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ENVISAT Microwave Radiometer (MWR) is an instrument designed and developed as part of the Envisat-1 satellite scientific payload, with Alenia Aerospazio engaged in the phase C-D as instrument Prime Contractor, leading an industrial consortium of European and American companies. The Flight Model of the Instrument was delivered to ESA at the end of July 1997, after successful completion of design, test and calibration activities. An Engineering Model of the instrument was also developed and completed in March 1997. The MWR output products are of prime importance for wind/wave products of the Radar Altimeter (RA-2) Instrument, part of the Envisat-1 payload, providing correction of atmospheric propagation data. The products are also useful for direct evaluation of brightness temperature in order to characterize polar ice, land surface properties and sea surface temperature. In order to achieve the required accuracy and sensitivity performance, an in- flight two-point calibration concept is adopted, with hot and cold calibration reference points for each frequency channel. Periodically the measurements of earth scene radiation are interrupted to allow the measurement of an on-board calibration load and of the deep cold space. The overall ground calibration tasks were performed through an iterative sequence of measurement and relevant model corrections, with an extensive instrument calibration in a thermal-vacuum environment, to derive the final radiometer model coefficients and to verify its performance in the expected in-flight environment. To achieve the required instrument calibration accuracy, extremely accurate blackbody target sources were required, in order to simulate the Earth scene and the deep space (for cold calibration), as seen by the radiometer during its in-flight mission. The definition, development and characterization of such blackbody targets were key aspects to achieving the required stimulus accuracy for proper calibration of the instrument. These tasks were jointly performed by the UK Meteorological Office (UKMO) and Alenia Aerospazio, and lead to the final calibration of the instrument with successful results. Within this paper an overview of the instrument calibration will be given; emphasis will be on the trade-offs and design requirements derived for the Calibration Targets, on their design and calibration, and finally on the achieved results and instrument performance.
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