spurious signal can appear completely unrelated to the surface features of the image, however, in other cases the atmospheric phase delay is caused by vertical inhomogeneity at low altitudes and this may result in fringes appearing to correspond with the topography. Persistent scatterer InSAR Persistent or permanent scatterer techniques are a relatively recent development from conventional InSAR, and rely on studying pixels which remain coherent over a sequence of interferograms. In 1999, researchers at Politecnico di Milano, Italy, developed a new multi-image approach in which one searches the stack of images for objects on the ground providing consistent and stable radar reflections back to the satellite. These objects could be the size of a pixel or, more commonly, sub-pixel sized, and are present in every image in the stack. That specific implementation is patented. Some research centres and companies, were inspired to develop variations of their own algorithms which would also overcome InSAR's limitations. In scientific literature, these techniques are collectively referred to as persistent scatterer interferometry or PSI techniques. The term persistent scatterer interferometry (PSI) was proposed by European Space Agency (ESA) to define the second generation of radar interferometry techniques. This term is nowadays commonly accepted by scientific and the end user community. Commonly such techniques are most useful in urban areas with many permanent structures, for example the PSI studies of European geohazard sites undertaken by the Terrafirma project. The Terrafirma project provides a ground motion hazard information service, distributed throughout Europe via national geological surveys and institutions. The objective of this service is to help save lives, improve safety, and reduce economic loss through the use of state-of-the-art PSI information. Over the last 9 years this service has supplied information relating to urban subsidence and uplift, slope stability and landslides, seismic and volcanic deformation, coastlines and flood plains. Producing interferograms The processing chain used to produce interferograms varies according to the software used and the precise application but will usually include some combination of the following steps. Two SAR images are required to produce an interferogram; these may be obtained pre-processed, or produced from raw data by the user prior to InSAR processing. The two images must first be co-registered, using a correlation procedure to find the offset and difference in geometry between the two amplitude images. One SAR image is then re-sampled to match the geometry of the other, meaning each pixel represents the same