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Retrieving soil moisture from spaceborne microwave radiometer observations often requires ancillary parameters such as surface vegetation opacity or vegetation water content (VWC). The conventional approach for deriving representative footprint- scale values of these parameters is to simply average the corresponding parameters of high-resolution pixels over the entire radiometer footprint. However, previous work has shown that simple averaging of ancillary parameter values may result in biased soil moisture retrievals from spaceborne radiometers. This letter uses a synthetic observing-system simulation experiment framework to theoretically demonstrate that ancillary VWC or canopy-opacity parameter values obtained with an alternative aggregation rule may significantly reduce the magnitude of soil moisture retrieval errors for highly heterogeneous vegetated areas.