The Sensitivity of InSAR Closure Phase to Spatial Variations of Soil Structure and Moisture as Revealed by FDTD Simulations

Large scale monitoring of soil moisture is important for environmental systems and agriculture, with both optical and synthetic aperture radar (SAR) remote sensing having become methods of choice for repeatably imaging the Earth’s surface. Besides SAR backscatter, the repeat pass interferometric SAR (InSAR) phase is also sensitive to soil moisture changes and has been proposed as an observable for soil moisture retrieval algorithms. The phase loop sum between three interferometric SAR images, called closure phase, is of particular interest for soil moisture retrieval due to its insensitivity to topography and atmospheric changes. A specialized 3-D finite-difference time-domain simulation tool is used to simulate SAR pixels containing a variety of different soil structures. Soil parameters such as inhomogeneity size, moisture gradients and surface roughness are investigated, and the observed closure phase is compared against the predicted results from an analytical model. Further, the sensitivity of closure phase to changes in soil moisture is compared for each of the soil structures under investigation. Finally, we construct a simple moisture regression problem and show that including polarimetry can enable the regression to solve for soil structure properties.