Multilayer Ionosphere Correction in Low-Frequency Interferometric SAR: Modeling, Inversion, and Application to BIOMASS

Early analysis of BIOMASS interferometric data acquired during the commissioning phase at Boreal and Antarctic latitudes revealed the presence of strong ionospheric disturbances that cause severe coherence losses. This phenomenon was investigated by studying the variation of the interferometric phase with respect to the squint angle. Results are consistent with the assumption of a complex vertical structure of the ionospheric phase, characterized by fast azimuth variations occurring at Km or sub-Km scale at multiple layers between the ground and the satellite orbit. To the best of our knowledge this phenomenon has never been considered in Synthetic Aperture Radar literature. This paper is therefore intended to present and discuss the results of our investigation on suitable correction methods needed to guarantee the generation of accurate geophysical products from BIOMASS interferometric data. The starting point of our analysis is a multi-layered model of the ionospheric phase as observed by varying the Radar Line of Sight (LoS) across different squint angles. The model is analyzed from a theoretical perspective to discuss the sensitivity of BIOMASS to ionospheric layering. Next, an inversion procedure is presented to estimate the ionospheric phase at an arbitrary number of layers directly from the complex coherence. The third element of our research consists in the formulation of correction methods to restore data quality given the estimated single-or multi-layer ionospheric phase. All aspects of the research are consistently supported by analysis of BIOMASS interferometric data acquired at different latitudes and affected by mild to very strong ionospheric disturbances. Results demonstrate that the methodologies presented in this paper are capable of compensating for the observed ionospheric effects in all cases analyzed so far.