This letter contributes a new approach to calibrating a tau-omega radiative transfer model coupled to land surface model output with low frequency (< 10 GHz) microwave brightness temperature (TB) observations. The problem of calibrating this system is generally poorly-posed because various parameter combinations may yield indistinguishable (least square error) results. This is theoretically important for a land data assimilation system since alternative parameter combinations have different impacts on the sensitivity of TB to soil moisture and misattribution of systematic error may therefore disrupt data assimilation system performance. Via synthetic experiments we demonstrate that using TB polarization difference to parameterize vegetation opacity can improve the stability of calibrated soil moisture/TB sensitivities relative to the more typical approach of utilizing ancillary information to estimate vegetation opacity. The proposed approach fully follows from the radiative transfer model, implemented according to commonly adopted assumptions, and reduces by one the number of calibration parameters.