ROSIS is a compact airborne imaging spectrometer which has been developed jointly by German industry and research organisations. The sensor makes use of a two-dimensional CCD array for imaging simultaneously 115 spectral bands of 512 picture elements perpendicular to the flight direction. The spatial resolution is 0.56 mrad, the spectral range is 430-860 nm and the spectral sampling interval is 4 nm. The driver for the design of ROSIS was its application for the detection of spectral fine structures in coastal and inland waters. This task determined the selection of spectral range, bandwidth, number of channels, radiometric resolution and tilt capability for sun glint avoidance. However, ROSIS can be used just as well for the monitoring of spectral features above land or within the atmosphere. It was used hitherto mainly for the scientific preparation of the imaging spectrometer MERIS to be flown on ESAs ENVISAT satellite. However,accurate radiometric calibration was limited by electronic problems. Hence the entire electronic system was re-designed and re-built. The analysis of laboratory data of the new ROSIS shows that these problems have been eliminated.