheric anisoplanatism occuring when observing wavefronts emerging from distinct line of sight. The targeted application is ultimately the (pre-)compensation of the atmospheric turbulence experienced by a laser beam during ground-to-satellite optical links. The purpose of the study is to evaluate the effectiveness of the uplink pre-compensation, if the downlink signal (received from the satellite) is used as a reference. Because of the point-ahead angle of the satellite, one expects some decorrelation between the downlink and the uplink signals, which, in turn, impacts the efficacy of the pre-compensation. The larger the beam, the smaller its divergence and the more sensitive it is to pointing errors. In this framwork, a test campaign was carried out in May 2018 at the Optical Ground Station (OGS) of the European Space Agency (ESA), to perform measurements of double stars featuring angular separations representative of the point-ahead angle of GEO/LEO satellites. The differential Tip/Tilt distortion between the double stars is used as an estimator of the typical decorrelation between the downlink and the uplink signal, hence the present study. The algorithm used to extract the tip-tilt error due to anisoplanatism is described, and the experimental results are compared to the numerical predictions. It is then shown how to estimate the jitter of the telescope, based on the common motion of two independent stars as seen in the focal plane of the telescope. Finally, the paper provides a methodology to determine the maximum transmitter aperture of a ground-based terminal, in case a tilt pre-compensation is applied based on the satellite signal.
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This study was supported by the European Space Agency (ESA) in the framework of the ARTES ScyLight programme. The authors also gratefully acknowledge the Instituto de Astrofisica de Canarias (IAC) as well as the sky quality group (http://www.iac.es) for their support during the measurements campaign.
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Alaluf, D., Armengol, J.M.P. Ground-to-satellite optical links: how effective is an uplink Tip/Tilt pre-compensation based on the satellite signal?. CEAS Space J (2021). https://doi.org/10.1007/s12567-021-00392-2