Validation of the Permanent Scatterers Technique in Urban Areas

Most of the earth surface has been monitored systematically first by ERS-1, then by ERS-2 and it will be with the satellite ENVISAT, thus creating long and consistent series of data. In urban areas and where exposed rocks are visible, it is possible to identify numerous back scatterers that do not change their signature with time (the Permanent Scatterers) and therefore they can be used as natural monuments to estimate the progressive motion of the terrain. The precision of the measurement is a small fraction of a wavelength (5.6cm) and millimetric motions are appreciable with good reliability. The atmospheric contribution is rather smooth spatially and independent from take to take, so that it can be identified and removed from the data using a proper processing, provided that the density of the PS is high enough as it happens in urban areas. Then, it is possible to obtain maps of subsidence with very high spatial sampling rate (more than 50 PS/km2, in urban areas) and high quality. The sub-millimetric accuracy of this technique has been checked by means of a comparison with GPS measurements where available and by cross-correlating dilation of metallic structures in Paris and Milano with temperatures over a six years period. The PS technique has been patented at POLIMI on May 1999 [1]. It has been validated by means of ERS SAR data and the main results have been published in the last two years by the same authors [2-8]. Another independent group has exploited the PS technique for scientific purposes recognizing its advantages with respect to the conventional DINSAR technique [9]. In this paper only the last main results of the PS technique achieved by means of ERS-1 and ERS-2 repeated observations are reported.