The PSIG procedure is a new approach to Persistent Scatterer Interferometry (PSI), which is implemented in the in-house PSI chain of the Geomatics Division of the CTTC. The PSIG procedure has been successfully tested over urban, rural and vegetated areas using X-band SAR data. This paper briefly describes the main steps of the procedure, mainly focusing on the two key processing steps of the approach. The first one is a selection of Persistent Scatterers (PS) consisting in a candidate Cousin PS (CPS) selection based on a phase similitude criteria that allows a correct phase unwrapping and a phase unwrapping consistency check. The second key element is a 2+1D phase unwrapping algorithm, which consists in a 2D phase unwrapping followed by a 1D phase unwrapping that allows the detection and correction of unwrapping errors. The results of the CPS selection and the 2+1D phase unwrapping obtained using a stack of 28 TerraSAR-X StripMap images over the metropolitan area of Barcelona are shown.