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The European Centre for Medium-range Weather Forecasts (ECMWF) brightness will use temperatures from the soil moisture and ocean salinity mission to analyze root zone soil moisture through a variational data assimilation system. The first guess is obtained from numerical weather prediction (NWP) model fields, an auxiliary database, and a land surface microwave emission model. In this paper, we present the community microwave emission model and research the first-guess errors in L-band brightness temperatures. An error propagation study is performed on errors introduced through: (1) uncertainties in the parameterizations of the radiative transfer model; (2) auxiliary geophysical quantities for the radiative transfer computations; and (3) an imperfect NWP model. It is found that the vegetation and dielectric models introduce uncertainties with a difference of up to 25 K between models. However, the biggest error in brightness temperature is likely related to the use of an auxiliary vegetation database, which results in differences of -20 to +20 K in our simulations. These potential errors are in many regions higher than the variance in brightness temperatures related to an imperfect NWP model.