InSAR
Interferometric Synthetic Aperture Radar. A technique that compares two or more SAR images of the same area taken at different times to detect surface deformation with millimeter-level precision. Used for monitoring ground subsidence, volcanic activity, earthquake displacement, and infrastructure stability.
Overview
Interferometric Synthetic Aperture Radar (InSAR) is a geodetic technique that compares phase information from two or more SAR acquisitions to measure surface topography or detect ground deformation with millimeter-level precision. Differential InSAR (DInSAR) isolates surface displacement by removing topographic phase, while Persistent Scatterer InSAR (PS-InSAR) tracks stable reflectors over years to build long-term deformation time series.
How It Works
InSAR exploits the fact that SAR sensors record both amplitude and phase. When two images are acquired from slightly different positions or times, the phase difference produces an interferogram — a map of fringes where each cycle represents half the radar wavelength of path-length change (~2.8 cm for Sentinel-1 C-band). For deformation monitoring, topographic phase is subtracted using a known DEM, and residual phase reveals ground movement along the satellite's line of sight.
PS-InSAR identifies pixels maintaining coherent phase over many acquisitions — buildings, rock outcrops — and uses them to measure displacement rates as small as 1 mm/year with sub-millimeter precision over time series spanning years.
Key Facts
- Each fringe in an interferogram represents half the radar wavelength of ground displacement (~2.8 cm for Sentinel-1 C-band).
- PS-InSAR can measure displacement rates as small as 1 mm/year over multi-year time series.
- Sentinel-1 is the primary data source for operational InSAR monitoring due to its free data and systematic 6-day revisit.
- The European Ground Motion Service (EGMS) uses InSAR to provide continent-wide ground deformation measurements for all of Europe.
- InSAR requires coherence between acquisitions — dense vegetation and water surfaces lose coherence rapidly.
Applications
Ground Subsidence Monitoring
Detecting land sinking from groundwater extraction, oil/gas production, mining, or tunneling in urban and industrial areas.
Earthquake and Tectonic Studies
Mapping co-seismic surface deformation and post-seismic relaxation to understand fault mechanics and seismic hazard.
Volcano Monitoring
Detecting magma chamber inflation/deflation by measuring centimeter-scale ground uplift or subsidence near volcanic centers.
Infrastructure Stability Assessment
Monitoring structural deformation of bridges, dams, buildings, and railway embankments at millimeter precision without physical instrumentation.
Limitations & Considerations
InSAR requires coherence between image pairs — vegetation, water, and agricultural fields cause decorrelation that destroys the phase signal. Atmospheric water vapor introduces path delays that can mimic or mask real deformation signals. The technique measures displacement only along the satellite line of sight, requiring ascending and descending passes to resolve full 3D motion. Phase unwrapping — converting cyclic fringes into absolute displacement — can fail in areas of rapid deformation or low coherence.
History & Background
InSAR was first demonstrated for topographic mapping using Seasat data in the 1980s. The technique gained prominence after the 1992 Landers earthquake in California, when Massonnet et al. produced the first satellite-based coseismic deformation map using ERS-1 data. PS-InSAR was developed by Alessandro Ferretti and colleagues at Politecnico di Milano in the early 2000s. The Sentinel-1 mission (2014) revolutionized InSAR by providing free, systematic, global SAR coverage with a 6-day revisit cycle.
Related Terms
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