Phase Optimization of Multi-looked Polarimetric Interferometric SAR Data: Introductory Experiments with Dual-pol Sentinel-1 SAR Images

Abstract

The exploitation of sequences of Synthetic Aperture Radar (SAR) images for the generation of ground surface deformation measurements represents nowadays a consolidated practice and is helpful for the analysis of several natural and man-made phenomena. Over the years, several multi-temporal Interferometric synthetic aperture radar (MT-InSAR) techniques have been developed and successfully applied, mostly for the analysis of independently processed single-pol SAR datasets. To mitigate the noise effects in the differential SAR interferograms, several noise-filtering techniques have been proposed, most of them working independently on single interferograms. Subsequently, new efforts have been made to extend these methods to include space-time information and improve the reliability of ground displacement measurements related to permanent scatterers (PS), coherent distributed scatterers (DS), and partially coherent scatterers. In the framework of the methods addressing the DS targets signal characterization and study, the Small Baseline Subset (SBAS), the Phase Triangulation algorithm (PTA), CAESAR, and other alternative multi-temporal approaches have been proposed in the literature almost over the last decade. In this context, an improved SBAS processing chain, which helped to considerably increase the deformation time-series retrieval capability of the original SBAS procedure and increase the number of detectable DS targets in mid-to-low coherent areas, was originally proposed in 2015. The research was developed by a group of scientists of IREA-CNR and benefited from the conscientiousness, competence, patience, and time of Mariarosaria Manzo, who was one of the co-authors of the work. The method complemented the extended minimum cost flow (EMCF) space-time phase unwrapping operations with an additional processing step that allowed an increase in the signal-to-noise ratio of a set of multi-temporal multi-looked interferograms by computing a set of optimized phases. The growing availability of SAR data regularly collected by the available SAR constellations, such as the EU Copernicus Sentinel-1 sensors, and the plan of new constellations with improved spatial resolutions and revisiting times as well as with the potential to collect data at different polarizations (dual-pol/full-pol data) fosters the development/adaptation of the existing methods to include in a unique framework these space-time approaches and the polarimetric InSAR (dual-pol/full-pol) techniques. The aim is to consider the different polarization channels to maximize the coherence of the generated interferograms considering spatial, temporal, and polarimetric information. Some preliminary tests have been run using a set of dual-pol Sentinel-1A SAR data in heterogeneous (urban/rural) regions. The joint exploitation of co- and cross-polar data, as well as of full-pol data, which can be included in this framework, is also valuable for better discerning between the different sources of information.