The Extreme Universe Space Observatory is an experiment to investigate the highest energy cosmic rays by recording the extensive air showers they create in the atmosphere. This will be done by recording video clips of the development of these showers using a large high-speed video camera to be located on the Japanese Experiment Module of the International Space Station. The video clips will be used to determine the energies and arrival directions of these cosmic rays. The accuracy of these measurements depends on measuring the intrinsic luminosity and the direction of each shower accurately. This paper describes how the accuracy of these measurements will be tested and improved during the mission using a global light system consisting of calibrated flash lamps and lasers located deep in the atmosphere.