Presented at the 12th Specialist Meeting on Microwave Radiometry and Remote Sensing of the EnvironmentSpaceborne radiometric measurements at L-band from the Aquarius instrument are compared to numerical model simulation. A calibration bias is estimated: of the order of 3K to 5K in vertical polarization, and of the order of 1.5 K or less in horizontal polarization. A bias temporal drift of about 1 K over five months is identified for all beams and polarizations. Measurements/model comparisons are also performed to assess the uncertainty of various aspects of the data processing that are critical for retrieving Sea Surface Salinity, such as the correction for Faraday rotation and the impact of sea surface roughness on the surface signal. Errors in the antenna pattern models are identified, that lead to significant biases in Faraday rotation angle. The precision of the model for surface roughness impact is estimated to be of the order of 0.25K over a large range of wind speeds, but is less accurate at the low and high end of the wind speed range, particularly for wind speeds much larger than 15 m/s.