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The Flat Target Transformation

This paper describes a practical method to calibrate the microwave imaging radiometer with aperture synthesis (MIRAS) in orbit using external targets. Together with the in-orbit validation of the antenna patterns, the method ensures the generation of valid and calibrated visibilities from raw measurements with minimum impact from instrument errors, particularly antenna errors. It has been possible to devise this strategy only after the derivation of the Corbella equation, which correctly describes how MIRAS works. The proposed in-orbit calibration and validation approach is fully based on this equation. In a markedly different way to the point target response method used to calibrate other imaging systems, the technique put forward is essentially based on measuring a ldquoflatrdquo target such as the cold sky near the Galactic poles. This is why it has been called the ldquoflat target transformationrdquo.