Analysis of Very High Resolution X-BAND Persistent Scatterer Interferometry Data over Urban Areas
Cuevas-González, M.1; Crosetto, M.1; Monserrat, O.1; Devanthery, N.1; Crippa, B.2
1Institute of Geomatics, SPAIN; 2University of Milan, ITALY

Persistent Scatterer Interferometry (PSI) is a satellite-based SAR remote sensing technique used to measure and monitor land deformation from a stack of interferometric SAR images. This work concerns very high resolution X-band PSI, focusing on the technical aspects of deformation measurement and monitoring over urban areas. The advantages of X-band PSI are manifold. PSI offers wide-area coverage associated with a relatively high spatial resolution. This allows studying an entire metropolitan area, obtaining a global outlook of the deformation phenomena and maintaining, at the same time, the capability to measure individual structures and buildings. Another important advantage is its sensitivity to small deformations of the order of 1 mm/yr.

The thermal expansion (thermal dilation) component of PSI observations, which is a result of temperature differences in the imaged area between SAR acquisitions, is a key technical aspect to be analysed. This is an interesting feature of PSI, which can be surely used to illustrate the high sensitivity of X-band PSI to very subtle displacements. However, the thermal dilation has to be separated from the deformation, which is often the signal of interest. We will propose a strategy to deal with the thermal dilation phase component, which involves further developing the standard two-parameter PSI model (deformation velocity and residual topographic error) with a third unknown parameter called the thermal dilation parameter estimated for each analysed Persistent Scatterer (PS). The map obtained from plotting this parameter for all PSs of a given area is called thermal map and provides interesting information on the thermal behaviour of the observed scene, especially of buildings and infrastructures. This work will i) describe the proposed PSI estimation parameter procedure; ii) discuss the proposed thermal expansion model; iii) outline the issue of parameter estimability, which is a very important issue for any PSI deformation monitoring application in urban areas; iv) illustrate different types of thermal maps for the metropolitan area of Barcelona, making a preliminary comparison between TerraSAR-X and CosmoSkyMed results.

As stated above, thermal dilation can have a strong impact on the PSI products, especially on the deformation velocity maps and deformation time series, if not properly handled during the PSI data processing and analysis. A comprehensive discussion of this key aspect will be also provided. Its importance is related to the fact that the PSI analyses are often executed using limited stacks of images, which may cover a limited period of time, e.g. several months only. These two factors (limited number of images and short covered period) make the impact of a non-modelled thermal dilation particularly critical. This will be illustrated considering different case studies based on TerraSAR-X and CosmoSkyMed PSI data.

Finally, to overcome the problems related to the thermal dilation component of PSI observations, a new data processing strategy will be described. This approach can be used to overcome the above mentioned problems of parameter estimability, which may occur using limited stacks of images or stacks that cover a limited period of time, e.g. several months only. The basic idea is exploiting only the PSs that are not affected (or affected in a negligible way) by thermal dilation which, in a first approximation are most of the PSs located on the ground. The full procedure to exploit such PSs will be described step by step. The pros and cons of the proposed approach will also be discussed. Case studies from the metropolitan area of Barcelona will be used to illustrate the effectiveness of the proposed data analysis strategy.