Abstract
The National Aeronautics and Space Administration 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 m<sup>3</sup>/m<sup>3</sup>). The validation approach also includes other (“sparse network”) <i>in situ</i> 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 <i>L</i>-band (1.4 GHz) radiometer-based SM record continues to meet mission requirements. The products are generally consistent with SM retrievals from the European Space Agency 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.
Highlights
The National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) mission has produced global soil moisture (SM) measurements since March 2015 [1]
The SMAP validation strategy benefited from two earlier missions that had a considerable focus on the validation of SM products: the (Japan Aerospace Exploration Agency) JAXA AMSR-E (Advanced Microwave Scanning Radiometer-Earth Observing System) instrument launched by NASA on the Aqua satellite in 2002 ([6]), and the SMOS (Soil Moisture and Ocean Salinity) satellite launched by ESA (European Space Agency) in 2009 ([7])
The validation of six years of SMAP SM products demonstrate that they meet the accuracy requirements set for the mission
Summary
The National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) mission has produced global soil moisture (SM) measurements since March 2015 [1]. AMSR-E validation efforts spurred the development of locally dense observation networks with surface SM measurements in hydrologic research watersheds for SM validation at the footprint scale of these satellites (tens of kilometers) (e.g., [8],[9],[10],[11],[12],[13],[14]). This trend continued with SMOS (e.g., [15],[16],[17],[18],[19]). When SMAP was launched in 2015, there was already a significant infrastructure of locally dense networks in place with respect to the remote sensing footprint size, due to these earlier efforts and active international cooperation
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