A new study assessing the quality of SMOS soil moisture estimates (L2 product) has recently been published !
This analysis, part of the ESA FRM4SM project, aims at investigating the sensitivity of SMOS performance to surface conditions. It uses in situ measurements from the International Soil Moisture Network as reference data. The comparison shows a wide range of agreement (here defined as ubMRSD values). The differences can be explained by i) the sensitivity of SMOS retrieval to certain surface conditions (such as vegetation, open water content, topography complexity and soil texture), ii) the characteristics of the in situ probes (calibration, installation, technology…) and iii) the differences in spatial and temporal scales between the two datasets.
The sensitivity to surface conditions has been extrapolated to the global scale, to obtain the following SMOS soil moisture uncertainty map (which can be interpreted as the upper limit due to the different contributions of the uncertainty budget) :
Much more information in the paper published by F. Gibon and al : here