Presented at the 2013 Aquarius/SAC-D Science Team MeetingConstraining dynamical systems with new information from ocean measurements, including observations of sea surface salinity (SSS) from Aquarius and SMOS, requires careful consideration of data errors that are used to determine ("weight") the importance of constraints in the optimization. Here such weights are computed as the reciprocal of the variance of data errors and are derived by comparing satellite SSS observations with ocean model output. The approach applied to the Aquarius L3, V2.0 and SMOS L3, OI global salinity maps. The resulting data error variance maps have a complex spatial pattern, ranging from less than 0.05 pss^2 to 1-2 pss^2 along the coasts and high latitude regions. Comparing the data-model misfits to the data errors indicates that the Aquarius and SMOS constraints could measurably affect estimated SSS values in several ocean regions, including the western boundary currents and parts of the ITCZ and equatorial regions. The findings are also discussed in reference to the Aquarius total error budget and model errors. In particular, derived errors are less than mission accuracy requirements in low and mid latitudes, but exceed in situ data errors, particularly at high latitudes.