The AMSU-A1 measures scene radiance in the microwave spectrum. Its data, in conjunction with the HIRS/3 data, is used to calculate the global atmospheric temperature and humidity profiles from the Earth's surface to the upper stratosphere, approximately at 2-millibar pressure altitude (48 km).
A complete end-to-end calibration is performed by the instrument provider. A complex internal calibration procedure guarantees that this calibration performance is maintained over the mission life time. Here the instrument scan mirror dwells to different scan positions which represents warm and cold references. The warm calibration reference is realised by an internal warm calibration load whereas for the cold reference a view to deep space is made. Knowledge and accuracy of warm calibration load brightness temperature is better than ± 0.2K