WindSat, an experimental sensor on the Coriolis mission launched in January 2003, is intended to measure the partially polarized emission from the ocean surface, and thereby indirectly measure the ocean surface wind vector. The WindSat calibration and validation process has two primary purposes. The first is to verify the WindSat radiometer absolute accuracy, which is driven by the calibration target accuracy and knowledge, the receiver performance, the antenna characterization and antenna pattern correction, and the geolocation and pointing processing. The second purpose is to validate the sea surface wind speed and direction environmental data products. A key part of this process to identify and quantify error sources and, if necessary, generate new sensor calibration coefficients, algorithms, and environmental data record (EDR) retrievals to bring the data products into specification. This paper focuses on the calibration of the brightness temperatures, the data product used to retrieve environmental parameters such as wind vector