Abstract
NASA’s Soil Moisture Active Passive (SMAP) mission has been validating
its soil moisture (SM) products since the start of data production on
March 31, 2015. Prior to launch, the mission defined a set of criteria
for core validation sites (CVS) that enable the testing of the key
mission SM accuracy requirement (unbiased root-mean-square error
<0.04 m3/m3). The
validation approach also includes other (“sparse network”) in situ SM
measurements, satellite SM products, model-based SM products, and field
experiments. Over the past six years, the SMAP SM products have been
analyzed with respect to these reference data, and the analysis
approaches themselves have been scrutinized in an effort to best
understand the products’ performance. Validation of the most recent SMAP
Level 2 and 3 SM retrieval products (R17000) shows that the L-band (1.4
GHz) radiometer-based SM record continues to meet mission requirements.
The products are generally consistent with SM retrievals from the ESA
Soil Moisture Ocean Salinity mission, although there are differences in
some regions. The high-resolution (3-km) SM retrieval product, generated
by combining Copernicus Sentinel-1 data with SMAP observations, performs
within expectations. Currently, however, there is limited availability
of 3-km CVS data to support extensive validation at this spatial scale.
The most recent (version 5) SMAP Level 4 SM data assimilation product
providing surface and root-zone SM with complete spatio-temporal
coverage at 9-km resolution also meets performance requirements. The
SMAP SM validation program will continue throughout the mission life;
future plans include expanding it to forested and high-latitude regions.