Vegetation and soil moisture inversion from SAR closure phases: First experiments and results

The inversion of soil moisture from Synthetic Aperture Radar (SAR) closure phases is intrinsically plagued by ambiguities that affect the moisture order. This work shows a characterization of the ambiguities and a way to solve for them with the help of interferometric coherence. This allows to properly constrain the inversion and to retrieve the moisture signal. A data set of ALOS-2/PALSAR-2 L-band images is used as an example of successful inversion at the scene level, with sub-kilometer resolution. The results are validated with soil moisture products based on ASCAT and show a high degree of correlation. The raw moisture derived by the algorithm could be immediately used to correct SAR interferometric phases; however, for applications that need absolute moisture levels, a calibration step is likely necessary. Unexpectedly, a good performance was observed over forested areas, which suggests a sensitivity of closure phases to tree moisture; at the same time, over pastures and agricultural fields the closure phase signal was found relatively weak. Additional research is needed to evaluate the applicability of the same measurements principle to shorter wavelengths and exploitation of potential synergies with backscatter and polarimetric information.