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The Special Sensor Microwave Imager Sounder (SSMIS), a new type of conically scanning microwave sounder, was launched by the Defense Meteorological Satellite Program in October 2003. Performance of the instrument and retrieval software was characterized in an extensive calibration/validation campaign. This paper describes results based on comparisons between SSMIS Lower Atmospheric Sounding (LAS) channel measurements and radiative transfer calculations based on conventional synoptic radiosondes, numerical weather prediction models, and special observations campaigns including dedicated lidar measurements and scientific radiosonde and dropsonde measurements. Retrieved lower atmospheric profiles were also directly compared with these data sources. Two significant sources of bias were identified. The emissivity of the primary reflector contributes to measured brightness temperatures, and the warm load calibration source is susceptible to uncompensated solar heating. Otherwise, it was determined that LAS channels are locally stable and accurately track atmospheric changes. Polarization of some channels was found to differ from the design. Several approaches were identified to mitigate sources of bias.