What Is Synthetic Aperture Radar Interferometry?

Synthetic aperture radar interferometry (InSAR) is a geodetic and remote sensing technique that extracts topographic or surface displacement information by analyzing the phase difference between two or more coherent SAR images acquired from slightly different positions or at different times. Because the phase of each SAR pixel encodes the precise round-trip distance from the antenna to the ground, any shift in that distance between two acquisitions appears as a measurable phase change in the interferogram. Millimeter-scale surface movements that are invisible to optical imagery can be detected through this phase signal over large geographic areas and without ground instrumentation.

InSAR was demonstrated operationally in the early 1990s after the European Space Agency launched ERS-1 in 1991 and built a systematic archive of repeat-pass SAR data. A landmark 1993 study by Didier Massonnet and colleagues used ERS-1 interferometry to map the coseismic deformation field of the 1992 Landers earthquake, establishing that spaceborne InSAR could deliver centimeter-accuracy deformation maps at regional scale. The ESA Technical Manual on InSAR Principles remains a standard reference for processing and interpretation guidelines used across the research community.

Phase, Coherence, and Interferogram Formation

An interferogram is produced by multiplying one complex SAR image by the complex conjugate of another. The resulting phase at each pixel contains contributions from the topographic path length difference (controlled by the spatial baseline between the two acquisition geometries), any surface displacement along the line of sight that occurred between acquisitions, atmospheric delay, and noise. A flat-earth phase term from the spheroid and a topographic phase term from a reference digital elevation model are typically subtracted from the raw interferogram to isolate the displacement signal. Coherence, a measure of signal similarity between the two images, quantifies whether the phase is reliable: high coherence (values near 1) indicates stable scatterers, while low coherence (values near 0) appears over vegetated surfaces, water, or areas with significant change between passes.

Differential InSAR and Time-Series Methods

Differential InSAR (DInSAR) removes the topographic contribution using an external elevation model and retains the deformation signal, enabling detection of ground motion caused by earthquakes, volcanic inflation, mining subsidence, and groundwater extraction. The USGS uses InSAR to map land subsidence in agricultural and urban basins where water or hydrocarbon extraction has caused measurable compaction. Time-series InSAR methods such as Persistent Scatterer InSAR (PSInSAR) and Small Baseline Subsets (SBAS) extend the technique by stacking many interferograms to separate seasonal, linear, and nonlinear deformation signals while suppressing atmospheric artifacts. These methods can resolve deformation rates below 1 millimeter per year when applied to a sufficiently large stack of acquisitions over coherent scatterers.

Topographic Mapping

Single-pass InSAR, in which two antennas separated by a fixed baseline acquire data simultaneously, eliminates temporal decorrelation entirely and produces digital elevation models (DEMs) of high precision. The Shuttle Radar Topography Mission (SRTM) in February 2000 used this principle with a 60-meter mast-deployed antenna aboard the Space Shuttle Endeavour to map 80 percent of Earth's land surface between 56°S and 60°N with approximately 16-meter vertical accuracy. The SRTM DEM remains a globally available reference dataset used across hydrology, geomorphology, and navigation. Airborne single-pass interferometers such as those operated by the German Aerospace Center (DLR) achieve sub-meter vertical accuracy over local areas.

Applications

Synthetic aperture radar interferometry has applications in a wide range of disciplines, including:

• Seismology: coseismic and postseismic deformation mapping for earthquake source characterization
• Volcanology: pre-eruptive ground inflation and subsidence monitoring using repeat-pass satellite observations
• Hydrology: groundwater basin subsidence and aquifer recharge monitoring
• Glaciology: ice sheet velocity mapping and grounding line migration detection
• Urban infrastructure: building and infrastructure settlement monitoring over cities and transport corridors