Resume of 7 years MOS in-orbit calibration - events, effects and explanations

Kurzfassung

The in-orbit calibration of the Modular Optoelectronic Scanner MOS on the Indian Remote Sensing Satellite IRS-P3 has delivered the actual radiometric recalibration coefficients with sufficient accuracy for most of the 18 spectral channels in the VIS/NIR spectral range during the 7 years mission time. This has been the basis for the thematic interpretation of the MOS data. The three different and independent in-orbit calibration methods: lamp calibration, sun calibration and ground target based (vicarious) calibration as well as different possibilities of dark signal determination and the extensive knowledge of instrument performance data and instrument characteristics from the lab measurements have enabled us to overcome all failures and difficulties of the instrument which occurred in orbit. The failure of the lamp and sun calibration equipment in September 2000 has been overcome by using the vicarious calibration and dark signal measurements at the earth night side at new moon. The failure of the thermo-electric cooling of the detectors in November 2002 could be overcome only by the knowledge of the temperature dependence of the spectral responsivity of the different spectral channels and its dark signals. Thus we are able to continue the determination of the time trend of the recalibration coefficients in spite of these problems. In the paper we will give a resume of the most important events concerning the in-orbit calibration during the mission time, try to find explanations for some effects and present the results of determining the recalibration coefficients and the accuracy reached under the concrete environmental and instrumental conditions in orbit.