Calibration of the NOAA AMSU-A Radiometers With Natural Test Sites

The Advanced Microwave Sounding Unit-A (AMSU-A) instruments onboard NOAA-18 and NOAA-19 are investigated by comparison of the measurements from the two satellites over Antarctica and the tropical ocean. Characteristics of the data over the two test sites are demonstrated. Thirty-day mean brightness temperatures at nadir over Antarctica from NOAA-18 and NOAA-19 measurements show that there is minimal diurnal variability ( <; 0.5 K) in the measurements during the Antarctic winter months. Therefore, these measurements provide a practical approach to determine the relative intersatellite/intersensor calibration biases. The resultant biases for the two satellites are <; 0.2 K for channels 1-5 and 8 and <; 0.6 K for the other channels. Monthly mean angular distributions of brightness temperatures at the four window channels 1-3 and 15 from the satellites over the tropical ocean are compared to those of simulations, which are obtained with an ocean model of radiative transfer. These AMSU-A data provide a unique opportunity for the investigation of the scan-position-dependent biases (or asymmetries) of the data. The biases are revealed by the difference between the observed brightness temperatures and the simulations. The results in this study provide a useful guide for the calibration and validation of microwave instruments.