  Soil moisture impacts exchanges of water, energy and carbon fluxes between
the land surface and the atmosphere. Passive microwave remote sensing at L-band
can capture spatial and temporal patterns of soil moisture in the landscape.
Both ESA and NASA have launched L-band radiometers, in the form of the SMOS and
SMAP satellites respectively, to monitor soil moisture globally, every 3-day at
about 40 km resolution. However, their coarse scale restricts the range of
applications. While SMAP included an L-band radar to downscale the radiometer
soil moisture to 9 km, the radar failed after 3 months and this initial
approach is not applicable to developing a consistent long term soil moisture
product across the two missions anymore. Existing optical-, radiometer-, and
oversampling-based downscaling methods could be an alternative to the
radar-based approach for delivering such data. Nevertheless, retrieval of a
consistent high resolution soil moisture product remains a challenge, and there
has been no comprehensive inter-comparison of the alternate approaches. This
research undertakes an assessment of the different downscaling approaches using
the SMAPEx-4 field campaign data
