Downscaling SMAP Brightness Temperatures to 3 km Using CYGNSS Reflectivity Observations: Factors That Affect Spatial Heterogeneity  
[PERSON]  
1 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA1 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA1 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA1 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA1 Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, USA11 While a variety of remotely sensed NSSM products are available, they generally have either fine spatial resolution or fine temporal resolution, but not both. Microwave radiometers, like SMAP and Soil Moisture and Ocean Salinity (SMOS) [5], retrieve NSSM at coarse spatial resolution (\(\sim\)40 km) and fine temporal resolution (\(\sim\)2-3 days). Microwave synthetic aperture radar (SAR), like Sentinel-1 [6] and the upcoming NASA-ISRO SAR (NISAR) mission [7], retrieve NSSM at fine spatial resolution (dozens of meters to 1 km) but coarse temporal resolution (\(\sim\)6-12 days). There is still a need for NSSM products with both fine spatial scale and fine temporal scale.  
We developed an algorithm to downscale SMAP TBs using Cyclone Global Navigation Satellite System (CYGNSS) reflectivity data. The purpose of downscaling SMAP TBs is to represent the spatial heterogeneity of TB at a finer scale than possible via passive microwave data alone, with the end goal of developing a 3 km NSSM product. In this paper, we discuss the SMAP/CYGNSS TB downscaling algorithm, including the complexities involved in collocating SMAP and CYGNSS data. We also investigate the factors that affect the spatial variability of downscaled SMAP/CYGNSS TBs and analyze which types of landscapes produce the most and least spatially heterogeneous TBs. Finally, we discuss the primary sources of uncertainty associated with downscaled SMAP/CYGNSS TBs. Deriving NSSM from SMAP/CYGNSS TBs will be addressed in a separate paper.