Since March 1996 the Modular Optoelectronic Scanner (MOS) of the German Aerospace Research Establishment (DLR) has provided spectral radiance data of the atmosphere-surface system in 18 spectral channels and up to 420 pixels in a 200 km swath from a 817 km sun synchronous polar orbit. The MOS consists of two imaging spectrometers, MOS-A (four channels with a halfwidth Delta lambda 1.4 nm in the O2 A absorption band) and MOS-B (13 channels between 400 and 1010 nm with Delta lambda 10 nm), and a camera, MOS-C, at 1.6 mu m. Beside the onground laboratory calibration as the basis for calculation of the spectral radiance data, a mission of such long duration requires a periodic recalibration or at least a stability check of instrument properties in orbit to support the reliability of the remote data. In this paper the two in-flight calibration methods will be described and their capabilities will be demonstrated on the basis of the real data during MOS's first year in orbit: the relative checking of the instrument parameters with internal lamps and the absolute calibration with the extraterrestrial sun radiation.