Exploitation of GNSS for calibrating space-borne SAR for the study of landsubsidence

Nowadays advanced multi-temporal interferometric approaches such as PSI (Persistent Scatterers Interferometry) derived from the processing of space-borne SAR (Synthetic Aperture Radar) images represent an effective tool to detect terrain movements and provide millimetric ground measurements over large scenes thanks to their wide-area coverage, non-invasiveness, and high accuracy. Nevertheless, PSI data lack of absolute reference both in time and space, as they are relative estimates measured along the sensors-to- target line of sight and referred to a chosen stable motionless reference point. In this work, a methodology to fix relative InSAR results into conventional geodetic reference systems through calibration with GNSS (Global Navigation Satellite System) data acquired from permanent stations is proposed. In particular, mean yearly velocities of PSI radar benchmarks are corrected with GNSS values by means of procedures commonly used in geodesy for combining crustal and local deformation studies. The operative method is tested in the area of Ravenna and Ferrara cities on the north-western Adriatic coast within the eastern alluvial plain of Po River, extensively affected by subsidence with strong spatial and temporal variations. The outcomes reveal the usefulness of the presented methodology for generating unique ground deformation maps over wide are using geodesy for aligning PSI data before SAR maps stacking.