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The Advanced Microwave Sounding Unit-A (AMSU-A) on the Meteorological Operations Platform A (METOP-A) Satellite was successfully launched in October 2006. The AMSU-A, which was provided by NOAA, is one of a new generation of total-power microwave radiometers which have been flown on the NOAA-15 to NOAA-18 Satellites since May 1998. A systematic postlaunch calibration and validation (cal/val) of the instrument performance was conducted with on-orbit data. A brief report of the postlaunch assessment of the instrument performance is presented in this paper. Scan-by-scan examination of the radiometric calibration counts is employed to confirm normal functioning of the instrument and to detect any anomalous events, such as lunar contamination (LC) in the space radiometric counts, which are detected, flagged, and corrected using an algorithm for detection and correction of LC in AMSU-A data. The correction algorithm provides a practical approach for scan-by-scan correction of the LC in AMSU-A data and improves the accuracy of operational calibration of the NOAA Level IB data. The long-term trends of the space and warm calibration counts, channel gains, and housekeeping temperature sensors are monitored. Temperature sensitivity (or NEDeltaT) values for individual channels are also examined. The monthly mean angular distributions of brightness temperatures from the METOP-A and NOAA-15, -16, and -18 over the tropical ocean region of 20deg S-20deg N are obtained and theoretically modeled to demonstrate that the ocean can be used as a cold reference calibration target. The establishment of a natural Earth calibration target is an important addition to the few tools available to date for cal/val of spaceborne microwave radiometers. The measured ocean brightness temperatures from the four satellites provide eight measurements across the diurnal time for a unique opportunity of studying the diurnal variation of ocean brightness temperatures which show a pattern of daytime cooling- - and nighttime warming.