The Impact of Closure Phases on InSAR Processing

Abstract

SAR interferometry is increasingly accurate in retrieving the surface deformation history over large areas of the Earth. Advancements have occurred in atmosphere delay compensation, both ionospheric and tropospheric: the remaining contributions of atmospheric propagation to the error budget are a few cm for deformation or a few mm/yr for deformation rates, for a typical Sentinel-1 case. We think that today a crucial aspect that needs consideration is the choice of interferograms to be processed to estimate the deformation. Many groups are limiting the processing to high-coherent pairs with short temporal separation: however, short temporal interferograms tend to be biased. This can be understood observing the presence of closure phases. Examining three subsequent images and building the closure phase, one typically founds inconsistencies in the order of a few degrees: scaling the error to the time span of a year the final deformation rate bias can easily reach several mm/yr. A possible physical interpretation of non-zero closure phase is related to moisture variations in semi-transparent media. The short term interferograms are biased because of the asymmetry of the moisture cycles combined with a non-linearity in the phase variations w.r.t. moisture. In our experiments with Sentinel-1 with a limitation of the temporal baseline at 72 days, we have observed biases of ~7 mm/yr on distributed targets. Here permanent scatterers served as the gold standard since their closure phase is always zero. We consider two main approaches to the closure phase problem in InSAR time series: using full-covariance estimators, based on the temporal covariance matrix, and modelling the underlying moisture process. Our preliminary experiences with full covariance matrices indicate a small bias (< 1 mm/yr) with respect to PS processing. Given the low-resolution of available moisture products, we are working on the inversion of moisture levels directly from closure phases. First L-band results are encouraging, compared to ground truth. If models and inversions are correct, the compensation of the modelled phase should reduce the path-dependency in the temporal integration and consequently the biases.