Polarimetric Persistent Scatterer Interferometry for Ground Deformation Monitoring with VV-VH Sentinel-1 Data

Abstract

With the launch of the Sentinel-1 satellites, it becomes easy to obtain long time-series dual-pol (i.e., VV and VH channels) SAR images over most areas of the world. By combining the information from both VV and VH channels, the polarimetric persistent scatterer interferometry (PolPSI) techniques is supposed to achieve better ground deformation monitoring results than conventional PSI techniques (using only VV channel) with Sentinel-1 data. According to the quality metric used for polarimetric optimizations, the most commonly used PolPSI techniques can be categorized into three main categories. They are PolPSI-ADI (amplitude dispersion index as the phase quality metric), PolPSI-COH (coherence as the phase quality metric), and PolPSI-AOS (taking adaptive optimization strategies). Different categories of PolPSI techniques are suitable for different study areas and with different performances. However, the study that simultaneously applies all the three types of PolPSI techniques on Sentinel-1 PolSAR images is rare. Moreover, there has been little discussion about different characteristics of the three types of PolPSI techniques and how to use them with Sentinel-1 data. To this end, in this study, three data sets in China have been used to evaluate the three types of PolPSI techniques’ performances. Based on results obtained, the different characteristics of PolPSI techniques have been discussed. The results show that all three PolPSI techniques can improve the phase quality of interferograms. Thus, more qualified pixels can be used for ground deformation estimation by PolPSI methods with respect to the PSI technique. Specifically, this pixel density improvement is 50%, 12%, and 348% for the PolPSI-ADI, PolPSI-COH, and POlPSI-AOS, respectively. PolPSI-ADI is the most efficient method, and it is the first choice for the area with abundant deterministic scatterers (e.g., urban areas). Benefitting from its adaptive optimization strategy, PolPSI-AOS has the best performances at the price of highest computation cost, which is suitable for rural area applications. On the other hand, limited by the medium resolution of Sentinel-1 PolSAR images, PolPSI-COH’s improvement with respect to conventional PSI is relatively insignificant.

Assessment of the Contribution of Polarimetric Persistent Scatterer Interferometry on Sentinel-1 Data

“…We first review in this section the contribution of this work with respect to previous PolPSI studies with S1 data [20]- [22]. The general findings about the increase in pixel density, and hence spatial coverage of the deformation results, are in agreement with the results already published.…”

“…Regarding the deformation results provided by the optimised polarimetric channel, we have found that the increased number of pixels helps find new active deformation areas which are missed by the VV data. In addition, as it was also shown in [22], the deformation rate can be estimated more accurately thanks to the increased pixel density. In our tests, this was especially helpful in the case of Alcoy, since this area exhibited the lowest density of PS among all test sites.…”

“…The projection vector ω can be expressed in many forms. The first version was introduced in [24], which has been used in most PolPSI studies [7], [8], [22],…”

“…However, since the launch of several satellites with extended polarimetric capabilities in late 2000's, such as ALOS-PALSAR, Radarsat-2, COSMO-SKYMED, and TerraSAR-X, some efforts were conducted to study how the additional polarimetric channels measured by them could help improve the performance of PSI. For that purpose, Polarimetric PSI (PolPSI) approaches have been proposed and evaluated with dual-pol TerraSAR-X data [7]- [14], dual-pol ALOS-PALSAR [14], quad-pol Radarsat-2 data [13], [15]- [19], and also recently with dual-pol S1 data [20]- [22].…”

“…In the end, an increase of pixel density was found, together with an overall temporal coherence better than the one provided by the VV channel alone. More recently, the PolPSI optimisations of amplitude dispersion, average coherence, and a spatially adaptive method, were evaluated with S1 data in three different sites of China [22]. An improved performance was obtained in terms of pixel density and less noisy interferometric phase, which led to more detailed and reliable deformation maps.…”