PERMANENT SCATTERER INTERFEROMETRY: A NEW TOOL FOR GROUND DEFORMATION MONITORING

Interferometric Synthetic Aperture Radar (InSAR) is a remote sensing tool capable of measuring small displacements of the earth's surface, across large areas. InSAR technology emerged in the mid-90's, after the launch of the ESA satellite ERS-1 and ERS-2, although the proof of concept study dates back to the 80's and the pioneer work of JPL researchers on SEASAT data. InSAR data measure the displacement within an area of interest, along the line of sight of the radar beam, between two SAR images acquired at different times. With SAR systems operating in the microwave domain, phase shifts between pairs of images enabled displacements to be measured to millimeter accuracy. As experience in the use of InSAR progressed, it became evident that accuracy was often severely affected by the atmospheric contribution to phase shift. This led researchers to devise methods for identifying the atmospheric contribution and, in 1999, the Politecnico di Milano pioneered a multi-interferogram approach that identified, quantified and removed atmospheric distortions, leaving displacement as the only remaining contribution to signal phase shift. Described as the Permanent Scatterers Technique (PS Technique or PSInSAR), it has been used in tectonics and for assessing geotechnical hazards such as landslides, settlement and subsidence.

This paper discusses the use of PSInSAR data in monitoring landslides, subsidence and other terrain instability issues. The basic mathematical model is presented and the potential of the technology and its drawbacks are discussed. Extensive processing of thousands of SAR scenes has demonstrated how multi-temporal data-sets can be successfully exploited for terrain monitoring, by identifying objects on the landscape that have a stable, point-like behaviour. These objects, referred to as Permanent Scatterers (PS), can be geo-coded and monitored for movement very accurately, acting as a “natural” geodetic network. The paper presents examples of applications of monitoring landslides, settlement and subsidence, using experience in Italy, US and Canada, and concludes with a discussion on future directions for PSInSAR analysis, taking into account the new radar sensors to be launched in the near future.