Performance of SMOS Soil Moisture Products Over Core Validation Sites

The European Space Agency (ESA) launched the Soil Moisture and Ocean Salinity (SMOS) mission in 2009; currently, multiple global soil moisture (SM) products are based on the measurements of its L-band (1.4 GHz) radiometer. We compared four SMOS products with each other: Level 2, Level 3, IC (INRA-CESBIO), and near real-time products. The comparisons focused on core validation sites (CVS), whose spatial representativeness errors allow the estimation of the SM product performance for bias-insensitive metrics [unbiased root-mean-square error (ubRMSE) and correlation (
$R$
), and anomaly
$R$
] with negligible uncertainty and for bias-sensitive metrics [mean difference (MD) and root-mean-square difference (RMSD)] with acceptable uncertainty. When the products were compared with CVS independently, the results showed that the ubRMSE,
$R$
, and anomaly
$R$
of the IC product were better than those of the other products, while the MD was larger. However, the differences between the performances were smaller when the products were assessed using only the data points when each product had a valid retrieval. This indicates that the algorithms have similar performance and that data screening and quality flagging of the retrievals markedly affects the performance. The NASA Soil Moisture Active Passive (SMAP) mission produces a similar SM product as SMOS using an L-band radiometer. The closeness of the ubRMSE,
$R$
, and the anomaly
$R$
performance of the IC product and the SMAP product (0.039 versus 0.041
$\text{m}^{3}/\text{m}^{3}$
, 0.80 versus 0.81, and 0.75 versus 0.75) demonstrate that the SMOS and SMAP radiometers can achieve similar SM sensitivity.