The SMAP and Copernicus Sentinel 1A/B microwave active-passive, high-resolution surface soil moisture product

Authors: DAS, N. - Jet Propulsion Laboratory; ENTEKHABI, D. - Massachusetts Institute Of Technology; DUNBAR, R.S. - Jet Propulsion Laboratory; CAUBELL, M. - Jet Propulsion Laboratory; COLLIANDER, A - Jet Propulsion Laboratory; YUEH, S. - Jet Propulsion Laboratory; JAGHUBER, T. - Goddard Space Flight Center; CHEN, F. - US Department Of Agriculture (USDA); Crow, Wade; O'NEIL, P.E. - Goddard Space Flight Center; WALKER, J. - Monash University; BERG, A. - University Of Guelph; Bosch, David; CALDWELL, T. - University Of Texas; Cosh, Michael; Holifield Collins, Chandra; LOPEZ-BAEZA, E. - University Of Valencia; THIBEAULT, M. - Universidad De Buenos Aires

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2019
Publication Date: 9/3/2019
Citation: Das, N., Entekhabi, D., Dunbar, R., Caubell, M., Colliander, A., Yueh, S., Jaghuber, T., Chen, F., Crow, W.T., O'Neil, P., Walker, J., Berg, A., Bosch, D.D., Caldwell, T., Cosh, M.H., Holifield Collins, C.D., Lopez-Baeza, E., Thibeault, M. 2019. The SMAP and Copernicus Sentinel 1A/B microwave active-passive, high-resolution surface soil moisture product. Remote Sensing of Environment. https://doi.org/10.1016/j.rse.2019.111380.
DOI: https://doi.org/10.1016/j.rse.2019.111380

Interpretive Summary: Accurate measurements of surface soil moisture are valuable for a wide range of agricultural applications including: irrigation scheduling, crop yield forecasting, drought assessment and fertilizer management. In January 2015 NASA launched the Soil Moisture Active/Passive (SMAP) satellite mission with the goal of improving our ability to globally measure surface soil moisture from space. A shortcoming in past satellite-based soil moisture products has been their coarse spatial resolution (typically > 30 km) which limits their value for certain agricultural applications. Therefore, a critical goal of the SMAP mission is the production of higher-resolution soil moisture products via the merger of high-resolution (active) radar observations with lower-resolution (passive) microwave radiometry. Using radar data acquired from the European Space Agency's Sentinel-1 satellites, this paper describes the production and validation of a high-resolution (1-km) SMAP surface soil moisture product that enhances our ability to monitor field-scale soil moisture patterns in agricultural regions.

Technical Abstract: Soil Moisture Active Passive (SMAP) mission of NASA was launched in January 2015. Currently, SMAP has an L-band radiometer and a defunct L-band radar with a rotating 6-meter mesh reflector antenna. On July 7th, 2015, the SMAP radar malfunctioned and became inoperable. Consequently, the production of high-resolution active-passive soil moisture product got hampered, and only ~2.5 months (April 15th, 2015 to July 7th, 2015) of data remain available. Therefore, during the SMAP post-radar phase, many ways were examined to restart the high-resolution soil moisture product generation of the SMAP mission. One of the feasible approaches was to substitute the SMAP radar with other available SAR data. Sentinel-1A/Sentinel-1B SAR data was found most suitable for combining with the SMAP radiometer data because of its nearly similar orbit configuration that allows overlapping of their swaths with a minimal time difference, a key feature/requirement for the SMAP active-passive algorithm. The Sentinel interferometric wide swath (IW) mode acquisition also provides the co-polarized and cross-polarized observations required for the SMAP active-passive algorithm. However, some differences do exist between the SMAP and Sentinel SAR data. They are mainly: 1) Sentinel has a C-band SAR whereas SMAP operates at L-band; 2) Sentinel has multiple incidence angles within its swath, and SMAP has one single incidence angle; and 3) Sentinel swath width is ~300 km as compared to SMAP with 1000 km swath width. On any given day, the narrow swath width of the Sentinel observations significantly reduces the overlap spatial coverage between SMAP and Sentinel as compared to the original SMAP radar and radiometer swath coverage. Hence, the temporal resolution (revisit interval) suffers due to narrow overlapped swath width and degrades from 3 days to 12 days. One advantage of using very high-resolution resolution Sentinel-1A/Sentinel-1B data in the SMAP active-passive algorithm is the potential of obtaining the disaggregated brightness temperature and thus soil moisture at a much finer spatial resolution of 3 km and 1 km at global extent. The assessment of high-resolution product at 3 km and 1 km using the soil moisture calibration and validations sites shows reasonable accuracy of ~0.05 m3/m3. The SMAP-Sentinel active-passive high-resolution product is now available to the public (new version released in October 2018) through NSIDC (NASA DAAC). The duration of this product is from April 2015 to current date