As a NPOESS risk reduction mission, WindSat was built on the heritage of other satellite microwave radiometers such as AMSR-E, TRMM/TMI, SSM/I and SMMR, and added the polarimetric capability enabling it to resolve the ocean surface wind vector. Its radiometer operates in discrete bands at 6.8, 10.7, 18.7, 23.8, and 37.0 GHz. The 10.7, 18.7 and 37.0 GHz channels are fully polarimetric, while the 6.8 and 23.8 GHz channels are dual polarized only (vertical and horizontal). Therefore WindSat has a wide range of land applications beyond ocean remote sensing. To this end, and to provide algorithm risk reduction for NPOESS future Microwave Imager/Sounder (MIS) mission, we developed new land algorithms that retrieve soil moisture and vegetation water content based on WindSat data. The new algorithms were validated using in-situ data acquired during soil moisture field experiments (SMEX) in US by soil moisture networks (during SMEX03 in Oklahoma and SMEX04 in Arizona), and by extensive manual area sampling (during SMEX05 in Iowa). The temporal coverage of our data is three months for SMEX03 and SMEX04, and one month for SMEX05. Despite this relatively short temporal coverage, these three experiment sites provided diverse land/vegetation covers (SMEX03-rangeland and winter wheat, SMEX04-sparse shrubland, SMEX05-agricultural domain with corn and soybean) and wet/dry soil conditions. In general, WindSat volumetric soil moisture retrieval uncertainty is about 4% with 0.4% bias at 50 km Horizontal Cell Size (HCS). Despite very different land covers and wide range of soil conditions, the retrieved soil moisture is highly correlated with in-situ data. Spatial patterns of the retrieved soil moisture are consistent with precipitation maps generated using NEXRAD radar data; and retrieved global soil moisture distributions are consistent with dry/wet patterns of climate regimes. In addition, good agreements are also obtained between WindSat retrieved Vegetation Water Content and AVHRR derived Green Vegetation Fraction data produced by NOAA/NESDIS.

WindSat algorithm validation results suggest that there is sufficient soil moisture sensitivity to satisfy the NPOESS soil moisture requirements if C- and X-band are included in the MIS channels. Equally important, applications will require a 1.8-m antenna for MIS to be useful for soil moisture sensing. Further research activity is needed to improve our modeling skills and expand the algorithm range of validity, particularly on RFI mitigation and in-situ validation.