Atomic Clock Ensemble in Space (ACES) is an ESA fundamental physics mission that establishes a very precise time scale in space. The main clock is a laser-cooled Cesium clock called PHARAO. Two days after launch, the ACES module was mounted on the ISS and now enables time transfer between ground clocks and the clock signal in space using two very precise and accurate synchronization methods: a microwave technique called Microwave Link (MWL) and an optical counterpart called European Laser Timing (ELT). Satellite Laser Ranging (SLR) stations within the International Laser Ranging Service (ILRS), which are equipped with a very precise clock or can establish fiber links to clocks operated at metrology institutes, are motivated to contribute to ELT. In the first evaluation phase, the Wettzell Laser Ranging System (WLRS) will be the main ground station, performing regular clock synchronization experiments between ground and ACES in space. We will provide an introduction to the ACES mission and the timer, which is used for time-tagging the ELT detection events on board the International Space Station (ISS). Furthermore, we will discuss the performance of ELT by showing the results of the calibration campaign completed before the ACES launch.