<title>Comparison of vicarious and in-orbit calibrations of the imaging spectrometer MOS</title>

Abstract

DLR's Modular Optoelectronic Scanner MOS on the Indian Remote Sensing Satellite IRS-P3 has been working now for almost 5 years in orbit. In September 2000 the power supply for driving the internal lamps and the sun calibration equipment failed so that we no longer have actual in-orbit calibration values. However the spectrometers themselves are still working and nadir remote data collection is running. To remedy this situation we have tried to use vicarious calibrations over the Sahara desert. The "Great Eastern Erg" near the border between Tunisia and Algeria has been selected for this purpose. Because we do not have any ground truth measurements from this area, we have investigated the correlation between the in-orbit sun and internal lamp calibration data and the upwelling radiance data of this area in the VIS/NIR-channels of MOS from May 1996 to August 2000. The vicarious calibration data were corrected with respect to actual sun irradiance only, but not to atmospheric conditions. Nevertheless there is a remarkably high correlation between the in-orbit calibrations and these vicarious calibrations. This enables us to continue to generate calibration data sets for MOS only by using actual vicarious calibration data in place ofthe in-orbit calibration data. Additionally the vicarious calibrations can be compared with the extrapolated results of the time trend of the radiometric sensitivity of all spectral channels which we found from the previous in-orbit calibrations. The results of all these investigations are presented in this paper.