The Microwave Sounding Unit (MSU) and Advanced Microwave Sounding Unit (AMSU) on board the National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites were designed to measure the atmospheric temperature from the surface to the lower stratosphere under all weather conditions, excluding precipitation. They offer a unique opportunity for monitoring climate change of the atmosphere and possibly holding answers to the question of how much the atmosphere has changed over the last several decades. However, the tropospheric temperature trends derived from these measurements are under significant debate, mostly caused by calibration errors. To obtain reliable climate trend, consistent and physics-based recalibration and reprocessing of the MSU and AMSU observations that aims at reducing the intersatellite differences are required. Recently, Zou et al. (2006) developed a nonlinear method to inter-calibrate NOAA satellites using simultaneous nadir overpasses (SNO) between satellite pairs and showed that the method provided accurate calibration at level 0 and remarkably reduces intersatellite differences for level 1b and gridded brightness temperature data. The method also removes warm target contamination in the brightness temperature differences of overlapping satellites which is critically important for obtaining reliable climate trend. This presentation will review previous studies using MSU observations for climate trend studies and then discuss the new calibration method and its accuracy. We will provide the updated tropospheric temperature trend results with the new calibration. We also discuss NESDIS/ORA future plans for recalibration and reprocessing of all MSU and AMSU channels using the SNO calibration methodology.