  The ESA's SMOS and the NASA's SMAP missions, launched in 2009 and 2015,
respectively, are the first two missions having on-board L-band microwave
sensors, which are very sensitive to the water content in soils and vegetation.
Focusing on the vegetation signal at L-band, we have implemented an inversion
approach for SMAP that allows deriving vegetation optical depth (VOD, a
microwave parameter related to biomass and plant water content) alongside soil
moisture, without reliance on ancillary optical information on vegetation. This
work aims at using this new observational data to monitor the phenology of
crops in major global agro-ecosystems and enhance present agricultural
monitoring and prediction capabilities. Core agricultural regions have been
selected worldwide covering major crops (corn, soybean, wheat, rice). The
complementarity and synergies between the microwave vegetation signal,
sensitive to biomass water-uptake dynamics, and optical indices, sensitive to
canopy greenness, are explored. Results reveal the value of L-band VOD as an
independent ecological indicator for global terrestrial biosphere studies.
