PSI makes measurements of ground movement on naturally occurring
permanent scattering points. Persistent scatterers are features
such as the roofs of buildings, metallic structures and prominent
natural features. In urban areas, there can be as many as
600 persistent scatterers per square kilometre. Uniquely,
this technique provides the motion history (up to 12 years)
for each indivdual persistent scatterer.
Data Used
PSI uses radar satellite data acquired by the ERS-1, ERS-2,
Radarsat and Envisat satellites. All of the UK, most of mainland
Europe and much of the US and Japan have sufficient data to
allow PSI processing, with new data being acquired regularly.
Due to the large volume of data the maximum area processable
in one block is currently 50 x 50km.
Removal of atmospheric and topographic influence
PSI overcomes atmospheric and topographic induced errors
(related to certain weather conditions and DEM accuracy) by
utilising 30 or more scenes to calculate:
An atmospheric correction which is calculated from the
30 scene archive that removes atmospheric artefacts from
the interferograms.
Measurements of customer specified points using artificial reflectors
Corner Reflectors (CRInSAR)
For measurements at specific, predetermined
points, inexpensive radar corner reflectors can be deployed,
acting as purpose-built persistent scatterers. NPA’s
work has contributed towards the development of Corner
Reflector InSAR as an ideal tool for monitoring the displacement
of engineered developments such as reservoirs, dams, bridges,
pipelines, buildings, or even areas of slope instability.
Compact Active Transponders (CATInSAR)
Currently developed Compact Active Transponders
(CATs) are automatic devices that transmit a radar signal
towards the overpassing radar satellite. This transmission
is recorded by the satellite in the same manner as a beam
reflected by a corner reflector, and allows the same precise
displacement measurements to be made. CATs are simpler to
install than corner reflectors, are less structurally vulnerable,
and promise a considerable cost saving. They are currently
being assessed by NPA as natural successors to corner reflectors
for customer specified measurement points.
In a project funded by the European Space Agency,
NPA were working with Thames Water, the world's third largest
water company, using corner reflector interferometry to monitor
the displacement of retaining banks associated with water
volume in their West London reservoirs. Seven inexpensive
reflectors were deployed around the Wraysbury reservoir near
London.
