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The National Oceanic and Atmospheric Administration (NOAA)-18 Polar-orbiting Operational Environmental Satellite (POES) was successfully launched with the fourth Advanced Microwave Sounding Unit-A onboard in May 2005. After launch, a systematic post launch calibration and validation of the instrument performance was conducted with on-orbit data. A brief report of the initial 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 in the space radiometric counts, which are accurately detected, flagged, and corrected using a physical model of lunar surface temperature and antenna patterns derived from on-orbit data. The long-term trends of the space and warm calibration counts, channel gains, and housekeeping temperature sensors are monitored. Temperature sensitivity (or the noise-equivalent ) values for individual channels are also monitored since launch. The long-term temporal trends of the monthly averages and angular distributions of brightness temperature measurements from the NOAA-16 and -18 over the Amazon rain forest region are obtained and compared to demonstrate that the Amazon rain forest can be used as a hot reference calibration target. The establishment of a land calibration target is an important addition to the few tools available to date for calibration and validation of spaceborne microwave radiometers.