  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.
