The Permanent or Persistent Scatterer (PS) approach allows the estimation of deformation time-series related to point-wise, high coherent scatterers on the ground based on processing long sequences of SAR data.
Persistent Scatterer Interferometry (PSI -sometimes also called Permanent Scatterer Interferometry) is a particular DInSAR technique. It exploits multiple SAR images acquired over a specific area in order to retrieve the deformation phase component over time. In general, a minimum number of 15 SAR acquisitions is needed for PSI processing. Due to the large number of necessary acquisitions, the deformation component of the interferometric phase observations can be estimated very precisely (in the order of a few mm/yr) and other phase contributions such as atmospheric disturbances and topographic height differences can be better estimated and removed.
PSI rely on so called Persistent Scatterer that are targets showing coherent phase behavior in time. Such targets are usually found on man-made structures such as buildings or bridges, or very stable features such as rocks. PSI is a technique that is therefore mainly used over urban or semi-urban terrain. Usually, PSs are selected based on their amplitude and phase power spectrum stability over time.
The main outcomes of a PSI analysis are a deformation velocity map and the displacement time-series of the single point targets, or PSs. The velocity map represents the deformation rate of the detected PSs in Line-of-Sight of the sensor, generally in mm/yr. Usually, subsidence, e.g. target moving away from the sensor, is represented in red, stable PSs in green and uplift, e.g. PSs moving toward the sensor in blue. The displacement time-series show for each PS the amount of the deformation, usually in mm, over the whole period of observation. Different phase model can be defined in order to retrieve the best possible estimate of the deformation, considering also seasonal displacements or breakpoints in the time-series.
Performing PSI analysis in both ascending and descending directions allows the fusion of the results in order to retrieve vertical and East-West component of the deformation. North-South deformation components cannot be retrieved due to the orbit configuration of the SAR satellites.
PSI finds use in a large range of thematic applications related to subsidence and long-term change monitoring, such as infrastructure monitoring, groundwater reservoir monitoring, monitoring of mining areas, landslide inventory and monitoring, as well as volcanology.