Objective: To develop technologies and to demonstrate a reliable optical communication uplink from an optical ground station to a satellite in low Earth orbit.Targeted Improvements: First ever demonstration that reliable optical feeder uplinks to telecommunication satellites in LEO are feasible. Description: Optical communication promises the possibility to transmit virtually unlimited amounts of data from optical ground stations (OGS) to future telecommunication satellites, without the need for licensing. However optical communication is facing two main problems, cloud coverage and atmospheric turbulence. While cloud coverage can be mastered by OGS redundancy (placing multiple OGS in meteorologically uncorrelated locations - meaning >500 km apart), the atmospheric turbulence problem on the uplink has not been mastered yet. The reason being that the orbital motion of a spacecraft (S/C) requires a small angular split between the transmitted and the received communication beams. This angular split, called point ahead angle (PAA), is tiny (up to 10 arcsec depending on the elevation angle of the LEO satellite), but it prevents deriving reliably wave-front distortions onthe uplink from probing the wave-front distortions on the downlink. The result of this is that the uplink beam cannot be pre-compensated and that the quality of service on the uplink becomes extremely bad and unreliable. The uplink beam experiences extreme intensityfluctuation when received at the spacecraft as well as outages that can last several milliseconds. It is impossible to base a reliable telecom service on such a poor channel quality. This activity will therefore develop and demonstrate technologies that enabletheprobing of the wave-front distortions on the uplink beam and their compensation by beam pre-distortion. In this activity a technology shall be utilised that was initially developed by the US MoD and that is widely used by astronomers to correct for atmospheric wave-front distortions, namely the creation of an artificial Sodium guide star in a small distance away from the telecommunicationsatellite that corresponds to the PAA. In tracking LEO satellites the PAA is not constant, but changes with the elevation of the S/C. The relatively fast movement of the S/C through the sky additionally increases the strength of atmospheric turbulence effect. This activity will develop reliable turbulence mitigation techniques for LEO optical feeder links under (24/7) day and night-time operation.It will perform a long-term test campaign with optical communication terminals on LEO satellites (e.g. T-OSIRIS, Optel-mu, Pixl, SmallCAT...) to determine the achievable link performance and reliability in varying atmospheric turbulence conditions and satellite elation angles.