Preliminary Study of Cramer-Rao Lower Bound for Subsurface Soil Moisture Estimation Using SoOp Reflectometry

Frequencies in the Very-High (VHF) to Ultra-High (UHF) range show potential for the remote sensing of soil moisture within the root-zone. This paper analyzes the Cramer-Rao Lower Bound (CRLB) for estimating soil moisture parameters using the SoOp Coherent Bistatic Scattering Model (SCoBi). CRLB defines the best achievable estimation variance for any unbiased estimator, hence allowing to identify optimal measurement configurations for soil moisture estimation. For different frequency, polarization and direction values SCoBi can model specular scattering surface reflection coefficients. Initial CRLB analysis are carried out using different combinations of a maximum of 120 measurements. The results indicate that surface soil moisture can reliably be measured while soil moisture values at 40 cm depth can be estimated within ± 4% accuracy if the surface and subsurface soil moisture is below 32.5% VSM. Its also shown that dual frequency measurements of soil moisture can greatly reduce the CRLB compared to using a single frequency.