SPACEBORNE SAR ANATOMY OF A CITY

The ensemble of st able rada r targets (the so-called Permanent Scatterers -PS-, [1], [2]) in urban area can be interpreted as the radar skeleton of a city. By means of the PS technique, developed at POLIMI, the skeleton radar reflectors are exploit ed for overcoming the difficulties related to the conventional S AR interferometry approach (phase decorrelation and atmospheric effects), achieving millimetre accuracy in monitoring possible relative displacements. Even if the PS technique is an operational tool since 2000, the physical nature of the targets is still a subject of investigation. In this work we present the last results of this research program obtained by using 120 ERS and Envisat repeated acquisitions on the area of Milano (Italy). First, the PS’s posi tion and their elevation with respect to the ground are estimated with high precision. Then the scattering pattern of targets as a function of their geometrical characteristics (dimensions and orientation) is analysed. The variation of the Radar Cross Section as a function of the acquisition geometry (normal baseline and Doppler centroid) is linked to the extension of the scatterer in range and azimuth. Temperature dependent amplitude variations have been exploited for individuating retro-reflecting periodic structures (Bragg scattering). Exploiting the auto-interferogram generated from an Envisat Alternating Polarization acquisition, odd and even bounces of radar echoes are identified thus separating dihedrals from spec ular or trihedral reflectors [8]. Finally, by means of the collected radar measurements, 5 principa l PS typologies have been identified: ground retro-reflectors (gratings), elevated retro-reflectors (roofs), dihedrals, poles and trihedrals.