Comparison of HH and VV Polarizations for Deformation Estimation using Persistent Scatterer Interferometry

摘要

Persistent Scatterer Interferometry technique (PSI) exploits time series of interferometric SAR data to estimate deformation over an area. The process involves analysing stable phase pixels from differential interferograms formed with respect to a single master. In this study, eight RADARSAT-2 C-band fine quad polarization images from 2014 to 2016 are used to identify Persistent Scatterer (PS) points and estimate deformation over Vijayawada city in India. With a full graph approach, 29 interferograms are generated. All the interferograms are unwrapped to aid stable deformation estimation. The prime objective of our research is to study the effect of HH and VV polarizations in PS point selection and deformation estimation. Initially, pixels with Amplitude Stability Index > 0.75 are selected as PS points. Later, PS pixels exhibiting temporal coherence > 0.75 and reflectivity > 1 are selected for deformation estimation. Before refinement, PS points for HH and W polarization are identified as 49409 and 48395 respectively, whereas after refinement, they are about 24063 and 34389. Atmospheric Phase Screen (APS) is estimated and removed using a filtering approach. After APS removal, mean line of sight (LOS) displacement and velocity maps are generated. A linear model is used to invert displacements into velocities. Velocity obtained using HH polarized data ranges from+ 16 to -14 mm/year, and for VV it ranges from+12 to -19 mm/year. The deformation trend is linear throughout the observation period except in October 2014. Deformation around this month shows up and down pattern. We attribute this observation to the devastating Hudhud cyclone that hit the state of Andhra Pradesh in October 2014. Atmospheric variations due to the presence of strong troposphere component varied radar signal delay before and after the cyclone. Excluding this phenomenon, time series plots of selected points showed linear deformation pattern in both polarizations.