Identification and calibration of ground system biases in ground to space laser time transfer

Laser time transfer is and attractive technique to transfer time ground to space with picosecond precision and systematic errors on the level of tens of picoseconds. Recently the European Laser Timing experiment is under construction in the frame of the European Space Agency mission Atomic Clock Ensemble in Space. The objective of this laser time transfer is synchronization of the ground based clocks and the clock on board the International Space Station with picosecond precision and the accuracy better than 50 picoseconds We are reporting on a progress in identification and calibration of the biases associated to both ground and space segment. To characterize the delays of a ground segment the Calibration Device has been developed and tested. It enables to calibrate the ground based laser systems for their systematic timing biases. As a result the ground to ground time transfer in an ELT experiment should be accomplished with systematic errors not higher than 25 picoseconds. To determine the delays of a space segment the new calibration procedure has been designed and tested as well. The calibration test results will be presented.