The SMAP satellite, launched on 31st January 2015, was aimed to provide global high resolution soil moisture maps at 9km spatial resolution by downscaling 36 km L-band radiometer brightness temperature observations using 3 km L-band radar backscatter observations, then retrieving soil moisture from the resulting downscaled brightness temperature. However, due to failure of the radar, NASA focused on other available active microwave satellites and selected Sentinel-1A/1B SAR data as an alternative to the SMAP radar to produce the downscaled brightness temperature product. However, this meant combining L-band SMAP brightness temperature and C-band Sentinel backscatter rather than the L-band SMAP backscatter. Consequently, this study evaluated the impact of using C-band rather than L-band backscatter in the downscaling process using the airborne L-band backscatter and brightness temperature data collected from the fifth Soil Moisture Active Passive Experiment (SMAPEx-5) in south-eastern of Australia. Accordingly, the downscaled brightness temperature of using the C-band and L-band combination which NASA has produced, and the L-band combination using the airborne backscatter data, have been compared with the airborne brightness temperature observations also at L-band. The results show minor differences from using C-band backscatter in place of the intended L-band backscatter.

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