Correction of the Phase Bias in Short-term, Multi-looked Interferograms

Abstract

With the advent of the European Commissions Copernicus two-satellite Sentinel-1 constellation, operated by ESA, space-borne Synthetic Aperture Radar (SAR) Interferometry (InSAR) has become a key geophysical tool for surface deformation studies. Since the completion of the constellation in 2016, data are acquired globally with a typical revisit period of 12 days, and every 6 days in Europe. The relatively short revisit time (compared to 35-days of previous ESA SAR satellites) is a significant advance because interferograms spanning a short interval maintain better coherence and allow a more accurate estimate of rapid deformation. The short revisit time also leads to a greater number of acquisitions, which is useful for statistical reduction of the noise contribution (e.g., due to atmospheric phase delay) in InSAR time series analyses. Despite the clear benefits, it has recently been shown that using the shorter-interval, multilooked interferograms can introduce a bias (also referred to as a fading signal) in the interferometric phase, which, in turn, biases the estimated velocities. This poses a dilemma we would like to include all short-interval interferograms and to carry out multilooking because this improves coherence and hugely improves coverage, but the velocities obtained from using these interferograms alone are not reliable. We developed an empirical mitigation strategy for correcting the phase bias based on the assumption that the change in strength of the bias in interferograms of different length is a constant ratio. We tested the algorithm over different study areas with 6-day and 12-day acquisition patterns and in various land-covers. We also compared the estimated velocities in western Turkey against results from a phase linking approach that has been shown to be almost insensitive to the phase bias. Our corrected velocities agree well with those from phase linking approach. Correction of the short-term interferograms for the phase bias is of great importance, particularly for InSAR processing systems aimed at studying geohazards over large areas, including the COMET LiCSAR system, ESAs Geohazard Exploitation Platform (GEP), and NASAs Advanced Rapid Imaging and Analysis (ARIA) system.