The Global Imaging Monitor of the Ionosphere (GIMI) is one of the scientific instruments aboard the Department of Defense's Advanced Research and Global Observation Satellite (ARGOS). GIMI contains two far-ultraviolet electron- bombarded CCD (EBCCD) cameras, with 9 degree(s)-square fields of view. GIMI is intended for far-UV imagery of upper atmospheric and ionospheric airglow and auroras, occultations of UV-bright stars by the neutral upper atmosphere, and deep-space celestial objects and diffuse far-UV background. To obtain quantitative information from the images, an extensive program of calibrations, prior to launch and in-orbit, was required. Laboratory calibrations before launch consisted of imaging monochromatic laboratory UV light sources whose integrated intensities were monitored by separately calibrated photon-counting detectors. The in- flight calibrations involved observations of UV-bright starts, for which previously obtained and calibrated UV- bright stars, for which previously obtained and calibrated UV spectra exist from previous UV astronomy satellites (such as OAO-2 and IUE). We describe the procedures used in obtaining data for calibration purposes, and for reducing and analyzing the calibration data. We also discuss procedures and results for determining the absolute sensitivities, and their variations with wavelength, position in the field of view, and on-orbit time during the ARGOS mission.