Ground deformations monitoring by persistent scatterer pairs (PSP) SAR interferometry

Synthetic aperture radar (SAR) interferometry is a powerful technology for measuring slow terrain movements due to landslides, subsidence, and volcanic or seismic phenomena. The extraction of this information is a complex task, because the phase of the signal is measured only modulo 2π and is affected by noise and systematic terms. The persistent scatterer (PS) approach brought important advances in the solution of this problem.
A new approach named persistent scatterer pairs (PSP) was recently developed for the identification of persistent scatterers in series of full resolution SAR images, and the retrieval of the corresponding terrain height and displacement velocity (M.Costantini, S.Falco, F.Malvarosa, F.Minati, “A new method for identification and analysis of persistent scatterers in series of SAR images,” in Proc. Int. Geosci. Remote Sensing Symp. (IGARSS), Boston MA, USA, 7-11 July 2008, pp. 449-452). The PSP technique overcomes problems related to the presence of atmospheric and orbital artifacts in the signal by exploiting only the relative properties of neighboring points both for identification and analysis of PS, thus removing the need for model-based interpolations starting from a preliminary set of measurements obtained by radiometric or low resolution analyses. It does not require data calibration or pre-selection of radiometrically stable points, thus being robust to errors in the pre-selection phase and to density of pre-selected points.
The PSP method is characterized by the exploitation of redundant information, which makes for a very good robustness to noise. Among other things, a new technique for robust finite-difference integration and phase unwrapping (which are key problems in SAR interferometry) was recently published. It uses redundant differential estimates (not only between nearest neighboring points) and multi-dimensional information (e.g. multitemporal, multi-frequency, multi-baseline), or external data (e.g. GPS measurements) (M.Costantini, F.Malvarosa, F.Minati, “A general formulation for robust and efficient integration of finite differences and phase unwrapping on sparse multidimensional domains,” in Proc. ESA Fringe 2009 Workshop (ESA SP-677, also at http://earth.eo.esa.int/workshops/fringe09), Frascati, Italy, Nov.-Dec. 2009).
The PSP method has been extensively tested on real SAR data. The obtained results show that the proposed approach is very effective. In particular, it is expected to obtain a higher density of persistent scatterer measurements than previous techniques, at least in the cases where the atmospheric artefacts are not very well described by the models used in standard approaches. Moreover, the method proved to be very robust to noise and disturbances.
In this work we analyze the qualifying characteristics of the PSP method and show the results obtained in a large sample of cases, and, in particular, the results obtained with high resolution COSMO-SkyMed SAR images. Thanks to the high resolution, the density of persistent scatterers increases dramatically, resulting in more detailed and accurate measurements by means of SAR differential interferometry techniques, which is particularly important in monitoring of infrastructures. Moreover, the COSMO-SkyMed constellation can provide an unprecedented frequency of interferometric observations, which allows monitoring faster movements.