Enhanced PSP SAR Interferometry Analysis of Structures and Natural Terrains: Method and Application to COSMO-SkyMed Data
Costantini, Mario; Minati, Federico; Trillo, Francesco; Vecchioli, Francesco
e-GEOS - an ASI/Telespazio company, ITALY

Synthetic aperture radar (SAR) interferometry is a powerful technology for measuring ground elevation and deformation due to landslides, subsidence, and volcanic or seismic phenomena. The persistent scatterer pair (PSP) approach was recently proposed [1] for the identification of persistent scatterers (PS) in series of full resolution SAR images, and the retrieval of the corresponding terrain height and displacement velocity.
In this work we will present a development of the PSP basic idea aimed at fully extracting the coherent information even from low intensity SAR signals. Basically, the proposed method is very effective at extracting the available information from each single pixel of the SAR image where you could expect a coherent signal, even when there are not strongly scattering structures and the expected signal is low, as in the case of rather smooth surfaces or natural terrains. We will show that an unprecedented high density of ground deformation measurements is obtained, and without the loss of resolution of distributed scatterer techniques [2].
The enhanced PSP procedure presented here exploits (according to the original PSP idea) only the relative properties of pairs of neighboring points ("arcs"). Differently from standard PS techniques ([3] and successive works), this makes the methods insensitive to spatially correlated signals such as atmospheric or orbital artifacts, and does not require model-based interpolations of a preliminary set of measurements. However, whereas the standard PSP algorithm was designed to analyze the minimum number of points and arcs necessary to identify most of the PS, the enhanced procedure, by means of an optimized search procedure more deeply explore all the available information, and makes possible to extract deformation and elevation information corresponding to practically all SAR image pixel where you could expect a coherent signal (also corresponding to smooth surfaces or natural terrains, provided that they do not change with time, like in the case of cultivations or dense vegetation).
As a matter of fact, SAR systems like COSMO-SkyMed, thanks to the short wavelength and the low noise level, provide a significant backscattering also from rather smooth surfaces or natural terrains. The sensitivity of the enhanced PSP approach allows extracting this signal and providing very dense ground deformation measurements also in these types of terrains.
The PSP method has been extensively validated and used for operational projects on low and high resolution SAR data over several sites covering most typologies of applications: from study of large areas affected by subsidence, landslides, earthquakes and volcanoes phenomena, up to monitoring of single structures, buildings, transport infrastructures, etc. The standard PSP analysis of COSMO-SkyMed himage stripmap data proved in several experiments its capability of providing ground deformation measurements with millimetric precision [4]. Moreover, the very good (metric) 3D localization of the measurement points allows distinguishing different displacements in the different parts of buildings and structures, which is essential for stability monitoring.
The enhanced PSP procedure presented in this work, while guaranteeing the same level of accuracy and resolution, is capable of providing more dense ground deformation measurements and, in particular, is able to much better cover areas not characterized by strong scatterers. A first example of this capability is given in Figure 1, where two sets of measurements obtained from the processing of a stack of 30 COSMO-SkyMed himage stripmap SAR images, acquired from February 2010 to February 2011, over the area of Naples, Italy (Piazza Plebiscito and Palazzo Reale, and a go-kart race), are shown. Very dense and precisely localized deformation measurements are found not only in correspondence of structures, but also over the pavements of the square and of the race track.
A second example, relative to the railway Padua - Venice, Italy, is shown in Figure 2, where the deformation measurements obtained by the standard and the enhanced PSP processing of a stack of 50 himage stripmap COSMO-SkyMed data., acquired in the period May 2009-Sep. 2011, are compared: the enhanced PSP analysis allowed selecting additional measurements (in red) with respect to the standard PSP analysis (yellow points). A measurement for almost each point of the image in correspondence of the railway is obtained (up to 200,000 PS/sqkm), allowing to characterize with continuity the structure.
The enhanced PSP technique is able to extract an amazing number of PS measurements in correspondence of scarcely vegetated and non cultivated areas. Figure 3 and Figure 4 show two examples relative to a copper mine in Russia. The average density of deformation (and elevation) measurements obtained with a stack of 30 himage stripmap COSMO-SkyMed SAR data, acquired in the period Apr. 2012 - Oct. 2012, is more than 10,000 PS/sqkm and locally it reaches more than 80,000 PS/sqkm. Note that no averages with loss of resolution were necessary to obtain the measurements over natural terrains.

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[1] 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, pp. 449-452, 2008.
[2] A. Ferretti, A. Fumagalli, F. Novali, C. Prati, F. Rocca, and A. Rucci, "Exploitation of distributed scatterers in interferometric data stacks," in Proc. Int. Geoscience Remote Sensing Symp. (IGARSS), Cape Town, South Africa, 2009.
[3] A. Ferretti, C. Prati, and F. Rocca, "Permanent scatterers in SAR interferometry," IEEE Trans. Geosci. Remote Sensing, vol. 39, no.1, pp. 8-20, 2001.
[4] M. Costantini, T. Chen, Y. Xu et al., "High Resolution Ground Deformations Monitoring by COSMO-SkyMed PSP SAR Interferometry: Accuracy Analysis and Validation," in ESA FRINGE Proceedings, Frascati, Italy, 2011.