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 calibrated by comparison of the measurements from the two satellites over the Antarctica, the tropical ocean, and the Amazon Rainforest, respectively. Characteristics of the data over the three test sites are demonstrated. Daily mean brightness temperatures over Antarctica from NOAA-18 and NOAA-19 measurements demonstrate that there are little diurnal differences in measurements during the Antarctic winter months from each satellite. Therefore these measurements provide a practical approach to obtain relative channel biases of inter-satellite data. The monthly mean angular distributions of brightness temperatures at the four window channels 1-3 and 15 from the satellites over the tropical ocean (0°S-20°N) are compared to demonstrate the close similarity between the two measurements. Similarly, data over the Amazon Rain Forest are also compared and theoretically modeled. Results in this study provide a useful guide for calibration and validation of microwave instruments.