This technical note summarize investigations relative to the spatial and temporal scale mismatches between SMOS and the in situ measurement (used as reference for calibration). First, we present an overview of the different caveats that one should consider when comparing coarse resolution (>10 3 km 2 ) satellite data with in situ measurements, in terms of error budget and uncertainties. Then, we develop a list of analysis to evaluate the sensitivity of the SMOS performance to different configuration (spatial and temporal) with the in situ measurement. Results show that more than just the location of the probe within the footprint, the characteristic of the surface condition around the probe plays a significant role in the level of agreement between the SMOS and the probe. In fact, favoring probes with strong similarities between surface conditions and what the satellite sees on average increases the level of consistency between datasets. The analysis of SMOS performance as a function of the degree of diversity of surface states (Shannon, Gini) points in the same direction, i.e. increasing the potential heterogeneity of the surface condition within the footprint increase the spatial scale mismatch. Using those outputs added to the representativeness indicator developed by TUWIEN has led to a subset of SMOS nodes (∼ 30) and probes that seems favorable for comparison with satellite. Other analyses show a sensitivity of the validation scores to the different strategies of colocation, i.e. spatial (in situ vs. satellite driven) and temporal (autocorrelation sampling window length and position) but need more investigation to be significant. Finally, a discussion and estimate of the range of values for the various contributors to the uncertainty budget was made. As not Work Package 5 REQ16 -Technical Note 3 ESA Fiducial Reference for Soil Moisture (FRM4SM) Issue 2.0