In the past decade considerable research has focused upon the accurate description of root zone soil moisture for the initialization of numerical prediction models (e.g., for weather or hydrology). The lack of in situ soil moisture measurements on the continental and global scales has motivated the development of two L-band microwave remote-sensing missions: the Soil Moisture and Ocean Salinity (SMOS) mission from the European Space Agency, and the Soil Moisture Active and Passive (SMAP) mission from NASA. At the Meteorological Research Division of Environment Canada, development work is focused upon the inclusion of these more direct measurements of near-surface soil moisture into the new version of the Canadian Land Data Assimilation System (CaLDAS). In this study we present the results of a series of observing system simulation experiments (OSSEs) in which we investigate several aspects related to the assimilation of L-band microwave soil brightness temperatures. In particular, we will discuss issues related to the impact of the horizontal resolution of the background or first-guess land surface predictions and the specification of the background error covariances.