Background

SAR Interferometry

The technique of repeat-pass spaceborne SAR interferometry was first demonstrated with data from the brief Seasat mission in 1978, and later with the Shuttle Imaging Radar Missions. With the sustained operation of the ESA ERS missions over a period of more than ten years, this technique has been validated and has delivered significant new scientific results in very diverse domains, ranging from volcanology to glaciology.

In some cases the results obtained rely on the availability of suitable SAR acquisitions over very long timescales (years) for monitoring slowly varying geophysical phenomena. A broad spectrum of new applications utilising this technique has been demonstrated, including digital elevation mapping, crustal motion monitoring, land subsidence monitoring and land cover classification.

Limitations

The suitability of repeat-pass SAR acquisitions for InSAR-based applications depends on several independent factors, including baseline and change of surface and atmospheric conditions between two acquisitions. When one or several of these factors are inappropriate (e.g. too large baseline, strong variations of water vapour content in the atmosphere, loss of coherence etc.), no InSAR information can be obtained from this data through the traditional InSAR techniques.

Example of Persistent Scatterer Interferometry output

Persistent Scatterers

In the late 1990’s it was discovered that some (usually) bright targets retain their phase and amplitude stability for a period of months or years, and that the phase information of these targets (here denoted Persistent Scatterers (PS)) can be exploited even on otherwise uncorrelated long time scale interferograms. This led to the development of innovative methodologies pioneered by the Politecnico di Milano that enable the detection of the relative positions of the PS in the direction of the satellite line-of-sight (LOS) with millimetric accuracy.

These methodologies are collectively referred to as PSI-methods, where PSI stands for Persistent Scatterer Interferometry.

With the PSI techniques it is possible to exploit the phase information present in large stacks of images and to reconstruct time-series of the relative PS positions since 1992 on areas covered by the ERS and Envisat SAR image archives. This can provide unique and indispensable information in many application fields, for example in ground subsidence detection, monitoring of potentially unstable structures and landslide monitoring.

Evidence of subsidence in Milano

Catastophic collapse in Paris

Uncertainty

The PSI-methods are a group of new and technologically advanced technologies for measuring relative position, which is why people outside the radar community generally have little or no experience in utilising the positional data produced by them. During the Fringe 2003 Workshop, “Advances in SAR Interferometry from ERS and ENVISAT Missions,” organised in ESRIN in December 2003 it was recognised that amongst the end-users of PSI-produced data there is uncertainty about the accuracy and trustworthiness of the measurements. This uncertainty was also perceived to be hindering both the wider acceptance of PSI-products and the development of the emerging commercial PSI-market.

ESA decided to act on the recommendation of the Fringe 2003 Workshop and initiated the PSIC4 Project in order to produce reliable information about the accuracy and dependability of the PSI-methodologies.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry