  Microwave remote sensing offers a powerful tool for monitoring the growth of
short, dense vegetation like soybean. As the plants mature, changes in their
biomass and 3-D structure impact the electromagnetic (EM) backscatter signal.
This backscatter information holds valuable insights into crop health and
yield, prompting the need for a comprehensive understanding of how structural
and biophysical properties of soybeans as well as soil characteristics
contribute to the overall backscatter signature. In this study, a full-wave
model is developed for simulating L-band backscatter from soybean fields.
Leveraging the ANSYS High-Frequency Structure Simulator (HFSS) framework, the
model solves for the scattering of EM waves from realistic 3-D structural
models of soybean, explicitly incorporating the interplant scattering effects.
The model estimates of backscatter match well with the field observations from
the SMAPVEX16-MicroWEX and SMAPVEX12, with average differences of 1-2 dB for
co-pol and less than 4 dB for cross-pol. Furthermore, the model effectively
replicates the temporal dynamics of crop backscatter throughout the growing
season. The HFSS analysis revealed that the stems and pods are the primary
contributors to HH-pol backscatter, while the branches contribute to VV-pol,
and leaves impact the cross-pol signatures. In addition, a sensitivity study
with 3-D bare soil surface resulted in an average variation of 8 dB in co- and
cross-pol, even when the root mean square height and correlation length were
held constant.
