The global ocean surface wind vector is a key parameter for short-term weather forecasting, the issuing of timely weather warnings, and the gathering of general climatological data. In addition, it affects a broad range of naval missions, including strategic ship movement and positioning, aircraft carrier operations, aircraft deployment, effective weapons use, underway replenishment, and littoral operations. WindSat is a satellite-based multi-frequency polarimetric microwave radiometer developed by the Naval Research Laboratory for the U.S. Navy and the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Integrated Program Office (IPO). It is designed to demonstrate the capability of polarimetric microwave radiometry to measure the ocean surface wind vector from space. The sensor provides risk reduction for the development of the Conical Microwave Imager Sounder (CMIS), which is planned to provide wind vector data operationally starting in 2010. WindSat is the primary payload on the Air Force Coriolis satellite, which was launched on 6 January 2003. It is in an 840 km circular sun-synchronous orbit. The WindSat payload is performing well and is currently undergoing rigorous calibration and validation to verify mission success. The WindSat radiometer has polarimetric channels at 10.7, 18.7 and 37.0 GHz. Dual-polarization channels at 6.8 and 23.8 GHz provide key data sea surface temperature and atmospheric water vapor. The WindSat design and ground processing focus on the primary mission of measuring the ocean surface wind vector. However, the unique data set produced by WindSat can be used for many environmental remote-sensing applications in the ocean, land, sea ice, and atmosphere discipline areas. This presentation will describe the WindSat sensor and mission objectives. Furthermore, the top –level WindSat radiometer performance will be presented.