Environmental effects on brightness temperature observation from an L-band radiometer

Measuring the microwave brightness temperature (TB) using a radiometer is important for estimating soil moisture (SM). However, no study has demonstrated the effect of environmental conditions on these measurements. With this technology being explored for use in precision agriculture, indoor application and/or utilization in environments close to buildings becomes a certain reality. Therefore, an experiment was conducted in a warehouse of concrete and steel construction to investigate this issue. An L-band microwave radiometer known as ELBARA III was used to measure the surface TB over a soil box at different incidence angles and moisture contents. An environmental correction equation was applied to offset the effect of the indoor environment on the TB observed by the sensor. Accordingly, the effect of the TB environment on the TB observations at different incidence angles and moisture contents were analyzed. The environment correction equation provided a substantial improvement in estimating the direct TB emitted from the soil relative to model estimates with a reduction in root-mean-squared error (RMSE) from 57 K to 4 K. Overall, the results demonstrated that the built environment had a substantial influence on the TB observed by the sensor, and that it was not possible to directly use indoor measurements for reliable SM retrieval. Use of the environment correction equation offers inspiration for SM retrieval from indoor measurements, but further studies are required. This result provides an early glimpse into the ability of microwave radiometers for SM monitoring in indoor environments.