Quantum key distribution via satellites enables the technology to be applied at transcontinental scale; nevertheless, in contrast to fiber systems, using a free-space optical communication channel presents certain extra technological  obstacles. The contribution of the acquisition, pointing and tracking system’s operation to the potential quantum bit error value is investigated in this research. The polarization extinction ratio measurements varying with the pointing  mirror angular position are reported, based on these data, the upper limit of the quantum bit error is predicted for several types of satellite passages.