Estimating Soil Moisture from SAR Interferometry with Closure Phases

Abstract

Since soil moisture influences the differential interferometric synthetic aperture radar phase, it would be advantageous to be able to use InSAR to estimate the moisture. Unfortunately, it hasn’t yet been demonstrated how to accomplish this in a precise and robust manner. Two major problems exist: firstly the relationship between moisture and phase variations is not yet well understood for a variety of ground covers. Secondly, various terms contribute to the interferometric phase and the soil moisture one is mostly one of the smallest, meaning that it is difficult to separate it from the other contributions and provide a reliable estimation. To address the second problem we use triplet of interferograms, canceling out all phase terms other than soil moisture. Closure phases, the triplet’s product, depend on soil moisture variations with a non-linear ambiguous relationship. We show that its inversion is feasible once the ambiguity is solved. We use ALOS-2 and UAVSAR data to estimate soil moisture using a simple physical model.