Permanent Scatterers technology: a powerful state of the art tool for historic and future monitoring of landslides and other terrain instability phenomena A. Ferretti Tele-Rilevamento Europa – T.R.E.S.r.l. A. Prati & F. Rocca Dipartimento di Elettronica e Informazione – Politecnico di Milano N. Casagli & P. Farina Dipartimento di Scienze della Terra – Universita’ di Firenze B. Young BRIAN YOUNG consulting ABSTRACT: This paper discusses the use of multiple image sets of SAR data acquired by past and current satellite platforms in monitoring landslides and other terrain instability issues. The basic mathematical model is presented, highlighting the critical parameters of interferometry in geological and geotechnical applications. The potential of the technology and its drawbacks related to availability of long temporal series of SAR acquisitions are also discussed. Extensive processing of many 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 and Canada, and concludes with a discussion on future directions for InSAR. 1. INTRODUCTION Landslides represent one of the most diffuse natural hazards in many parts of the world, threatening and influencing the socio-economic conditions of many countries (Schuster, 1996). Due to the difficulties of putting countermeasures into effect in terms of mitigation works, a deep knowledge of landslide distribution and state of activity is required, especially for those situations where property and infrastructure are exposed. Conventional methods used for detecting and monitoring slope instability could benefit from the use of remote sensing systems due to the rapid and easily updatable acquisitions of data over wide areas, which reduce both field work and costs (Soeters and Van Westen, 1996). Recent advances in optical and radar imagery capabilities, e.g. high spatial resolution, stereoscopic acquisition and high temporal frequency acquisitions; the development of new robust techniques based on the interferometric analysis of radar images, such as the Permanent Scatterers Technique (Ferretti et al., 2001), and the possibility of integrating these data within a Geographical Information System (GIS) have dramatically increased the potential of remote sensing for landslide investigations (Farina et al, in review). This paper addresses the use of interferometric methods for measuring ground movement. Interferometric Synthetic Aperture Radar (InSAR) is a remote sensing tool capable of measuring small displacements of the earth’s surface, across large areas. The first generation of InSAR technology emerged in the mid-90’s and was referred to as Differential InSAR, or DInSAR. It measured the displacement within an area of interest (AOI), along the line of sight of the radar beam, between two points in time represented by two SAR images, or scenes. 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 DInSAR progressed, it became evident that accuracy was often severely affected by the atmospheric contribution to phase shift and, at that time, the ability to remove it did not exist. 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