SAR Interferometry and TRE’s proprietary techniques

InSAR is a remote sensing technique that can be used to accurately measure ground displacement.

Radar sensors mounted on satellites transmit microwave signals toward a target area, some of which are reflected back to the satellite. These ‘back scattered’ signals are read and stored by the satellite sensor to form radar images of the target area. Sophisticated software then compares pairs of images of the same target to detect changes in the ground surface, such as displacement, that have occurred in the time span between the two acquisitions. InSAR is a non-intrusive, non-destructive technology that measures relative displacement over time, with sub-centimeter accuracy.
InSAR is however limited by the impossibility of removing errors introduced by atmospheric effects and is only able to measure total displacement and average displacement rates; it cannot distinguish between linear and non-linear movement.

PSI technologies first emerged in 1999 when the Polytechnic University of Milan (POLIMI), in Italy, produced and patented its PSInSAR™ algorithm.

This technique is a significant evolution of conventional InSAR whereby:

• A multi-image data set is used
• Atmospheric and orbital errors are essentially removed
• Sub-pixel radar reflections are analyzed
• Linear and non-linear deformation patterns are identified
• Time histories of movement are generated for every radar target

The use of multi-image datasets makes it possible to identify stable reflectors, referred to as permanent scatterers, or PS, which are points on the ground that consistently return stable signals to the satellite sensor. These PS allow ground displacement velocities to be measured with millimeter accuracy. PS typically correspond to objects on man-made structures such as buildings, bridges, dams, water-pipelines, antennae, as well as to stable natural reflectors (e.g. exposed rocks).

The PSInSAR™ algorithm was licensed exclusively to TRE for world-wide application.

Ten years after PSInSAR™, TRE has developed a new algorithm, namely SqueeSAR™, which represents a further advancement for satellite data analysis and a breakthrough in Earth observation capabilities. 
Beyond PS, we noticed that distributed scatterers also exist and that they too can be used for monitoring ground displacement. Distributed scatterers or DS consist of an extensive area where the back-scattered energy is less strong in some way, but statistically homogeneous within the area. Hence using our SqueeSAR™ algorithm, it is also possible to ’process’ this energy and detect the movement of areas namely dominated by DS, with the same accuracy as analysis with PS.
DS displacement time series are indeed less noisy too. DS typically correspond to debris areas, non-cultivated lands and desert areas.

It is also important to highlight that the PSInSAR™ processing chain is maintained and used within the SqueeSAR™ algorithm: the result is an enhancement of our information output capacity, meaning PS plus DS, to gain an enhanced insight into ground deformation and associated surface movements.

SqueeSAR™ algorithm in summary:

• Ground points identified: PS and DS
• High density of ground measurement points identified in urban areas (PS)
• High density of ground measurement points identified in non-urban areas (PS and DS)
• Time series provided for each ground point (PS and DS)
• Millimetre accuracy on ground displacement values
• Time series standard deviation reduces – i.e. coherence increases and noise decreases
• Increased confidence on ground behaviour due to increased coverage of points – especially significant for landslides, outcrops and generic areas with low reflectivity