A SAR signal contains amplitude and phase information. Amplitude is the strength of the radar response and phase is the fraction of one complete sine wave cycle (a single SAR wavelength). The phase of the SAR image is determined primarily by the distance between the satellite antenna and the ground targets.
Interferometric SAR (InSAR) exploits the phase difference between two complex radar SAR observations of the same area, taken from slightly different sensor positions, and extracts distance information about the Earth's terrain.
By combining the phase of these two images after coregistration, an interferogram can be generated where phase is highly correlated to the terrain topography and deformation patterns can be mapped. If the phase shift related to topography is removed from the interferograms, the difference between the resulting products will show surface deformation patterns occurred between the two acquisition dates. This methodology is called Differential Interferometry (DInSAR).
The following interferogram of Bam, Iran shows the terrain deformation following a M6.6 earthquake on December 26th 2003, which killed 26,271 people and injured an additional 30,000. The images were acquired from ENVISAT ASAR on December 3rd 2003 and February 11th 2004 with a baseline (spatial separation between satellite orbits) of 14 m. The coloured fringes map the deformation of the surface of the Earth in the direction of the view from the satellite in units of the radar wavelength (2.8 cm) between colour cycles.
Figure 1: ENVISAT Interferogram of Bam, Iran
Persistent Scatterer Interferometry (PSI) is a branch of interferometry that exploits point scatterers, with strong radar backscatter, over a long time period (years) to provide a phase history of the point target over time. Persistent scatterers can be small, usually manmade, features that remain very correlated over time.
Conventional DInSAR can have limitations with respect to discrimination between the effects of displacement and atmospheric signature. PSI techniques can overcome such limitations by relaxing usual baseline and temporal constraints and maximising the number of useable interferograms, which can then be used to calculate mean trends over time from a large history of interferograms. Only the targets with sufficiently high coherence are considered, resulting in reduced pixel density.
Figure 2: Terrain Deformation Map of Murcia, Spain Using the PSI Technique. The Maximum Subsidence (Red) is About 5 cm per Year. (image courtesy of Altamira Information TerraFirma project)
Applications of InSAR include:
- geophysical monitoring of natural hazards: earthquakes, volcanoes and landslides
- time-series analysis of surface deformation: subsidence and structural stability
- glacier motion analysis
- digital elevation mapping.
ESA's Next Generation User Services for Earth Observation (ngEO) can be used to search for suitable interferometric datasets, taking baseline, Doppler centroid and burst synchronisation criteria into account.
For an introduction to interferometric concepts, please see ESA's InSAR Principles: Guidelines for SAR Interferometry Processing and Interpretation (ESA TM-19).