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

Abstract

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 pplications 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 ap- plicability of the same measurements principle to shorter wavelengths and exploitation of potential synergies with backscatter and polarimetric information.