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Frequently Asked Questions

Persistent Scatterer Interferometry (PSI) - Frequently Asked Questions

Persistent Scatterer Interferometry, or PSI, revolutionises the precision surveying of ground and structural motions. Using this powerful space-based technique, the motion of individual structures and ground features can be measured to millimetre-level relative accuracy over whole cities or regions in a single process.

The key advantages of PSI are:

Non-invasive surveying of large areas in a single process.
Cost-effective.
Helps prioritise expensive ground-based survey.
Sub-millimetre relative accuracy.
Global data archive means measurements can be made back in time, providing an area's motion history.

The power of PSI resides in the archive of radar data that stretches back to 1992. With repeat acquisitions having been (and continuing to be) made at the rate of nearly one a month over most areas (each 'scene' covers an area of 100km x 100km), the signal-phase location of each individual Persistent Scatterer (PS) can be derived over time. This results in the motion history of each PS within an array, as well as an interpolated 2D map displaying the average annual velocities for the whole region under investigation.

+ What is a Persistent Scatterer?
+ Which regions are covered?
+ What are the detectable rates of movement?
+ Are measurements absolute?
+ How is it possible to look back in time?
+ What is a feasibility study?
+ Specifications

What is a Persistent Scatterer?

The PSI process employs the data acquired by European radar satellites to identify pre-existing features on the ground that provide a strong and time persistent reflection of the emitted radar signal back to the satellite. Such arrays of Persistent Scatterers (PSs) tend to comprise buildings, pylons, lamp-posts, even bare rocks - in fact anything displaying suitable reflective and geometric properties. A dense urban environment, for example, will exhibit many hundreds of PSs per square kilometre, whereas more rural environments may contain a few tens of points per square kilometre.

Which regions are covered?

Since 1992, the 'ERS' range of European radar satellites have acquired a world-wide data archive totalling approximately 1.5 million scenes, increasing daily as new acquisitions continue to be made. This means that PSI can be confidently applied to most areas in Europe, with a measurement sampling rate of nearly 10 per year at every single PS. A feasibility study is recommended for non-urban areas and regions outside Europe where PS density or scene availability could be too low for the application. In such cases artificial PSs (corner reflectors or Compact Active Transponders, CATs) can be deployed for site-specific surveys.

What are the detectable rates of movement?

Some phenomena such as clay shrink/swell cause slow ground motions, while others such as coal mining cause much faster movements. The maximum amount of motion that PSI can detect between two successive satellite data acquisitions is approximately 1 cm (determined by inherent functional parameters such as the radar's operating wavelength and data acquisition frequency). Assuming a maximum number of data acquisitions per year, this equates to a maximum of 10 cm detectable movement per year. measurement point moves by more than this amount and/or moves strongly non-linear in time (e.g. a dramatic land movement after a period of stability), a measurement gap results.

Are measurements absolute?

PSI measurements are not absolute but relative, in both location and time, to a predetermined 'reference' PS and date, respectively. Ideally, the reference PS should be known to be stable to make sense of the motion displayed by the remaining PSs, though this can be corrected via post-processing if necessary.

How is it possible to look back in time?

The PSI process relies on a large volume of satellite radar data, normally around 70 to 100 archived scenes, dating back to 1992. The availability of these data is thanks to the European Space Agency's ERS-1 satellite (operating from 1991 to 1997) and its successor, ERS-2 (1995 to present). These satellites have collected a regularly updated archive of more than 1.5 million scenes, now being augmented by the Agency's Envisat satellite (2002 to present). In terms of PSI, every scene in the archive over a given site yields a motion measurement at every PS in that scene.

What is a feasibility study?

The PSI process relies on the serendipitous distribution of pre-existing features on the ground that happen to behave as PSs. In other words, the practitioner has no direct control over what is, and what is not a PS. In urban areas where suitable features are plentiful and sufficient radar data resides in archive, processing can be undertaken with a good degree of confidence. However, in rural areas or regions where data coverage is relatively low, or topography is extreme, the confidence for producing a worthwhile result might diminish. To prevent unnecessary financial outlays for full processes that might yield poor results, NPA has devised a PSI feasibility approach. This involves the procurement of a small number of scenes from archive, some basic radar processing and an amplitude thresholding to determine that sufficient ground features reflect strongly and persist throughout the scenes used.

Specifications:

Potential time coverage: 1992 - 2005
Potential area coverage: Worldwide
Typical point coverage: 100-200 / km2 over urban areas and 10s / km2 over rural areas
Data sources: SAR (Synthetic Aperture Radar) acquired by ERS-1, ERS-2, Envisat (operated by ESA) or Radarsat (operated by Radarsat & CSA)
Measurement direction: Sensor line-of-sight (23° from vertical for ERS/Envisat)
Precision of average annual velocity: ~ ± 0.1mm / year
Relative movement resolution: Sub-millimetric to millimetric, depending on parameters such as number of data scenes, atmospheric conditions, local topography, distance from the reference location.
Elevation precision: better than 1m
Relative spatial accuracy: ± 5m in E-W, ± 3.5m in N-S
Absolute spatial accuracy: ~ 15m

Output format

The generic PSI data product includes:

1) Table of PS time series and average annual displacement rate

delivered as tabular data in dBase IV (.DBF) format
includes for each PS, geographic point co-ordinates, displacement time series, average annual displacement rate, height error and average annual displacement rate error

2) Processing summary report (metadata)

includes a number of quantitative and some qualitative site specific information, such as listing of data used, reference scene, reference point location co-ordinates and basic ground motion statistics
hardcopy velocity map

3) Map of average annual displacement rates (provided on request)
4) Reference multi-image reflectivity map (MIR) image (provided on request)

For full details see PSI Deliverables