Millimeter ground deformation retrieving from high-resolution PAZ SAR images with a combined correction of unwrapping and long-wavelength errors: A case study in Alcoy, Spain

PAZ mission is an X-band synthetic aperture radar (SAR) satellite launched by Spain in February 2018, capable of routinely acquiring images with high spatio-temporal resolution. In this paper, we explore the potential of spotlight-mode (HS) PAZ images for monitoring small-scale ground deformation with millimeter-level accuracy, utilizing 21 HS PAZ images acquired over the Alcoy basin (SE Spain) between September 2019 and February 2021. Phase unwrapping and long-wavelength atmospheric delays significantly impede high-accuracy estimation of small-scale ground deformation in the study area. To address these issues, we first propose an approach for correcting phase unwrapping errors in interferograms by incorporating constraints from both spatial and temporal domains. Subsequently, we propose a block-based correction algorithm based on principal component analysis (PCA) to mitigate long-wavelength errors in the interferograms. Our results demonstrate that the proposed method can effectively eliminate long-wavelength errors in interferograms after both traditional phase-based method and GACOS corrections, reducing the standard deviation of the interferograms by up to a maximum of 68.4 %, and a value of 37.7 % on average. External validations from GPS and topographic measurements confirm that the deformation time series derived from the proposed method show an accuracy higher than 3 mm. This means millimeter-level precision is achieved in small-scale ground deformation measurements using PAZ SAR images. The analysis of the ground deformation derived from PAZ images reveals 41 active deformation areas in the Alcoy basin between September 2019 and February 2021, each that exhibits a deformation rate exceeding 5 mm/year. Through independent component analysis (ICA) and k-means clustering, we identify three distinct temporal evolution patterns corresponding to landslide activities, land subsidence, and land settlements. This study serves as a methodological blueprint for high-accuracy ground deformation estimation using high-resolution PAZ imagery, offering valuable complementary data to conventional medium-resolution SAR systems (e.g., Sentinel-1) in ground deformation monitoring applications.